Contents:
Strobe Lights and Design Guidelines, Useful Circuits, and Schematics
Copyright (c) 1994, 1995, 1996, 1997, 1998
All Rights Reserved
Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied:
Thanks to Don Klipstein (email: don@misty.com) for his comments and additions to this document. His Web site (http://www.misty.com/~don/) is a valuable resource for information relating to lighting technology in general and also includes additional articles dealing with strobe principles and design.
All modern electronic flash units (often called photographic strobes) are based on the same principles of operation whether of the subminiature variety in a disposable pocket camers or high quality 35 mm camera, compact separate hot shoe mounted unit, or the high power high performance unit found in a photo studio 'speed light'. All of these use the triggered discharge of an energy storage capacitor through a special flashtube filled with xenon gas at low pressure to produce a very short burst of high intensity white light. The typical electronic flash consists of four parts: (1) power supply, (2) energy storage capacitor, (3) trigger circuit, and (4) flashtube. An electronic flash works as follows: 1. The energy storage capacitor connected across the flashtube is charged from a 300V (typical) power supply. This is either a battery or AC adapter operated inverter (pocket cameras and compact strobes) or an AC line operated supply using a power transformer or voltage doubler or tripler (high performance studio 'speed' lights). These are large electrolytic capacitors (200-1000+ uF at 300+ V) designed specifically for the rapid discharge needs of photoflash applications. 2. A 'ready light' indicates when the capacitor is fully charged. Most monitor the voltage on the energy storage capacitor. However, some detect that the inverter or power supply load has decreased indicating full charge. 3. Normally, the flashtube remains non-conductive even when the capacitor is fully charged. 4. A separate small capacitor (e.g., .1 uF) is charged from the same power supply to generate a trigger pulse. 5. Contacts on the camera's shutter close at the instant the shutter is fully open. These cause the charge on the trigger capacitor to be dumped into the primary of a pulse transformer whose secondary is connected to a wire, strip, or the metal reflector in close proximity to the flashtube. 6. The pulse generated by this trigger (typically around 4-10 KV depending on the size of the unit) is enough to ionize the xenon gas inside the flashtube. 7. The xenon gas suddenly becomes a low resistance and the energy storage capacitor discharges through the flashtube resulting in a short duration brilliant white light. The energy of each flash is roughly equal to 1/2*C*V^^2 in watt-seconds (W-s) where V is the value of the energy storage capacitor's voltage and C is its capacitance. Not quite all of the energy in the capacitor is used but it is very close. The energy storage capacitor for pocket cameras is typically 100-400 uF at 330 V (charged to 300 V) with a typical flash energy of 10 W-s. For high power strobes, 1000s of uF at higher voltages are common with maximum flash energies of 100 W-s or more. Another important difference is in the cycle time. For pocket cameras it may be several seconds - or much longer as the batteries run down. For a studio 'speed light', fractional second cycle times are common. Typical flash duration is much less than a millisecond resulting in crystal clear stop action photographs of almost any moving subject. On cheap cameras (and probably some expensive ones as well) physical contacts on the shutter close the trigger circuit precisely when the shutter is wide open. Better designs use an SCR or other electronic switch so that no high voltage appears at the shutter contacts (or hot shoe connector of the flash unit) and contact deterioration do to high voltage sparking is avoided. Note that for cameras with focal plane shutters, the maximum shutter speed setting that can be used (X-Sync) is typically limited to 1/60-1/120 of a second. The reason is that for higher shutter speeds, the entire picture is not exposed simultaneously by the moving curtains of the focal plane mechanism. Rather, a slit with a width determined the by the effective shutter speed moves in front of the film plane. For example, with a shutter speed setting of 1/1000 of a second, a horizontally moving slit would need to be about 1/10 of an inch wide for a total travel time of 1/60 of a second to cover the entire 1.5 inch wide 35 mm frame. Since the flash duration is extremely short and much much less than the focal plane curtain travel time, only the film behind the slit would be exposed by an electronic flash. For shutter speed settings longer than the travel time, the entire frame is uncovered when the flash is triggered. See the chapter: "Complete Strobe Schematics" for typical circuit configurations of both battery and line powered electronic flash units of various sizes. Red-eye reduction provides a means of providing a flash twice in rapid succession. The idea is that the pupils of the subjects' eyes close somewhat due to the first flash resulting in less red-eye - imaging of the inside of the eyeball - in the actual photograph. This may be done by using the main flash but many cameras use a small, bright incandescent bulb to 'blind' the eyes when the shutter is pressed to meter, then it goes off and the flash preserves the 'closed' pupils. This approach works. Using the main flash would require sub-second recycle time which is not a problem if an energy conserving flash is used (see the section: "Vivitar Auto/Thyristor 292 energy conserving automatic flash". However, it would add significant additional expense otherwise (as is the case with most cameras with built in electronic flash). A separate little bulb is effective and much cheaper. Failure of red-eye reduction or the automatic exposure control circuits will probably require a schematic to troubleshoot unless tests for bad connections or shorted or open components identify specific problems. However, some of these use fairly simple circuits with mostly standard components and can be traced without too much difficulty. For red-eye in particular, It is also possible for that extra incandescent light bulb to be burnt out but good luck replacing it! Remotely triggered 'fill flashes' use a photocell or photodiode to fire an SCR (or light activated SCR) which emulates the camera shutter switch closure for the flash unit being controlled. There is little to go wrong with these devices.
Automatic electronic flash units provide an optical feedback mechanism to sense the amount of light actually reaching the subject. The flash is then aborted in mid stride once the proper exposure has been made. This means that the flash duration will differ depending on exposure - typically from 1 ms at full power to 20 us or less at close range. Inexpensive automatic flash units just short across the flashlamp with an SCR or second internal 'quench' tube (an internal small xenon tube that looks like an oversize neon indicator lamp) triggered by a photosensor. See the sections starting with: "Vivitar Auto 253 electronic flash circuit". With these units, the same amount of energy is used regardless of how much light is actually required and thus low and high intensity flashes drain the battery by the same amount - and require the same cycle time. The excess energy is wasted. Note that it is not the distance to the subject that matters but the amount of total light energy reflected back to the sensor. The travel time of the light has nothing to do with controlling exposure. More sophisticated units use something like a Gate TurnOff Thyristor (GTO) to actually interrupt the flash discharge at the proper instant. See the section: "Vivitar Auto/Thyristor 292 energy conserving automatic flash". These use only as much energy as needed and the batteries last much longer since most flash photographs do not require maximum power. Furthermore, when using low power flashes, the cycle time is effectively zero since the main energy storage capacitor does not discharge significantly. Therefore, multiple shots can be taken in rapid succession. Failure of the automatic exposure control circuits will probably require a schematic to troubleshoot unless tests for bad connections or shorted or open components identify specific problems. However, some of these use fairly simple circuits with mostly standard components and can be traced without too much difficulty though the compactness of modern flash units makes this somewhat more of a challenge.
There are two potential hazards in dealing with the innards of electronic flash and other xenon strobe equipment: 1. The energy storage capacitor. Even on small pocket camera electronic flash units, these are rated at 100-400 uF at 330 VDC. This is 5-20 W-s which is enough to kill you under the right (wrong?) conditions. Hot shoe or side mounted electronic flash units have energy storage capacitors which are usually larger - typically 300-1000 uF or more. High performance studio speed lights may have 10 times this capacity and at much higher voltages resulting in even greater energy storage. Xenon strobes for pumping of solid state laser rods and other industrial and scientific applications may use many KV power supplies with 1000s of W-s energy storage capacitors - touch one of these and you will be but a puff of vapor in the wind... High voltage with high energy storage is an instantly deadly combination. Treat all of these capacitors - even those in tiny pocket cameras with respect. Always confirm that they are discharged before even thinking about touching anything. On larger systems especially, install a shorting jumper after discharging just to be sure - capacitors have been known to recover a portion of their original charge without additional power input. Better to kill the power supply than yourself if you forget to remove it when powering up. 2. Line connected (no power transformer) have all the dangers associated with AC line power in addition to the large power supply and energy storage capacitors. Always use an isolation transformer when probing line connected systems. However, keep in mind that the power supply filter capacitors and energy storage capacitors remain just as deadly. Additional important safety information regarding shock, excessively bright light, ultraviolet radiation, heat and fire hazards, and other hazards is available at: http:/www.misty.com/~don/xesafe.html. Reading and following these recommendations and heeding the warnings is especially important when working with high power strobes.
These guidelines are to protect you from potentially deadly electrical shock hazards as well as the equipment from accidental damage. Note that the danger to you is not only in your body providing a conducting path, particularly through your heart. Any involuntary muscle contractions caused by a shock, while perhaps harmless in themselves, may cause collateral damage - there are many sharp edges inside this type of equipment as well as other electrically live parts you may contact accidentally. The purpose of this set of guidelines is not to frighten you but rather to make you aware of the appropriate precautions. Electronic construction, testing, and troubleshooting can be fun and rewarding and economical. Just be sure that it is also safe! * Don't work alone - in the event of an emergency another person's presence may be essential. * Always keep one hand in your pocket when anywhere around a powered line-connected or high voltage system. * Wear rubber bottom shoes or sneakers. * Wear eye protection - large plastic lensed eyeglasses or safety goggles. * Don't wear any jewelry or other articles that could accidentally contact circuitry and conduct current, or get caught in moving parts. * Set up your work area away from possible grounds that you may accidentally contact. * Know your equipment: TVs and monitors may use parts of the metal chassis as ground return yet the chassis may be electrically live with respect to the earth ground of the AC line. Microwave ovens use the chassis as ground return for the high voltage. In addition, do not assume that the chassis is a suitable ground for your test equipment! * If circuit boards need to be removed from their mountings, put insulating material between the boards and anything they may short to. Hold them in place with string or electrical tape. Prop them up with insulation sticks - plastic or wood. * If you need to probe, solder, or otherwise touch circuits with power off, discharge (across) large power supply filter capacitors with a 2 W or greater resistor of 10-50 ohms/V approximate value (e.g., for a 200 V capacitor, use a 2K-10K ohm resistor). Monitor while discharging and/or verify that there is no residual charge with a suitable voltmeter. * Connect/disconnect any test leads with the equipment unpowered and unplugged. Use clip leads or solder temporary wires to reach cramped locations or difficult to access locations. * If you must probe live, put electrical tape over all but the last 1/16" of the test probes to avoid the possibility of an accidental short which could cause damage to various components. Clip the reference end of the meter or scope to the appropriate ground return so that you need to only probe with one hand. * Perform as many tests as possible with power off and the equipment unplugged. For example, the semiconductors in the power supply section of a TV or monitor can be tested for short circuits with an ohmmeter. * Use an isolation transformer if there is any chance of contacting line connected circuits. A Variac(tm) is not an isolation transformer! The use of a GFCI (Ground Fault Circuit Interrupter) protected outlet is a good idea but will not protect you from shock from many points in a line connected TV or monitor, electronic flash or strobe, or the high voltage side of a microwave oven, for example. (Note however, that, a GFCI may nuisance trip at power-on or at other random times due to leakage paths (like your scope probe ground) or the highly capacitive or inductive input characteristics of line powered equipment.) A fuse or circuit breaker is too slow and insensitive to provide any protection for you or in many cases, your equipment. However, these devices may save your scope probe ground wire should you accidentally connect it to a live chassis. * Don't attempt repair work, construction, or testing when you are tired. Not only will you be more careless, but your primary diagnostic tool - deductive reasoning - will not be operating at full capacity. * Finally, never assume anything without checking it out for yourself! Don't take shortcuts!
A working electronic flash or strobe may discharge its capacitors fairly quickly when it is shut off but most DO NOT do this. Furthermore, do not assume that triggering the flash fully discharges either the power supply filter or main energy storage capacitors fully - especially if it is a sophisticated automatic unit. The main filter capacitors in the low voltage power supply may have bleeder resistors to drain their charge relatively quickly - but resistors can fail. Don't depend on them. For battery powered equipment in particular, efforts may have been made NOT to bleed the energy storage capacitor to conserve on battery power should another shot be desired at a future time. Some units even keep the flash fully charged when supposedly turned off! The technique I recommend is to use a high wattage resistor of about 5 to 50 ohms/V of the working voltage of the capacitor. This will prevent the arc-welding associated with screwdriver discharge but will have a short enough time constant so that the capacitor will drop to a low voltage in at most a few seconds (dependent of course on the RC time constant and its original voltage). Then check with a voltmeter to be double sure. Better yet, monitor while discharging. Obviously, make sure that you are well insulated! For the power supply filter capacitors or main energy storage capacitors, which might be 400 uF at 350 V, a 2 K ohm 25 W resistor would be suitable. RC=.8 second. 5RC=4 seconds. A lower wattage resistor (compared to that calculated from V^^2 / R) can be used since the total energy stored in the capacitor is not that great (but still potentially lethal). The discharge tool and circuit described in the next two sections can be used to provide a visual indication of polarity and charge for TV, monitor, SMPS, power supply filter capacitors and small electronic flash energy storage capacitors, and microwave oven high voltage capacitors. Reasons to use a resistor and not a screwdriver to discharge capacitors: 1. It will not destroy screwdrivers and capacitor terminals. 2. It will not damage the capacitor (due to the current pulse). 3. It will reduce your spouse's stress level in not having to hear those scary snaps and crackles.
A suitable discharge tool for each of these applications can be made as quite easily. The capacitor discharge indicator circuit described below can be built into this tool to provide a visual display of polarity and charge (not really needed for CRTs as the discharge time constant is virtually instantaneous even with a multi-M ohm resistor. * Solder one end of the appropriate size resistor (for your application) along with the indicator circuit (if desired) to a well insulated clip lead about 2-3 feet long. For safety reasons, these connections must be properly soldered - not just wrapped. * Solder the other end of the resistor (and discharge circuit) to a well insulated contact point such as a 2 inch length of bare #14 copper wire mounted on the end of a 2 foot piece of PVC or Plexiglas rod which will act as an extension handle. * Secure everything to the insulating rod with some plastic electrical tape. This discharge tool will keep you safely clear of the danger area. Again, always double check with a reliable voltmeter or by shorting with an insulated screwdriver!
Here is a suggested circuit which will discharge the high voltage power
supply filter capacitors and main energy storage capacitors of most types
of electronic flash units and strobe lights. This circuit can be built
into the discharge tool described above.
A visual indication of charge and polarity is provided from maximum input
down to a few volts.
The total discharge time is approximately 1 second per 100 uF of capacitance
(5RC with R = 2 K ohms).
Safe capability of this circuit with values shown is about 500 V and 1000 uF
maximum. Adjust the component values for your particular application.
(Probe)
o-------+
In 1 |
/
\ 2 K, 25 W Unmarked diodes are 1N400X (where X is 1-7)
/ or other general purpose silicon rectifiers.
\
|
+-------+--------+
__|__ __|__ |
_\_/_ _/_\_ /
| | \ 100 ohms
__|__ __|__ /
_\_/_ _/_\_ |
| | +----------+
__|__ __|__ __|__ __|__ Any general purpose LED type
_\_/_ _/_\_ _\_/_ LED _/_\_ LED without an internal resistor.
| | | + | - Use different colors to indicate
__|__ __|__ +----------+ polarity if desired.
_\_/_ _/_\_ |
In 2 | | |
o-------+-------+--------+
(GND Clip)
The two sets of 4 diodes will maintain a nearly constant voltage drop of about
2.8-3 V across the LED+resistor as long as the input is greater than around
20 V. Note: this means that the brightness of the LED is NOT an indication
of the value of the voltage on the capacitor until it drops below about 20
volts. The brightness will then decrease until it cuts off totally at around
3 volts.
Safety note: always confirm discharge with a voltmeter before touching any
high voltage capacitors!
A variety of failures are possible with electronic flash units. Much of the circuitry is similar for battery/AC adapter and line powered units but the power supplies in particular do differ substantially. Most common problems are likely to be failures of the power supply, bad connections, dried up or deformed energy storage or other electrolytic capacitor(s) and physical damage to the to the flashtube or other components.
* Power source - dead or weak batteries or defective charging circuit, incorrect or bad AC adapter, worn power switch, or bad connections. Symptoms: unit is totally dead, intermittent, or has excessively long cycle time. Test and/or replace batteries. Determine if batteries are being charged. Check continuity of power switch or interlock and inspect for corroded battery contacts and bad connections or cold solder joints on the circuit board. * Power inverter - blown chopper transistor, bad transformer, other defective components. Symptoms: unit is totally dead or loads down power source when switched on (or at all times with some compact cameras). No high pitched audible whine when charging the capacitor. Regulator failure may result in excess voltage on the flashtube and spontaneous triggering or failure of the energy storage capacitor or other components. Test main chopper transistor for shorts and opens. This is the most likely failure. There is no easy way to test the transformer and the other components rarely fail. Check for bad connections.
WARNING: Line powered units often do not include a power transformer. Therefore, none of the circuitry is isolated from the AC line. Read, understand, and follow the safety guidelines for working on line powered equipment. Use an isolation transformer while troubleshooting. However, realize that this will NOT protect you from the charge on the large high voltage power supply and energy storage capacitors. Take all appropriate precautions. * Power source - dead outlet or incorrect line voltage. Symptoms: unit is totally dead, operates poorly, catches fire, or blows up. Spontaneous triggering may be the result of a regulator failure or running on a too high line voltage (if the unit survives). Test outlet with a lamp or circuit tester. Check line voltage setting on flash unit (if it is not too late!). * Power supply - bad line cord or power switch, blown fuse, defective rectifiers or capacitors in voltage doubler, defective components, or bad connections. Symptoms: unit is totally dead or fuse blows. Excessive cycle time. Test fuse. If blown check for shorted components like rectifiers and capacitors in the power supply. If fuse is ok, test continuity of line cord, power switch, and other input components and wiring. Check rectifiers for opens and the capacitors for opens or reduced value.
WARNING: the amount of charge contained in the energy storage capacitor may be enough to kill - especially with larger AC line powered flash units and high power studio equipment. Read and follow all safety guidelines with respect to high voltage high power equipment. Discharge the energy storage capacitors fully (see the section: "Safe discharging of capacitors in electronic flash units") and then measure to double check that they are totally flat before touching anything. Don't assume that triggering a flash does this for you (especially for automatic units). For added insurance, clip a wire across the capacitor terminals while doing any work inside the unit. Better to blow a fuse than you if you should forget to remove it. * Energy storage capacitor - dried up or shorted, leaky or needs to be 'reformed'. Symptoms: reduced light output and unusually short cycle time may indicate a dried up capacitor. Heavy loading of power source with low frequency or weak audible whine may indicate a shorted capacitor. Excessively long cycle time may mean that the capacitor has too much leakage or needs to be reformed. Test for shorts and value. Substitute another capacitor of similar or smaller uF rating and at least equal voltage rating if available. Cycling the unit at full power several times should reform a capacitor that has deteriorated due to lack of use. If the flash intensity and cycle time do not return to normal after a dozen or so full intensity flashes, the capacitor may need to be replaced or there may be some other problem with the power supply. * Trigger circuit - bad trigger capacitor, trigger transformer, SCR (if used), or other components. Symptoms: energy storage capacitor charges as indicated by the audible inverter whine changing frequency increasing in pitch until ready light comes on (if it does) but pressing shutter release or manual test button has no effect. Spontaneous triggering may be a result of a component breaking down or an intermittent short circuit. Test for voltage on the trigger capacitor and continuity of the trigger transformer windings. Confirm that the energy storage capacitor is indeed fully charged with a voltmeter. * Ready light - bad LED or neon bulb, resistor, zener, or bad connections. Symptoms: flash works normally but no indication from ready light. Or, ready light on all the time or prematurely. Test for voltage on the LED or neon bulb and work backwards to its voltage supply - either the trigger or energy storage capacitor or inverter trans- former. In the latter case (where load detection is used instead of simple voltage monitoring) there may be AC across the lamp so a DC measurement may be deceptive.) * Trigger initiator - shutter contacts or cable. Symptoms: manual test button will fire flash but shutter release has no effect. Test for shutter contact closure, clean hot shoe contacts (if relevant), inspect and test for bad connections, test or swap cable, clean shutter contacts (right, good luck). Try an alternate way of triggering the flash like a cable instead of a the hot shoe. * Xenon tube - broken or leaky. Symptoms: energy storage and trigger capacitors charges to proper voltage but the manual test button does not fire the flash even though you can hear the tick that indicates that the trigger circuit is discharging. Some xenon tubes have "getters", which are silver or dark silver coatings of a highly reactive metal, deposited on the inner surface of the flashtube at one or sometimes both ends. Less frequently, a getter may be found on a metal surface such as one of the electrodes inside the tube, but not on the tubing inner surface. The getter "gets" any traces of air or water vapor in the flashtube. If a flashtube with a getter is broken or leaky, the getter will be corroded into a powdery gray-white form. If you know there is a getter and it is corroded badly, the flashtube is no good. Please note that unrelated glass discoloration or staining that resembles corroded getters can occur in a heavily used or moderately abused flashtube that still works. Inspect the flashtube for physical damage. Substitute another similar or somewhat larger (but not smaller) flashtube. A neon bulb can be put across the trigger transformer output and ground to see if it flashes when you press the manual test button shutter release. This won't determine if the trigger voltage is high enough but will provide an indication that most of the trigger circuitry is operating.
The unit may be totally dead or take so long to charge that you give up. For rechargeable units, try charging for the recommended time (24 hours if you don't know what it is). Then, check the battery voltage. If it does not indicate full charge (roughly 1.2 x n for NiCds, 2 x n for lead-acid where n is the number of cells), then the battery is likely expired and will need to be replaced. Even for testing, don't just remove the bad rechargeable batteries - replace them. They may be required to provide filtering for the power supply even when running off the AC line or adapter. For units with disposable batteries, of course try a fresh set but first thoroughly clean the battery contacts. See the sections on batteries. The energy storage capacitor will tend to 'deform' resulting in high leakage and reduced capacity after long non-use. However, you should still be able to hear the high pitched whine of the inverter. Where the unit shows no sign of life on batteries or AC, check for dirty switch contacts and bad internal connections. Electrolytic capacitors in the power supply and inverter may have deteriorated as well. If the unit simply takes a long time to charge, cycling it a dozen times should restore an energy storage capacitor that is has deformed but is salvageable. This is probably safe for the energy storage capacitor as the power source is current limited. However, there is no way of telling if continuous operation with the excessive load of the leaky energy storage capacitor will overheat power supply or inverter components.
In addition to what is covered in this document, various circuits and design
guidelines are available at: http://www.misty.com/~don/donflash.html.
Among the features are:
* How to build your own trigger coil.
* A repetitive trigger circuit.
* Various parts sources including the use of Radio Shack stuff.
* Guidelines for estimate voltages and energy levels for flashtubes.
This site is constantly evolving so more interesting articles will likely
appear in the future.
Here are some general guidelines for the design of a small (5-20 W-s) battery
or line operated strobe.
Most small flashlamps will operate on about 300 V (some as low as 250 - or
less). If the flashlamp voltage is too low, the tube may not fire reliably
or at all. If the flashlamp voltage is too high, spontaneous firing or
damage and/or shortened flashlamp life due to excessive current may be
the result. For power, you will need one of the following:
1a. An inverter putting out about 300 VDC from your battery. Some of the
cheap disposable cameras use as little as 1.5 V but don't expect too much
battery life. There are zillions of simple inverter designs that will
work using either discrete transistors or ICs with some minimal external
components. The easiest way to obtain the inverter is to rip one out of
a dead camera. Try garage sales, flea markets, thrift stores, or your
Aunt Patty's attic. Typical cost for a cheap pocket camera from these
sources is $.50 to $2. I don't know what your Aunt charges. Otherwise,
you can build one easily. The only difficult part is finding a suitable
transformer. They are easy to wind but don't expect great efficiency
unless extreme care is taken in the design. For designing IC based DC-DC
convertors, check out companies like Maxim and Linear Technology. These
generally only require minimal external components like capacitors,
diodes, and an inductor or two - but often no transformers.
WARNING: If left charging for longer than needed to get the ready light
to come on, the actual voltage on the energy storage capacitor may
approach 400 V with some of these cameras! Take even more care.
1b. A line operated voltage doubler for 110 VAC (just a rectifier for 220
VAC). When the peak voltage of the AC line is considered, these supplies
will provide about 300-320 VDC. Common 1N4005/6/7 silicon rectifiers
and small (e.g., 16 uF) 250 V electrolytics can be used for the doubler.
Include a surge limiting resistor of about 22 ohms in the common as well
as a current limiting resistor in the output (before the energy storage
capacitor) to allow the flashlamp arc to quench (e.g., 100-1000 ohms).
A line fuse, power switch, and power indicator are also essential.
Warning: this is a non-isolated line operated power supply - see safety
guidelines. Do not connect triggering circuit directly - use capacitive
or transformer coupling for safety.
2. An energy storage capacitor. A 200 uF capacitor charged to 320 V will
give you 1/2*C*V*V = 10 W-s. Xenon flashlamps are rated in terms of both
maximum flash energy and maximum average power (as well as others but for
small strobe units - under 25 W-s or so - these are the most critical).
These ratings should not be exceeded. For example, a tube rated at 20 W-s
flash energy and 5 W average power could be flashed at most once every 4
seconds at a 20 W-s level or at most once every second at a 5 W-s level.
Use a smaller capacitor for more frequent flashing. While photoflash rated
capacitors are desirable, you should be able to get away with any good
quality electrolytic for this type of modest power application. Note
that the typical pocket camera flash uses a 100-400 uF capacitor and puts
out quite a lot of light.
3. A trigger circuit. This is usually a HV pulse transformer into whose
primary you discharge a small capacitor - .1 uF at 100-300 V is typical.
The high voltage secondary is designed to put out 4-10 KV depending on
flashlamp size and type. If the voltage of the trigger pulse is too
low, the flashlamp may not fire or may fire erratically. If the
trigger voltage is too high, there may be arcing to the flashlamp
electrodes or other components resulting in possible damage. The
trigger output is connected with a short wire to an electrode (wire,
foil, or metal reflector) that is in close proximity to the xenon tube.
The high voltage pulse ionizes the xenon gas mixture allowing the
storage capacitor to discharge through it. Trigger transformers are
available from places like Mouser Electronics. These can also be
constructed relatively easily. See the article: "How to build your own
trigger coil" at http://www.misty.com/~don/donflash.html. Although not
very compact, a TV or monitor flyback or automotive ignition coil will
also work as a trigger transformer.
An SCR can be substituted for physical switch contacts where electronic
control of the trigger is desired. For the battery powered unit, there
is no issue of line isolation and the cathode of the SCR can be tied
directly to the ground of your logic circuits. However, with the line
operated strobe, isolation is essential for safety - use capacitor or
transformer coupling, or an optoisolator.
The common photographic strobe is not really designed for very short flash
duration. While a typical electronic flash is much much shorter than one
of those antique flash bulbs, it is still long compared to what is possible.
Typical flash duration for a full power flash is under a millisecond with
the range of automatic units going down to 20 microseconds or less for a
minimum energy flash. One of those antique flash bulbs, on the other hand,
had a flash duration of between 5 and 20 milliseconds. For most common
photography, 1 millisecond or less is for all intents and purposes,
instantaneous. However, if you want to freeze the blades of a rotating
turbine or stop bullet in flight, even 20 microseconds is way too long.
Some of the highest speed photographs using the light source to control
exposure have been taken with spark gaps operating at many KV resulting
in flash durations as low as fractions of microseconds. Even higher speed
photography is possible using electronic image tubes. The first instants
of conventional or nuclear detonations have been captured using this type
of technology.
For more information on high speed photography, see the classic works by
Harold E. ("Doc") Edgerton. The following are just some general comments:
Several design parameters influence flash intensity, duration, and maximum
repeat rate. However, the relationships are not linear as a flashlamp is
a gas discharge device with complex nonlinear resistance characteristics.
It is necessary to consult the flashlamp manufacturer's data sheets to do
any detailed design.
1. Voltage. For a given energy, flash duration varies inversely with
flash lamp voltage. The higher the voltage, the shorter the flash.
2. Capacitor size in uF. Total flash light output is proportional to the
energy storage capacitor uF rating. However, both the peak intensity
and the flash duration will increase with a larger capacitor.
3. Impedance of discharge path. Since the circuit when triggered is basically
a capacitor discharging into a low impedance load, both the duration and
peak intensity are affected. In addition, for higher capacity strobes
especially, controlling this impedance is critical to achieving optimal
light output as well as maximizing the life of the flash lamp. Excessive
peak discharge current as well as reverse current due to overshoot and
ringing reduces flash lamp life through damage to the electrodes. Too
much instantaneous current and the flashlamp may explode.
4. Flashlamp design. The diameter, length, material, gas pressure, and
electrode construction, etc. all affect the performance and power
handling capabilities.
5. Cooling. Convection, forced air, and liquid (water or oil) cooling may
be used. Dramatically higher average power is possible using liquid
flow cooling if the flash lamp design will permit this.
Flashlamp manufacturers publish very detailed data sheets for their products. For high power strobe design, all this information is essential. However, when building small strobe units (under 20 W-s), my general rules-of-thumb are: 1. Use a 250 - 350 V power supply for the energy storage capacitor. Depending on your application, this can be a battery or AC adapter powered inverter, transformer/rectifier power supply, a line operated voltage doubler for 110 VAC or a simple line rectifier and filter capacitor for 220 VAC. 2. Use a trigger transformer capable of 4-5 KV or more pulse output. The actual output trigger pulse voltage can be controlled by the voltage on the trigger capacitor. This is usually obtained from a voltage divider off of the energy storage capacitor. Too low and it won't flash reliably. Too high and arcing to nearby components may occur. 3. Follow the flashlamp manufacturer's ratings for maximum flash energy and average power. If you ripped the flashlamp out of something like a pocket camera, limit your flash energy to that provided by the capacitor contained in the unit or 10 W-s per inch of flashlamp length if the capacitor value is unknown. Limit the average power to this maximum energy every five seconds or the actual minimum full power cycle time if this known. 4. Use a photoflash rated capacitor if available but any good quality capacitor will probably work fine. No inductor is needed for these low power applications. For a 320 V power supply, flash energy is just about 5 W-s per 100 uF of energy storage capacitor rating. 5. Keep lead lengths between the energy storage capacitor and the flashlamp reasonably short (a few inches is fine). Minimize the length of the wire from the trigger transformer and make sure that it is well insulated and not in proximity to any other components. 6. Make sure human contact with all line connected and high voltage components is impossible during operation or at any time when a charge is present on the power supply or energy storage capacitors - by packaging everything in a plastic or grounded metal box, for example. 7. Always use capacitor, transformer, or optical isolation when triggering line powered strobe units from low voltage logic circuits or anything that a human may contact. This is recommended in general as it will assure that no high power transients find their way back into sensitive electronic circuits. 8. Don't neglect the essential power switch, fuse(s), and indicator lights. For logic controlled or computerized strobes, a mechanical test button using a hard set of contacts (i.e., across the SCR) is highly desirable. The guidelines above will adequately handle typical small to medium size strobes - perhaps to 50 W-s or so depending on the extent to which the flashlamp maximum energy specifications exceed the power input you are using and the characteristics of other circuit components. For higher power strobes, it is essential that appropriate flashlamps are used with photoflash rated capacitors. A series inductor - matched to the flashlamp, capacitor, and voltage - is critical to preserving the life of some flashlamps (perhaps beyond one flash!) and achieving maximum flash intensity. The flashlamp manufacturer's datasheets are probably the best source of this information. Also see the section: "High power (laser pump) strobe circuit". The series inductor is often needed for laser pumping applications and other applications where the quantity of energy and/or the peak current are particularly great for the size of the flashtube.
Cycle time on commercial electronic flash units is typically 1 to 10 seconds depending on design and the battery's state of charge. How can this be improved without overstressing the circuitry? Without redesigning the inverter circuit for higher power and using a larger flashtube, the only variable you have to play with is the size of the energy storage capacitor: * Cycle/charge time will be proportional to its uF rating. * Flash energy will be proportional to its uF rating. * Light output will be proportional to flash energy. * Perceived brightness will be a sub-linear function (not a proportional one) of flash energy. A simple approximation is flash energy raised to the .5 power (squareroot). Thus, depending on your needs, reducing flash energy may still result in adequate brightness. For example, cutting the uF rating of the energy storage capacitor to 1/4th of its original value will still result in about 1/2 the perceived brightness (not 1/4 the brightness). However, if you need the same intensity (i.e., to use the same F-stop), then the distance between the flash and the subject will have to be reduced by half in this case. Since power dissipation is still limited by the inverter, the flashtube should not overheat. The only concern is that the trigger capacitor has enough time to charge up - check its time constant and reduce its charging resistor if necessary to assure that the voltage on the trigger capacitor is high enough (close to what it would be for the unmodified circuit). What you DON'T want to do is use a higher voltage on the input. That would almost certainly blow the inverter transistor (either immediately or from overheating) and/or the transformer, energy storage capacitor, or flashtube. Where reducing the size of the energy storage capacitor is not adequate, here are some guidelines for more extensive redesign: (From: Don Klipstein (don@misty.com or klipstei@netaxs.com)). 1. For faster flash rates, you want lower energy levels to avoid overheating the flashtube. The smaller U-shaped tubes may take about 5 watts of average power at faster flash rates, and about 4 watts at really fast flash rates. This means you probably want a flash energy under 1 W-s. The efficiency is also lower at lower energy levels, giving you more heat and less light. With lower energy levels (under about 2 W-s or so with a smaller U-shaped tube), the tube works better with unusually high voltages near or even above 400 volts. You would also use less capacitance, to get your desired flash energy with a higher voltage. You will probably have better flash extinguishing with more voltage and less capacitance. 2. Add an inductor in series with the power feed into the energy storage capacitor. This makes the capacitor hardly recharge at all for a few milliseconds, allowing the flashtube to extinguish. A 15 watt fluorescent lamp choke ballast will probably work for this. This goes in series with the power feed to the capacitor, not in series with the flashtube. CAUTION: This inductor may cause a voltage overshoot of the energy storage capacitor - probably to your favor if the capacitor can take the extra voltage. Use two capacitors, with the inductor between their positive terminals, if the power feed requires a capacitor load. The first capacitor can be the larger value original energy storage capacitor. The second capacitor will be the low uF value one used for flashing, and will need to withstand extra voltage. For a typical variable rate stroboscope circuit, see the section: "Commercial stroboscope".
The typical integrated or camera mounted electronic flash unit has a flash duration of under 1 ms at full power. This is short compared to a flash bulb and adequate for most common photographic applications. However, when attempting to freeze high speed machinery or other rapid action, this may be way too long. All other factors being equal, flash duration is roughly proportional to the size - uF rating - of the energy storage capacitor. Where lower flash energy is acceptable and/or the strobe can be moved closer to the subject and/or faster film can be used, the normal energy storage capacitor in your electronic flash can be replaced with one that is smaller. Flash duration and energy will then be reduced in proportion to the ratio of the capacitor's uF ratings. Using a higher voltage will enable the uF rating of the capacitor to be decreased and still achieve the same total light output - the required uF (and flash duration) goes down as 1 / (V * V). Of course, since the same energy is involved, the physical size of the capacitor doesn't change much. There is no free lunch :-). For example, the typical small electronic flash unit uses a capacitor voltage of about 300 V. Designing a strobe with a 3 KV energy storage capacitor will permit its uF rating and flash duration to be cut by a factor of 100! High voltage flashtubes and capacitors must be used but the basic principles of operation of these strobes are unchanged. Power to charge the capacitors can be provided by a line operated transformer or high frequency inverter either directly using a rectifier or doubler, or diode-capacitor voltage multiplier. For ideas, see the chapters on helium neon lasers in the document: Lasers: Safety, Diode Lasers, Helium Neon Lasers, Drive, Info, Links, Parts as the operating voltage requirements for HeNe lasers are similar. Where fast cycle time is not critical or your required flash energy is modest, one of the sample circuits may be acceptable. Those pictures of bullets in flight were likely made with air spark gap light sources with 10s of KV on the energy storage capacitors resulting in flash durations in the microsecond range. OK, you have to go to the FAQ at the second site below, specifically: http://www.pacwest.net/byron13/sam/strbfaq.htm There is a schematic of a Vivitar Auto/Thyristor 293 which is an energy conserving flash. Unfortunately, I wasn't willing to totally rip it apart so some of the circuit is a bit estimated :-). I guestimate that you should be able to get 10 short flashes into your 20 ms or so at 50 percent efficiency. What about the fact that these are discrete? Will this result in unacceptable artifacts in the photos?
If you long for the blur of a real flash bulb, this may be for you!
In some cases, simply adding an inductor in series with the flash tube can
provide some increase in flash duration. However, where you want 20 ms
instead of less than 1 ms, this is not going to work. If the inductor is too
small it won't do much of anything. Once it starts to have an effect, the
effect will be to simply cut off the flash.
What should work better (and I have not tried this) is to add a high current
constant current driver between the capacitor and the tube. For example,
assuming a small flash with say 500 uF at 300 V results in roughly a 200 A
peak current assuming a 1 ms flash duration. This is an equivalent resistance
of about 1.5 ohms! To extend the flash duration to 20 ms requires dropping
the current to 10 A.
One way to do this is with a constant current series regulator set for 10A:
+-------+ R1
+300 o---+----|- |-|---------/\/\------+-----------+
| +-------+ 1 | |
| FL1 | |
Main | +-----------+ |
Energy + _|_ C1 | Constant | + _|_ C2
Storage --- 500uF | Current | --- 10uF
Capacitor | 350V | Regulator | - | 350V
| +-----------+ |
| | |
| | |
Return o---+--------------------------------+-----------+
The use of a high frequency switchmode (buck) converter will almost certainly
be necessary unless you have some really HUGE transistors floating around in
your junk box. The problem isn't the voltage or current rating - a common
BUT12A would meet these requirements - but rather maximum power and SOA (Safe
Operating Region). Peak power dissipation in a linear regulator would be
about 2,500 W!
C1 provides a sink for the flash tube current until the regulator can start
up. It may be necessary to play with their values to achieve reliable
operation. An alternative is to bias the transistor from a separate power
source prior to triggering.
Also see the section: "Driving continuous output xenon arc lamps".
Details are left as an exercise for the student :-).
These approaches can probably be applied to regular xenon flash lamps as well. See the section: "Lengthening the duration of a flash". (From: (macy@global.california.com)). Take a look at the Xenon arc tubes made by ILC of Sunnyvale which is used in lighting those invasive tiny little arterial cameras. The light comes from a xenon arc tube and is transferred down the arteries via fiber optics. I believe ILC used to provide these tubes on the open market (may still be) through Edmund Scientific, but they were too difficult for the general hobbyists and the market was zilch. As I recall, they fire at around 300-400 volts and then the arc sustains at something like 20 volts 10-20 Amps or such. The drivers for these consist of the lower voltage high power drive with a transformer winding in series with the output lead (capable of the high current). The high voltage would "spark plug" the arc until it fired, then the low power supply sustained the arc producing *very* bright, pure light. You could make such a driver where a lower voltage cap stores horrendous energy and use a "tickler" drive to fire the arc. Probably even get away with using the shorter duration arc you now have. This tube is made for constant high power. The arc electrodes are mounted inside a cylinder shape about 1 1/4 inch diameter by 1 1/2 inch length. The electrodes are mounted along the axis of the cylinder with a parabola reflecting cup and tripod mounting of the outgoing electrode (to not block the light much) The whole thing is clamped for heat exchange and electrical contact. Originally, the "bulb" was used for headlights on tanks because they survive shock so well. [I'd heard some guy put a row on top his 4 W sport and when he turned them on, he could see 2 miles down the road. - makes a good story but I can't confirm it.] Sometime in the 70's I met the man at Eimac (division of Varian) who invented the tube and he was looking for other applications. He had mounted it inside a 16mm projector to make a brighter, cooler light source. You could actually freeze the frame without having to dim because there was so little infrared in the light energy. It is bright. I saw a movie being projected overhead on a 16 foot screen in a drafting room and you could watch the movie easily! Plus the colors made for a beautiful rendition of the film. He saw this as a great potential market, until ..well the tube's light is so potent that it makes ozone and the ozone would "eat" up the aluminum parts on the projectors in 2 to 6 months. so that idea died. At least it finally found its way into a very useful application - medical.
If there is any doubt as to the polarity of a flashtube you are using in a new design, the following will help to confirm proper direction - running a flashtube with reverse polarity will adversely affect performance and life. (From: Don Klipstein (don@misty.com or klipstei@netaxs.com)). 1. Any red markings or "+" markings indicate the anode. 2. With no markings but electrodes of unequal size, the larger one is the cathode. 3. If both ends look identical but the trigger electrode is closer to one electrode, or more coupled to one electrode, then that electrode is the cathode. 4. If the tube looks symmetric except for having a getter at only one end, it is probably preferable to make the getter end the cathode, especially if any getter material exists on the electrode itself. Any vaporized getter metal forms positive ions easily, and will be attracted to the cathode. Metal vapor released around the anode is more likely to condense all over the tube and discolor it. 5. If both ends of the flashtube look alike except for one electrode being shinier and with rounded edges, then the shiny electrode with rounded edges is the anode, and the steel-gray (tungsten) sharply cylindrical one is the cathode.
It is usually not possible to determine all parameters of the inverter transformer when reverse engineering pocket camera strobes. (The following is from: Kevin Horton (khorton@tech.iupui.edu)) This is *always* the kicker. I have devoting heavy amounts of time into figuring out how these transformers work. They are very, very special. *nothing* else will work in their place, or if it does, it'll be woefully inefficient. They are usually .4" or so cubed, but may be larger. The gap on the core seems to be pretty critical- it limits the overall current that the circuit will draw. In one particular strobe I disassembled, they had a 100 pf cap coming from the output of the HV winding directly tied to the base of the drive transistor! I finally figured out why: it controlled the frequency vs voltage of the oscillator, hence giving it more current as it was completing a charging cycle! I've disassembled many of these small transformers. Unlike most ferrite transformers, these are usually held together by dipping them in wax, rather than varnish. Some transformers have the primaries wound on the core, while others have it on the outside. I haven't figured out exactly why this is. However, one one transformer I took apart, the feedback and drive windings were wound on the core; bifilar. The feedback was 11 turns, while the primary was 10. Both were #24. On top of that was thousands of turns of #40 or so wire. It seems that the small sizes play a part in the efficiency of these transformers; since the magnetic field is contained in such a small core area, the losses are small.
To obtain 300 VDC from the 115 VAC line requires a voltage doubler; 450 VDC requires a tripler. This also applies to the use of alternative power sources with the following caveats: * When using a 12V to 115 VAC inverter to provide the input to voltage multiplier circuits, cheap units produce a squarewave output with 115 V peak (since the RMS and peak value of a squarewave are the same). Therefore, the circuit will take this value and multiply it by 2 or 3 (or whatever) rather than the peak of a sinusoid (which is 1.4 times greater). Thus, a tripler will be needed to obtain 300 VDC when using one of these inverters rather than a doubler in the case of normal line input. * Driving these circuits from a gasoline powered generator should not be a problem unless its voltage becomes excessive - in which case the flashes may not be coming only from the xenon tube! Such a unit (actually probably an alternator) should produce a decent sinusoid unless it is actually a low voltage generator and inverter (as above). There are several considerations with respect to the design of these circuits. Capacitor size (uF value and voltage rating) is the one that generally has the most impact on performance and cost. Depending on the circuit, the required voltage rating may be anywhere from the peak of the AC waveform to the maximum output voltage of the circuit (both with a safety factor). Using the highest value will always be safe and not that expensive for modest size capacitors. For the capacitance value: * If too small, output power will be inadequate. Damage can even result if one or more electrolytic capacitors becomes reverse biased under heavy load. * If too large, cost will be excessive and performance will not improve significantly beyond a certain uF value. For a classic voltage doubler, the main consideration is the ripple (as determined by capacitor uF value and load) and what the diodes and current limiting resistor can provide. This is because the caps in this case are really just filtering opposite sides of the half wave rectifiers. (Where they are then charging another larger cap (via some isolation diodes) but no series resistance, this may get somewhat messier.) However, for anything with more stages or stages arranged where some of the capacitors are effectively in series with the output, analysis can become interesting (translation: I am not about to attempt it here!). In these cases, the impedance of the capacitors at the line frequency (60 Hz in the U.S.) will affect the power available before the output drops and/or has excessive ripple. My very rough rule of thumb just treats the impedance of the capacitors like a series resistance. Then, I would select the capacitor value so that this resistance is small compared to the needs of the circuit. A 1 uF capacitor has an impedance (magnitude) of about 2.65K ohms at 60 Hz. A 10 uF cap is 265 ohms. The 22 uF capacitors in the tripler described in the section: "Higher power photoflash with SCR trigger" have an impedance of about 120 ohms. Consider a load of 100 W at 350 VDC (average - which would be a high power strobe indeed). The load resistance is then: R = V*V/P = 1.22 K. Since this is large compared to the capacitor impedance - even if all capacitors are assumed to be in series - I wouldn't expect very much improvement with the use of larger capacitors. Keep in mind that this is an edge of the envelope calculation so a factor of 2 (or 20) either way is possible (and likely!). The ESR (Effective Series Resistance) of smaller electrolytic capacitors is also higher. This may result in excessive heat dissipation in the capacitor. There is also a 'ripple current' rating for capacitors which should not be exceeded. However, if your capacitors are from Radio Shack, this particular specification is probably not available :-). A surge limiting resistor on the line input should be provided to limit the peak current through the diodes and capacitors. Once a particular circuit has been constructed, test it under a dummy load which simulates the expected average power. If the output voltage drops excessively and/or there is too much ripple, try increasing the capacitor uF values (not all of them may need to be changed.) Check the waveform on each capacitor with a scope (you MUST use an isolation transformer for this!). The voltage must NEVER go negative for an electrolytic capacitor. Feel the capacitors for evidence of excessive heating. Also see the section: "Voltage doubler design considerations".
"I have a problem. I am using a standard voltage doubler (2 diodes, 2 capacitors) in a strobe circuit. The doubler consists of two 4.7 uF 450 V caps and two 1N4005 diodes. The timing circuit is a neon-bulb relaxation oscillator that triggers an SCR, which in turn dumps a .1 uF cap into a trigger coil to fire the flashtube. The flashtube gets a 47 uF cap discharged through it, which equals about 2.5 watt-seconds. The problem is that the 4.7 uF doubler capacitors overheat and fail! The doubler voltage is 325 volts with no load, so a 450 V rating should be adequate. Should I be using more capacitance for these?" (From: J. M. Woodgate (jmwa@thenet.co.uk)). Well, you really haven't given enough information. The problem is likely to be that you are exceeding the ripple current rating of the caps. I guess that you are running the neon and SCR from the doubler and your neon takes a thump of current when it fires, even if you have set the duty cycle down so that the average current is low. Higher value capacitors usually do have a higher ripple-current rating. But first you need to find how much ripple-current you are producing, and this depends on the cap. value (at least to some extent). Very roughly, the ripple current is pi times the DC current, and you should look at the load current waveform with a scope and take the maximum value as the DC value, to be sure of not over-running the capacitors.
In what some would call 'the good old days', it was common to carry around a bag full of flashbulbs to provide photographic illumination. These were a pain to use, resulted in disposal issues, and were somewhat hazardous when used 'according the label directions' - exploding flash bulbs were common. Of course, in the truly good old days, they had those exposed palettes with flash powder. They were really good at starting fires.... Individual flashbulbs were single use sources of light containing a wad of fine magnesium (or other active metal) wool with an electric or mechanical trigger. These produced an intense white light of relatively long duration (perhaps 40 ms as compared to 1 ms or less for an electronic flash). With the march of technology individual flash bulbs gave way to to flashcubes, flashbars, and other multi-use source of light (at least for the weekend camera bug - the professional photographer still needed the higher power of individual bulbs), While the electronic flash has been used professionally for at least 40 years, it is only within the last 10 years or so that all but the least expensive camera comes with a built-in electronic flash as standard equipment. However, some people still would like to continue to use their older equipment for which no low cost electronic flash upgrade may exist. The information in this chapter is directed toward these die-hards. While written specifically for replacement of the Polaroid SX-70 Flashbar, little or no modification should be needed to interface to nearly any camera or lamp holder originally designed for electrically triggered flash bulbs. The remainder of this chapter is based on material from: George Holderied (holderied@ubaclu.unibas.ch)). His web page: The Hacker's Guide to the SX-70 provides all sorts of useful information including more details on the flashbar retrofit described below.
I have built a flashbar eliminator. It works fine with my Polaroid SX-70.
As they stop selling flashbars in the US, it may be interesting for Polaroid
fans to build an interface for common electronic flashes.
The flashbar contains five glass bulbs on each side that are filled with
magnesium wool in an oxygen atmosphere. The magnesium is ignited by an
electric pulse. The glass bulbs are plastic coated to prevent them from
exploding. There is also an outer plastic wall which is another safety
shield and corrects the light color.
The flashbar is contacted from the front (active) side.
+----------------------------+
| A A A A A |
| 1 2 3 4 5 | <- Flashbulbs
| U U U U U |
+----------------------------+
H123456 <- Contacts
The wider contact (H) to the left shorts two contacts in the camera to indicate
the presence of a flashbar. Contacts 1, 2 and 3 go each to one side of the
bulbs 1, 2 and 3. Contact 4 is the common contact that goes to one side of
each bulb. Contacts 5 and 6 go each to one side of the bulbs 5 and 6.
The flashbar wiring is shown below:
(4)o---+-----+-----+-----+------+
| | | | |
(B1) (B2) (B3) (B4) (B5)
| | | | |
| | | | |
(1) (2) (3) (5) (6)
There are no electronics in the flashbar. The camera knows which bulb has
been fired by measuring the resistance across the bulb. A good bulb has a
resistance of a couple of ohms whereas a dead one has almost infinite
resistance.
There are several reasons to use an electronic flash instead of the flashbars: * Flashbars cost about US$4 and are becoming harder to obtain. * They end up in the garbage. * Bulb flashes, due to their longer duration, cause a dark spot in your field of view if looked into, that lasts for several minutes. You can easily find cheap electronic flashes that replace flashbars. A used one should not cost more than two flashbars. However, if you already have an electronic flash that you want to use instead of the bars, you need an interface. Flashbars have a fixed light output that reaches to a distance of about 3 meters (10 feet). The SX-70 exposure control is based on the focused distance. The camera's maximal aperture (f-stop) is 8. It is recommended to use an electronic flash with the same light output as a flashbar. That flash would have a guide number of 75 (9,5 feet * 8), or a metric Leitzahl of ca. 25 (3.2 meters * 8) at 150 ASA.
A flashbar-compatible camera outputs an electric pulse of about 4V at 1.5A. However, an electronic flash has a voltage of about 250 V between the trigger contacts that are shorted to trigger the flash. The interface is based on the general optoisolated remote trigger from a low voltage logic or signal. See the section: "Optoisolated remote trigger from low voltage logic or signal".
+-----------------------+-------o Flash Trigger (+)
| |
\ |A
/ 1M __|__
\ _\_/_ Thyristor
| / | TIC106
+-----+-------+ G | |K
22 | | | | |
(4) o--+-/\/--+ | _|_. | 2.2uf | |
| | | '/_\ _|_ | |
\ __|__ |/ C | --- | |
3.5 / _\_/_ ->| | Z6.3 | | |
\ | |\ E | | | |
| | | +-------+------------|--+
(3) o--+------+ +-----------------+--------+ |
| | |
Optocoupler / _|_ |
4N33 typ. 1K \ 100pF --- |
/ | |
| | |
+--------+--+-------o Flash trigger (-)
The 3.5 Ohm resistor tells the camera that there is a unused flashbulb in between the contacts 4 and 3. The 22 ohm resistor limits the current through the coupler's LED. On the output side an electrolytic cap (2.2 uF) is charged from the flash's trigger voltage to 6.3 V. This voltage is limited by a zener diode. When triggered, the coupler's phototransistor discharges the capacitor into the thyristor's gate and fires the thyristor and thus the flash. The whole circuit easily fits into an empty flashbar.
The little inverter in those units cannot put out enough power to charge the normal energy storage capacitor any faster. It is quite easy to replace the inverter with a voltage doubler off the AC line (with a current limiting resistor! Warning: non-isolated power supply). Using a smaller energy storage capacitor would also permit a much higher flash rate at reduced brightness and this would prolong the life of the flashtube as well. With too high a repetition rate at high power, the problem is heat dissipation in the tube. Above a flash rate of once every couple of seconds, your poor little tube will degrade fairly quickly and it may not turn off properly as well due to overheating of the electrodes. It will probably be necessary to use an SCR instead of a set of switch contacts to allow triggering from a 555 timer or other logic level input. For a basic constant frequency strobe, a relaxation oscillator using a unijunction transistor or neon lamp, an astable multivibrator built from a couple of general purpose transistors, or a counter operated from the AC line zero crossings or a crystal oscillator would be perfectly adequate. However, a very simple repeating trigger can be made from a motor driven cam operated microswitch. Using a variable speed motor would implement a basic adjustable frequency stroboscope with no additional electronic components. For a simple electronic modification: (From: William "Chops" Westfield (billw@cisco.com)). "In fact, some types of disposable (or other) camera electronic flash units can be converted to repetitive flashers (not quite a strobe, but useful as a safety-beacon sort of thing) by connecting a couple neon bulbs or a 130 V (or better) 'Sidac' or diac across the existing trigger contacts. This is a nice trivial modification." The resistance of the trigger capacitor charging circuit will affect the repetition rate and the RC time constant must be long enough for the main energy storage capacitor to charge to a high enough voltage for the xenon tube to fire reliably. Details are left to the student :-). However, I wonder about flashtube life if the original energy storage capacitor is used. So, you might want to replace it with a smaller one if adequate for your needs. In any case, if you retain the inverter, use an AC adapter or other power supply instead of batteries for testing at least. Otherwise, let me know which battery company's stock I should buy!
These are often seen in safety related applications - warning lights, for example, where a typical cycle might be two flashes .2 seconds apart with a .8 second dead time. Here is one approach to designing a strobe that will double (or multiple) flash from a battery powered inverter of limited capacity. Charge a large buffer storage capacitor from the DC-DC converter, then have its output feed a smaller flash energy storage capacitor through a resistor small enough to give you a fast recharge but large enough to allow the flashlamp arc to quench. Building the DC-DC converter is pretty easy and you should be able to make it run off of a battery without any problem. You can use a simple power oscillator feeding a home-wound step-up transformer. With the energy buffer, the inverter only needs to satisfy the average power requirements of the multi-flash cycle. See the section: "General strobe circuit design" For example, a small unit using a 100 uF 330 V capacitor for the flash could use a 1000 uF cap. for buffer storage separated by a 250 ohm power resistor. That would provide a 100 ms or so cycle time. The 1000 uF cap provides a reservoir between the relatively low power DC-DC converter and the tube as long as you do not flash too quickly - faster than your DC-DC converter can keep up. This should be much easier than trying to interrupt the 10s-100s of amperes of current flowing in the tube during the flash.
Here are two simple circuits for generating a continuously repeating strobe trigger. It is assumed that the power supply and xenon tube can handle the average power requirements for the minimum desired cycle time. Inadequate energy storage capacitor charging power will result in erratic or reduced intensity flashing. Excessive heating caused by too high a repeat rate may lead to damage to circuit components and/or the flashlamp or may result in the arc not quenching properly between flashes. Inadequate trigger charging power (RC time constant too long) will result in missed or erratic triggering.
This is a common 555 timer operated in astable mode. For a detailed
description of the circuit operation, see the 555 timer datasheet any
databook which includes the 555 timer chip.
Component values have been selected to cover a range of about 1 to 10 seconds
between flashes.
+--+
| V R1 R2
+5 to o--+-----------------+---+-+/\/\---+-/\/\--+
+15 VDC | | | 200K 5K |
| | | |
| +------|---|------+ |
_|_ C3 | |8 |4 | |
--- .1 uF | +---------+ | |
| | 2| VCC R |3 | |
| +-----+---|TR Q|--------------|---------o Trig+
| | | U1 |7 | |
| _|_ C1 | 555 DIS|--------------+
| --- 100 5| |6 | | (Positive edge trigger
| | uF +---|CV G THR|---+ | to optoisolator or
| | _|_ +---------+ | | isolated SCR gate.)
| | --- C2 |1 | |
| | | .01 uF | | R4 |
GND o--+----+-----+--------+ +---/\/\---+---------o Trig-
5K
This is a simple relaxation oscillator using a common NE2 neon indicator
lamp. As drawn, the repeat rate should be adjustable from about .1 to 10 Hz.
Adjust component values for your particular needs. (Note: use of audio
taper potentiometer would help to linearize the adjustment range.)
+---+
R1 R2 V |
+150 VDC o----/\/\----/\/\/-+----+--------+ +------o Out+
50K 20M | | |
| +++ |
| |o| NT1 |
| |o| NE2 __|__ SCR1
| +++ _\_/_ M21C, FOR3G,
| | C2* / | TIC106 typ.
+_|_ C1 +----||---+---' |
___ 2 uF | .01 uF | |
- | / 400 V / | (To trigger
| \ R3 \ R4 | capacitor/
| / 10K / 1K | transformer)
| \ \ |
| | C3* | |
Return o-----------------------+--------+----||---+------+------o Out-
.01 uF
400 V
The neon indicator, NT1, is a negative resistance device. It becomes conductive when the voltage across it exceeds about 90 V but then only requires about 60 V to maintain conduction. This circuit has an RC network formed by R1, R2, and C1. C1 charges through R1 and the repeat rate adjustment potentiometer, R2. When its voltage exceeds the NE2 breakdown voltage, C1 discharges through NT1 resulting on a pulse on R3 coupled by C2 to the gate of SCR1. The SCR fires and discharges the trigger capacitor (not shown) into the trigger transformer of the strobe firing circuit. Once the voltage across NT1 has decreased below about 60 V, it turns off and the cycle repeats. Since the voltage across NT1 is swinging between about 60 and 90 V (out of 150 VDC total), the repeat frequency should be between 4 and 5 times 1/(RC). It is assumed that SCR1 (Out+/-) takes the place of the shutter contacts, is the SCR, or in parallel with the SCR shown in the strobe circuits shown elsewhere in this document. For other values of VPP between about 100 and 300 V, adjust resistance values appropriately. Note that C3* and C4* are essential to provide safety isolation for line powered strobes. Please note that the characteristics of neon lamps sometimes change with age, temperature, and use. The SCR should have a sensitive gate since some neon lamps do not reliably conduct more than a few milliamps when they ionize in a relaxation oscillator.
Here is a circuit for an optoisolated trigger interface. This will permit
control of line-connected (non-isolated) strobes from logic or other lower
voltage signals. This is probably the safest way to deal with the isolation
and safety issues as the insulation resistance of typical optoisolators is
several KV (7.5 KV for the specific part shown).
Another important reason to consider this approach is to assure reliable
triggering in an electrically noisy environment. Such interference may be
external (e.g., power cables, digital busses) or internal. Even where there
are only two flashtubes, the current pulse when one of these fires could
falsely trigger the other. Minimizing the length of sensitive trigger wiring
and low pass filtering the trigger signals (i.e., RCs on the input to the SCR)
will help. However, the best way to prevent false triggering is to use light
rather than electrical signals to trigger the flash heads. Instead of running
long wires with low level signals, use individual fiber optic cables for each
channel between its LED and photodetector (rather than an opto-isolator) in
the circuit below. Such a design will even be immune to the EMP resulting
from a nuclear blast - should you care :-).
This basic design would be suitable for a wide variety of applications
requiring microprocessor, PIC, or PC control. A multiheaded strobe pulsing
to a musical beat (high power color organ) could be implemented by triggering
several strobe units from an audio amp's speaker output via audio filters
of various cutoff or bandpass frequencies.
VPP
o
|
\ R3
/ VPP.GT.10V: R3~=(1K x VPP)-5K.
(+5 to +10 V) \ VPP.LT.10V: R3=1K, R4 not needed.
IN1 (DC) o------+ /
C1 | R1 |
IN2 (AC) o--||--+--/\/\--+-------+ +-------+-----+ +----o Out+
.01 uF| 220 | | OPTO1 | | C2 | 1 uF |
| | +--|-------|-+ / +_|_ |
\ __|__ |__|__ |/ C| R4 \ ___ __|__ SCR1
R2 / D1 _/_\_ |_\_/_-> | | 5K / - | _\_/_ M21C, FOR3G,
1K \ 1N4148 | | | |\ E| | | / | TIC106 typ.
| | +--|-------|-+ +-----+ | |
| | |PC713V | | | |
GND o-----+--------+-------+ Typ. +-------|-----+----+ | (To trigger
| | | | capacitor/
| R5 \ C3_|_ | transformer)
| 1K / --- |
| \ 100| |
| | pF | |
+-----+----+--+----o Out-
The input signal may be DC coupled resulting is a high level triggering the strobe or AC coupled resulting in a positive edge trigger. R1 provides current limiting to the optoisolator's LED and R2 minimizes any possibility of electrical noise turning on the optoisolator. Change the values of R1 and R2 for a different input voltage range. D2 provides reverse voltage protection for the LED. For VPP greater than 10 V, the voltage divider formed by R3 and R4 charges C2 to about 5 V. This is the most common case where VPP is derived from the strobe power supply and is typically 300 V. The time constant for this RC network is under 5 ms so it will not affect high speed repeat operation. C2 assures that there will be enough current from the optoisolator to trigger SCR1 even with the high value resistor which may be used for R3 to minimize power dissipation with a large VPP. For a VPP of less than 10 V, the circuit can be simplified to just a current limiting resistor (leave out R4). When current flows through OPTO1's LED, it turns on the phototransistor which allows C2 to discharge into the gate of SCR1 which is connected to the trigger capacitor and transformer of the flashlamp firing circuit (not shown). To minimize the possibility of false triggering, locate the optoisolator circuit in close proximity to the SCR. R5 and C3 are included to reduce the SCR's sensitivity to any electrical noise pickup as well. VPP must be a positive DC voltage referenced to the terminal Out-. In most cases, this will be the energy storage capacitor's positive terminal.
Have you seen the 'new' ball used for the New Year's celebration on top of the tower in Times Square? Is uses something like 144 computer controlled xenon lamps. Sort of gives you something to strive for! Several approaches can be taken in designing such systems depending on the needs: * If only one flashlamp is to fire at any given time, a single energy storage capacitor can be shared by multiple flashlamps assuming the distance between it and any flashlamp is not excessive (probablyless than a couple of feet). * Where multiple flashlamps may fire in an arbitrary sequence (but the average rate is known), each flashlamp can be connected to its own energy storage capacitor fed through a current limiting resistor from one high capacity power supply. * If the maximum rate for each flashlamp is known, each head can have have its own independent inverter. For example, the flash units from disposable pocket cameras can be modified with beefed up heatsinks for the chopper transistor, suitably sized energy storage capacitors, and remote triggering capability. Hybrid systems using a combination of these techniques are also possible. In all cases, each flashlamp must have its own trigger transformer. An optoisolated SCR can then be controlled from a logic level signal - the output of a PC's parallel port or a dedicated bus. For long runs, use Schmitt Trigger gates or differential line drivers/receivers to prevent false triggering due to interference from the high voltage and high current pulses associated with each flashlamp's firing.
(The following is from: Kevin Horton (khorton@tech.iupui.edu)) I'm building a super strobe bar! It has 8 strobe tubes under computer control. (Actually a PIC processor, but hey, computer is a computer. I have all the stuff done except the control section, and I only have 2 of the 8 strobe units done due to the fact that I haven't found any more cheap cameras at the thrift store! (One Saturday morning's worth of garage sales and flea markets would remedy that! --- sam). It runs on 12 V, at up to 6 A, and can fire the tubes at a rate of about 8-10 times per second. The storage cap is a 210 uf, 330 V model; it gets to about 250 V to 300 V before firing; depending on how long it has had to charge. Because of this high speed, the tubes get shall we say, a little warm. (Well, maybe a lot warm --- sam). I have it set up at the moment driving two alternating 5 W-s tubes. I'm pumping them quite a bit too hard, as the electrodes start to glow after oh, about 5 seconds or so of continuous use. I know, a high class problem, indeed! My final assembly will have 8 tubes spaced about 8 inches apart on a 2x4, with a Plexiglass U-shaped enclosure with a nice 12 V fan blowing air through one end of the channel to cool the inverter and the tubes. Stay tuned. The following schematics provide some details of this design: * Inverter - High power 12 V to 300 V inverter for high repeat rate medium power strobes: inverter.gif. * Trigger - Opto-isolated logic level trigger for general electronic flash and strobe applications: trigger.gif.
(The following is from: Kevin Horton (khorton@tech.iupui.edu))
I have developed a cool little transformer circuit that seems to be
very efficient. I built this inverter as tiny as I could make it.
It runs off of 3V, and charges up a little 1 uf 250V cap all the way up in
about 30 seconds; drawing about 5 to 8 mA in the process. The numbers by the
windings tell the number of turns. The primary and feedback windings are
#28, while the secondary is #46. Yes, #46! I could hardly tell what
gauge it was, as it was almost too small to measure with my micrometer!
It may be #44 or #45, but at these sizes, who knows? I used a trigger
transformer for the wire. I used all the wire on it, to be exact; it
all *just* fit on the little bobbin. The primary went on the core first,
then the secondary, and finally the feedback winding. This order is
very important. I used a ferrite bobbin and corresponding ferrite 'ring'
that fit on it. The whole shebang was less than 1 cm in diameter, and
about 3-5 mm high! I gave it a coat of wax to seal things up, and made
the inverter circuit with surface-mount parts, which I then waxed onto the
top. There are two wires in, and two wires out. It's enough to run a
neon fairly brightly at 1.2 V, with a 3 ma current draw.
Vcc o----+--------------+ T1
| 6T )||
\ #28 )|| +-------o HV output
R1 / )||(
47K \ +---+ ||(
/ 2N4401 | ||(
| |/ C ||( 450T
| +--| Q1 ||( #46
| | |\ E ||(
| | | ||(
+--+ +--------+ ||(
| | |17T )||(
C1 _|_ | |#28 )|| +-------o HV return
.001 uF --- | | )||
| +-----------+
| |
Gnd o----+----------+
This schematic is also available as: teeny.gif.
This circuit will generate over 400 VDC from a 12 VDC, 2.5 A power supply or
an auto or marine battery. While size, weight, and efficiency are nothing to
write home about - in fact, they are quite pitiful - all components are readily
available (even from Radio Shack) and construction is very straightforward. No
custom coils or transformers are required. If wired correctly, it will work.
Output depends on input voltage. Adjust for your application. With the
component values given, it will generate over 400 V from a 12 V supply and
charge a 200 uF capacitor to 300 V in under 5 seconds.
C1 1 uF D2 1N4948 R2
+------||------+ T1 1.2KV PRV 1K 1W
| | +-----|>|-----/\/\---+------o +
| R1 4.7K, 1W | red ||( blk |
+-----/\/\-----+------+ ||( |
| yel )||( +_|_ C2
+ o----------------------------------+ ||( --- 300 uF
| red )||( - | 450 V
| +--------------+ ||( |
| Q1 | ||( blk |
6 to 12 | |/ C +--------------------+------o -
VDC, 2A +----| 2N3055 Stancor P-6134
D1 _|_ |\ E 117 V Primary (blk-blk)
1N4007 /_\ | 6.3 VCT Secondary (red-yel-red)
| |
- o------------+------+
1. Construction can take any convenient form - perf board, minibox, etc.
Make sure the output connections are well insulated.
2. C1 must be nonpolarized type - not an electrolytic.
3. D1 provides a return path for the base drive and prevents significant
reverse voltage on the B-E junction. Any 1 A or greater silicon diode
should be fine.
4. C2 is shown as typical energy storage capacitor for strobe applications.
5. D2 should be a high speed (fast recovery) rectifier. However, for testing,
a 1N4007 should work well enough. R2 limits surge current through D2.
6. The polarity of the input with respect to the output leads is important.
Select for maximum voltage by interchanging the black output wires.
7. Mount Q1 (2N3055) on a heat sink if continuous operation is desired. It
will get warm. Any general purpose NPN power transistor should work. For
PNP types, reverse the the polarities of the power supply and D1, and
interchange one set of leads.
8. Some experimentation with component values may improve performance for
your application.
9. When testing, use a variable power supply so you get a feel for how much
output voltage is produced for each input voltage. Component values are
not critical but behavior under varying input/output voltage and load
conditions will be affected by R1 and C1 (and the gain of your particular
transistor).
10. WARNING: Output is high voltage and dangerous even without large energy
storage capacitor. With one, it can be lethal. Take appropriate
precautions.
11. For your less intense applications, a fluorescent lamp can be powered
directly from the secondary (without any other components) in place of
a flash lamp! This works reasonably well with a T5-13W or T8-15W bulb
but Q1 does get quite hot so use a good heat sink.
12.
| | |
---+--- are connected; ---|--- and ------- are NOT connected.
| | |
Here, the intention is to trigger a strobe should a light beam be interrupted
or completed. Any of the electronic flash schematics can be modified for
this purpose.
Any light source with sufficient IR content can be used to generate the beam
including an incandescent lamp, IR (light) emitting diode (IR LED/IRED), or
IR laser diode. A small convex lens will greatly increase the range of any
of these light sources.
The circuit below should work for the detector. Its output may need to
be put through one (light beam completed) or two (light beam interrupted)
inverting Schmitt Trigger gates (e.g., 74LS14) to clean up its output and
provide the proper polarity. It should be AC coupled to the gate of an
SCR. The SCR will substitute for the camera's X-sync contacts and
fire the strobe. Note that if this is a line operated unit, capacitor
(or transformer) coupling is essential for providing the very important
line isolation barrier absolutely required for safety.
This approach can also be used to implement a slave flash. However, for such
a high intensity application, a Light Activated SCR (LASCR) would be suitable
and result in a simpler circuit (as in 1 component).
For the case where the strobe is supposed to fire when the light beam
is interrupted, when the light beam is unbroken, the photodiode is
illuminated providing current to keep the transistor on and its output
is low. When the beam is broken, the output goes high, is cleaned up
by the Schmitt Trigger gates creating a rising edge to provide a pulse
to trigger the SCR.
Any common IR or visible photodiode can be used for PD1. Sources include
optoisolators, photosensors from dead VCRs, and optomechanical computer mice.
Vcc o-------+---------+
| | +------o Out+
\ \ |
/ R1 / R3 |
\ 3.3K \ 470 (1 or 2 Schmitt __|__ SCR1
/ / Trigger Gates) _\_/_ M21C, FOR3G,
| | +-----+ +-----+ C1 /| TIC106 typ.
__|__ +---| ST1 |----| ST2 |---||---' |
Light beam ---> _/_\_ Q1 | +-----+ +-----+ .001 uF |
PD1 | B |/ C 600 V |
+-------| 2N3904 | (To trigger
| |\ E | capacitor/
\ | | transformer)
/ R2 | |
\ 27K | |
| | C2 |
+---------+------------------------||-----+------o Out-
_|_ .001 uF
- 600 V
1. CAUTION: Capacitors provide needed isolation barrier for line connected electronic flash units. Make sure they have adequate specifications. 2. For detecting a light beam being completed, logical inversion is needed. Therefore, use an inverting Schmitt Trigger or a single 74LS14 inverter. 3. For detecting a light beam being interrupted, no inversion is needed. Therefore, use a non-inverting Schmitt Trigger or 2 74LS14 inverters.
This circuit was printed on the back of the Radio Shack trigger coil blister
pack. It is suitable for various stroboscopic, signaling, engine timing,
scientific, and other similar (relatively) low intensity applications.
The power supply for this strobe can be either a voltage doubler operating
from the AC line (caution - no isolation) or a battery powered inverter. An
isolated SCR trigger circuit can be easily substituted for the firing button.
See the sample circuits elsewhere in this document.
(Original schematic provided by Robert Bullock (robert_bullock@pobox.tbe.com)).
R1
+V o----/\/\-----+-----------+---------------------+
250 ohm | | +|
2W | R2 / _|_
| 47K \ Fire button | | |
| 1/2 W / S1 || |
| | _|_ Trigger || | Flashlamp
| +--- ---+ T1 +---|| | FL1
| | | ||( || | RS 272-1145
| C2 _|_ +-+ ||( 4 || _ |
| .0022 uF --- )||( K |_|_|
C1 +_|_ 400 V | )||( V |
2 to 20 uF --- | )||( -|
400 V - | +---------+ +-+ |
| | RS 272-1146 | |
| R3 / | |
| 150K \ | |
| 1/2 W / | |
| | | |
Gnd o-------------+-----------+----------------+----+
Applied Voltage (+V): 200 to 300 VDC
Parts list:
C1 - Energy storage capacitor. 2 to 20 uF, 400 V.
C2 - Trigger capacitor. 0.0022 uF, 400 V.
R1 - 250 ohm 2 W.
R2 - 47K ohm 1\2 W.
R3 - 150K OHM 1\2 W.
S1 - Firing switch. SPST momentary pushbutton.
T1 - Trigger coil (transformer), 4 KV, Radio Shack 272-1146.
FL1 - Flashlamp, Radio Shack 272-1145.
Note: I could not find the trigger coil (RS part number 272-1146) in the latest
Radio Shack Catalog - the flashlamp was there - so I do not know if it is still
available from them. However, I don't see any reason why R2 and R3 cannot be
combined into one resistor (at R2's location - 200K, 1W) permitting the use of
a trigger transformer with a single terminal for the drive and HV return (more
common) should the one from Radio Shack be unavailable.
This schematic was traced from an electronic flash unit removed from an
inexpensive pocket camera, a Keystone model XR308. Errors in transcription
are possible.
Note that the ready light is not in the usual place monitoring the energy
storage capacitor voltage. It operates on the principle that once nearly
full charge is reached and the inverter is not being heavily loaded, enough
drive voltage is available from an auxiliary winding on the inverter
transformer to light the LED. It is also interesting that the trigger
circuit dumps charge into the trigger capacitor instead of the other way
around but the effect is the same.
Inverter Flashtube
+------------------------------+---------------------+--+--------+---+
| 1 K Ready LED | S1 Power | | | |
| +--/\/\-----+--|<|-----+ | ______ On | +-+ T2 +-+ |
BT1 _ | R1 | IL1 | | | \___| )||( |
3 V ___ | || +------|--/\/\/---+ | C1 | __ Off )||( +|FL1
2-AA _ | ||(2 .4 | R2 10 | Energy | | )||( _|_
___ | || +-------------+ | Storage | +-------+---+ ||( | | |
| | ||(5 .2 | | +| 280 uF | | ||( || |
+---+ || +------+ | __|__ 330 V | S2 Fire -| ||( || |
| ||(1 | | _____ | (Shutter) | +--|| |
+---+ ||( | C3 | | | +-----+ Trigger || |
3)||( 142 -|47 uF | -| | | | || _ |
<.1 )||( _|_ 6.3 | | | R1 \ _|_ C2 |_|_|
)||( ___ V | | | 1M / --- .02 uF |
+-+ || +-+ | | | | \ | 400 V -|
C| 4 T1 6 | +| | | | / | |
B|/ | | | D1 | | | | |
+--| 2SD879 +--------------|<|--+----------------+-----+--------------+
| |\ Q1 | | HV Rect. |
| E| | | |
| +-------------+------|------------------+
| |
+-------------------------+
1. The inverter boosts the battery voltage to about 300 V. This is rectified by D1 and charges the energy storage capacitor, C1. 2. The LED, IL1, signals ready by once C1 is nearly fully charged. 3. Pressing the shutter closes S2 which charges C2 from C1 through T2 generating a high voltage pulse (4-5KV) which ionizes the xenon gas in the flashlamp, FL1. 4. The energy storage capacitor discharges through the flashlamp.
1. The inverter transformer winding resistances measured with a Radio Shack
DMM. Primary resistance was below .1 ohms.
2.
| | |
---+--- are connected; ---|--- and ------- are NOT connected.
| | |
This schematic was traced from an electronic flash unit removed from an
inexpensive Kodak pocket camera. Errors in transcription are possible.
Designs similar to this are used by a wide variety of small photoflash units.
D1 Flashlamp
+------|>|-----+---------+---------------------------+
Inverter | HV Rect. | | FL1 |
+------+--------------------+ | | C3 |
| | Q1 | | | \ .047 +|
| |C PNP | | | Energy / R3 +---||---+ Trigger _|_
\ \| (ECG12) | | | Storage \ 3.3M| | | | |
/ R1 |------------+ | | 200 uF / | +--+ | T2 || |
\ 150 /| | | | +| 330 V | +--+oo+--+ || |
| |E T1 || +-+ | | __|__ | | +--+ | || +--|| |
| | ||(2 | | _____ C2 +-----+ IL1 | ||( || |
+--||--+----+ ||( | | | | | NE2 | ||( || _ |
| C1 1)||( 80 | | -| | | Ready +-+ ||( |_|_|
| .33 <.1 )||( | | | \ | )||( |
| )|| +-------+ | | / R2 | Shutter )||( |
| +--+ ||(3 | | | | \ 20M |- )||( -|
| | 4 ||( .2 | | | | / | S2 +-+ || +-+ |
| | || +----+ | | | | | | | |
| | 5 | | | | | +--------+ |
+------------------------+ | | | | | |
| +-----+---------+-----+---+-----------------+
| ___/ ____| |
| | | | : |
+------------------------||||------+ : S1 Power |
| | |___/ _________|
BT1 3V 2AA
1. The inverter boosts the battery voltage to about 300 V. This is rectified by D1 and charges the energy storage capacitor, C2. 2. The trigger capacitor, C3, charges through R3 and T2. 3. The neon bulb, IL1, signals ready by flashing at about 6 Hz. 4. Pressing the shutter closes S2 which discharges C3 through T2 generating a high voltage pulse (4-5KV) which ionizes the xenon gas in the flashlamp, FL1. 5. The energy storage capacitor discharges through the flashlamp.
1. Transistor was unmarked but ECG12 should be a suitable choice.
2. Resistances of T1 measured with Radio Shack DMM.
3. The power switch, S1, disconnects both the supply to the inverter
and the return for the trigger to prevent accidental triggering
with power off.
4.
| | |
---+--- are connected; ---|--- and ------- are NOT connected.
| | |
This schematic was traced from an electronic flash unit removed from a Kodak disposible 'Funsaver with Flash' pocket camera. Thanks to Bill Kennedy (email: hue@node1.frontiernet.net for performing the reverse engineering and providing this diagram. * Kodak Funsaver Flash Schematic in GIF format. I have also reverse engineered the flash in a Far East (probably) clone of this camera. The only obvious differences seem to be: C1 omitted, R1 = 200 ohms, R2 = 4.7M. Operation is essentially identical.
1. Pressing the 'charge' button (S1) enables base drive to the inverter transistor (Q1, 2SD965). The inverter boosts the battery voltage to about 300 V. This is rectified by D1 (1N4007) and charges the energy storage capacitor, (C2, 160 uF, 330 V). 2. The trigger capacitor, C3, charges through R2. 3. The neon bulb, IL1, signals ready by flashing at about a 5 Hz rate. 4. Pressing the shutter closes S2 (fire) which discharges C3 through T2 generating a high voltage pulse (4 to 5KV) which ionizes the xenon gas in the flashlamp, FL1. 5. The energy storage capacitor discharges through the flashlamp. Note that the battery is never actually disconnected from the inverter. Thus, a failure (shorted) of Q1 would result in draining the battery and potential overheating - I do not know if this has ever happened! WARNING: If left on charge for longer than needed to get the ready light to come on OR if run on greater than 1.5 V, the actual voltage on the energy storage capacitor can be much greater than the nominal 300 V. It is not known at what point the capacitor or other components blow up but needless to say, this becomes even more dangerous!
Here is a sample schematic for a typical line operated medium power
electronic flash unit. Cycle time is under 1 second.
D1 R2 Flashlamp
+---|>|---+--/\/\/--+---------+------+--------+-------------------+
| 1N4005 | 250 | | | | FL1 |
| +| 10W | | | | |
| _|_ C1 | \ +++ \ +|
Power | ___ 25 uF | Energy / R6 |o| IL1 / R3 Trigger _|_
S1 | | 200 V | Storage \ 91K |o| NE2 \ 1.5M* | | |
H __/ __| -| | 400 uF / +++ / T2 || |
| R1 | +| 450 V | | Ready | || |
N ---------/\/\---+ __|__ | | | C4 || +--|| |
| 27 | _____ C3 +------+ +-----+--||--+ ||( || |
AC Line | 5W +| | | | |.1 uF | ||( || _ |
115 VAC | _|_ C2 -| | | | +-+ ||( |_|_|
| ___ 25 uF | \ \ | )||( |
| | 200 V | / R7 / R4 | Shutter )||( |
| -| | \ 180K \ 3M |- )||( -|
| D2 | | / / | S2 +-+ || +-+ |
+---|<|---+---------+ | | | | | |
1N4005 | | R5 | | | | |
+---------+--/\/\---+-----+------+--------+ |
Doubler | 1.5M* |
| |
+-----------------------------------+
1. The doubler consisting of D1, D2, C1, and C2, boosts the AC line voltage to about 320 V. This charges the energy storage capacitor, C3 through R2. R1 limits inrush current to the doubler. 2. The trigger capacitor, C4, charges through R3, R5, and T2. 3. The neon bulb, IL1, signals ready by coming on when C3 is charged to about 270 V. 4. Pressing the shutter closes S2 which discharges C4 through T2 generating a high voltage pulse (4-5KV) which ionizes the xenon gas in the flashlamp, FL1. 5. The energy storage capacitor discharges through the flashlamp. R2 limits current from doubler to allow flashlamp arc to quench.
1. CAUTION: Line operated power supply is not isolated - use with care.
Fuse and power-on indicator not shown. * R3 and R5 provide protection
from line for trigger circuit - do not remove!
2. Flash energy is about 20 W-s. Adjust component values for desired
application.
3. For rapid cycle times, make sure flashlamp is rated for adequate
average power dissipation (e.g., 25 W for 1 second repeat). Forced
air cooling may be required for sustained operation at full power.
4. Trigger transformer, T2, available from places like Digikey and Mouser.
5. Shutter contacts, S2, may be replaced with SCR for electronic control
of flash trigger.
6.
| | |
---+--- are connected; ---|--- and ------- are NOT connected.
| | |
Here is a sample schematic for a typical line operated moderately high
power electronic flash unit. The power supply uses a tripler to generate
approximately 420 V for the energy storage capacitor.
An SCR allows a safely isolated logic or sensor signal to easily trigger
the strobe.
Cycle time is under 2 seconds.
D3 R2 Flashlamp
+-------+--|>|--/\/\--+----+------+--------+-------------------+
| |1N4007 270 | | | | FL1 |
| | 2W | | | | |
_|_ | | \ +++ \ +|
/_\ D1 | | / R5 |o| IL1 / R3 _|_
|1N4007 | C3 | \ 91K |o| NE2 \ 1.5M Trigger | | |
| | 500 uF | / +++ / || |
| | 450 V | | | Ready | T2 || |
+----+ +_|_ C2 +__|__ | | | C4 +--|| |
| | ___ 22 uF _____ +------+ +-----+--||--+ ||( || |
| | - | 450 V - | | | |.1 uF | ||( || _ |
C1 _|_+ _|_ | | | | | +-+ ||( |_|_|
22 ___ /_\ D2 | | \ \ __|__ )||( |
450 V | - |1N4007 | | / R6 R4 / _\_/_ SCR1 )||( |
| | | | \ 270K 1M \ / | TIC106 )||( -|
| | | | / / | | +-+ +-+ |
H o---+ +-------+---+---------+ | | | | | | |
R1 | | | | | | | | |
N o----------/\/\------+ +----+---------+-----+--+---+--------+---+
22 | |
Tripler C5* | |
Trigger + o---||---+ |
.001 uF |
600 V |
|
C6* |
Trigger - o---||---------+
.001 uF
600 V
1. The tripler consisting of D1, D2, D3, C1, and C2, boosts the AC line voltage to about 420 V. This charges the energy storage capacitor, C3 through R2. R1 limits inrush current to the tripler. 2. The trigger capacitor, C4, charges through R3 and T2. 3. The neon bulb, IL1, signals ready by coming on when C3 is charged to about 360 V. 4. Applying a positive edge between Trigger + and - turns on the SCR which discharges C4 through T2 generating a high voltage pulse (5-8KV) which ionizes the xenon gas in the flashlamp, FL1. 5. The energy storage capacitor discharges through the flashlamp. R2 limits current from doubler to allow flashlamp arc to quench.
1. CAUTION: Line operated power supply is not isolated - use with care.
Fuse and power-on indicator not shown. * C5 and C6 provides protection
from the line for trigger circuit - do not remove! As an added safety
precaution, the use of an optoisolator or optoisolated SCR is recommended
for the trigger circuit.
2. Flash energy is about 45 W-s. Adjust component values for desired
application.
3. For rapid cycle times, make sure flashlamp is rated for adequate
average power dissipation (e.g., 50 W for 1 second repeat). Forced
air cooling may be required for sustained operation at full power.
4. Trigger transformer, T2, available from places like Digikey and Mouser.
5.
| | |
---+--- are connected; ---|--- and ------- are NOT connected.
| | |
The Vivitar Auto 253 is a typical small inexpensive automatic electronic flash. As is typical of these designs, the flashlamp is paralleled with a quenchtube. This is a small discharge tube that looks something like an oversized neon indicator light (but probably xenon filled). The quenchtube is triggered at a time after the main flashlamp fires which is determined by the light reflected from the subject and terminates the flash when adequate exposure has been achieved. The actual trigger circuit using an SCR to pulse a trigger transformer applying a 4-5 KV pulse to a foil wrapping on the quenchtube. Typical flash duration for small automatic electronic flash units vary from about 1/50,000 second for a minimum energy (closeup) flash to 1/1000 second for a maximum energy (distant subject or manual) flash. The power supply portion of this unit is interesting as well. It can operate on either AC (220 V, it would seem from the circuit) or a 9 V battery. For AC, a simple half wave rectifier produces about 320 VDC needed by the flashlamp. On DC, it uses an inverter that operates on a 9 V battery rather than the 3 V which is typical of many cheap pocket cameras. This results in a fairly rapid cycle time of about 2 seconds. The ready light looks like an ordinary NE2 neon bulb but must have a different gas mixture as it does not turn on until nearly full charge is reached on the energy storage capacitor. There appears to be no voltage divider. In addition, there is another lamp that provides a nice green illumination for the flash 'computer' dial. This looks like a neon indicator lamp but with an internal phosphor coating. I have observed the spectrum of these things. I have seen two different gas fills in these that emit UV that makes the green-glowing phosphor do its stuff. One bulb type about the size of an NE-2H uses a mixture of neon and xenon. GE made those things (I don't know if anyone else ever did), which are called NE-2G lamps. The other type, a much smaller one that I found in Radio Shack's 272-708 green neon "cartridge", uses a mixture of neon and krypton. (Don Klipstein (don@misty.com or klipstei@netaxs.com)). The Vivitar schematic is split into two parts with FL1, C1, and L1 duplicated to improve readability.
AC D3
+-o IN o-|>|--+
S1 | |
DC AC | D1 |X D2 Flashlamp
+--o o +-o o +----|>|----+--|>|--+----------+-------+-------------------+
| /..|.. / | | | | | FL1 |
| | +| | | | LT1 | | | |
| | ___ +---|------+ +++ ||C \ \ +|
| | _ | | |o| ||C L1 / R5 / R3 _|_
| | ___ BT1 | | |o| ||C \ 1.2M \ 3.3M | | |
| | _ 9V | | +++ ||C / / Trigger || |
| | -| | C3 | | | | | || |
| +---+ T1 +-+--||--+ | | | | C2 T2 +--|| |
| ||(3 220 | \ R2 | | +--||--+ ||( || |
+-------+ ||( pF | / 1,2M | Energy | | .047 | ||( || _ |
| 2)||( 118 | \ | Storage | | uF +-+ ||( |_|_|
| R1 <.1 )||( | | | 380 uF | Ready | )||( |
/ 4.7K )|| +---+ | | +| 350 V +++ | Shutter )||( |
\ +--+ ||(5 | +----+ __|__ |o| IL1 |- )||( -|
/ | 4 ||( .2 | | _____ C1 |o| | S2 +-+ +-+ |
| | +--------+ | | +++ | | | |
| |/ C 1 | | | -| | +------+--+-----+ |
+--| Q1 | | | | | | | |
| |\ E 2SB324 | +_|_C4 | | | / R4 _|_ C5 |
| | | ___ | | | \ 3.3M --- 100 pF |
+----|-----------+ - | 10 | | | / | |
| Inverter | uF |Y | | | | |
+----------------+----+-------+----------+-------+---------+---------+
1a. DC 9 V: The inverter boosts the battery voltage to about 300 V. This is
rectified by D1/D2 and charges the energy storage capacitor, C1, through
the inductor, L1.
1b. AC 220 V: The line input is rectified by D3 and D1 resulting in about
320 V peak which charges the energy storage capacitor, C1, through the
inductor, L1.
2. The trigger capacitor, C2, charges through R3, R4, and T2.
3. The neon bulb, IL1, signals ready by glowing when the energy storage
capacitor is nearly fully charged.
4. Pressing the shutter closes S2 which discharges C2 through T2
generating a high voltage pulse (4-5KV) which ionizes the xenon
gas in the flashlamp, FL1.
5. The energy storage capacitor discharges through L1 and the flashlamp.
|
--- Power Supply --- | ---------- Automatic exposure control-----------
|
D2* Flashlamp Quenchtube R6
X o---|>|---+-------+---------+---------/\/\/\---------------------+-----+
| FL1* | | | |
_|_ | | QT1 | |
Main | | | ||C _|_ Quench | |
Trigger || | ||C L1* | | | Trigger C6 _|_ |
From T2 || | ||C | o || .047 uF ___ /
---|| | | | ||------------------+ T3 | \ R7
|| | | | o || )|| | / 1.2M
|| _ | +_|_ |_|_| )|| C7 | \
|_|_| ___ C1* | )|| +----||---+ |
| - | 380 uF | )||( .047 uF | |
| | 350 V | )||( | |
| | | )||( | |
Y o---------+-------+---------+ +--+ +--+------|-----+
| | | | |
| +----------+ | |
/ | | | |
R10 \ __|__ SCR1 | | |
1K / 470K _\_/_ M21C | | /
Flash Sensor \ LS1 R8 C9 / | 200 V | | \ R8
Power Opening | +----+----+-' | .8 A _|_ C8 | / 2.2M
--------------- CDS | +++ | | | --- 100 | \
Low 1/8" Light | |/| / _|_ | | pF | |
High 1/32" Sensor | |\| \ --- | | | |
Man Closed --> | |/| / |.05 | | | |
| +++ | | uF | | | |
+---+ +----+----+----------+------+-----+
| | |
| _|_ C10 __|__
| --- 100 _\_/_ D4
| | pF |
| | |
+--------+---------+
The power inputs, X and Y, may come from the Vivitar Auto 253 power supply circuit (above), other battery/AC adapter powered inverter, or other AC line operated power supply. 1. Quench trigger capacitor, C7, charges from energy storage capacitor, C1, through voltage divider formed by R7 and R8. 2. Flashlamp is triggered by 4-5 KV pulse from main trigger transformer, T2 (not shown) when shutter contacts close. 3. Because of series inductor, L1, voltage across flashlamp drops abruptly when current starts flowing. (Note: I am calling this an inductor - from appearance only - as it is unmarked. It may just be a small high current resistor). 4. This negative step is coupled by C6 to cathode of the quench trigger thyristor, SCR1. The anode-to-cathode voltage does not change but the cathode becomes negative with respect to the energy storage capacitor negative (common) terminal which feeds the gate circuit. 5. CDS light sensor, LS1, R8, and C9 form an RC network with a time constant inversely proportional to the light reflected off of the subject. Voltage on SCR1's gate increases as C9 charges. 6. When enough light has been detected indicating proper exposure, SCR1 is triggered dumping C7 through quench trigger transformer. Resulting 4-5 KV pulse ionizes (xenon) gas in quenchtube. 7. The quenchtube has a lower voltage drop than the flashlamp and thus bypasses any charge remaining on C1 around FL1 terminating the light output.
1. Cycle time is independent of flash duration as energy storage capacitor
is always discharged nearly fully by either flashlamp or quenchtube.
2. Components in automatic exposure circuit denoted with * are duplicated
from power supply section to improve readability of schematic.
3. Part numbers are consistent with the Vivitar Auto 292 schematic but have
no correlation with actual Vivitar designations.
4.
| | |
---+--- are connected; ---|--- and ------- are NOT connected.
| | |
An energy conserving electronic automatic flash only uses as much energy from the energy storage capacitor as is needed for the particular photographic situation base on light reflected from the scene. This is in contrast to lower cost units (like the circuit shown in the section: "Vivitar Auto 253 electronic flash circuit") that simply dump the excess charge. With these, closeups use the same amount of the battery's capacity and require the same recycle time as long distance shots. With an energy conserving flash, battery life is greatly extended and recovery from low power flashes is instantaneous. Although I did not totally reverse engineer this unit - not being willing to sacrifice my Vivitar 292 as I might never be able to get it back together after analysis - the construction is not what you would call modular - I was able to determine some of its basic operating principles. There is an SCR - it looks like a regular SCR - in *series* with the negative lead of the xenon flashlamp. The SCR part number is Mitsubishi CR3DZ-8. I was not able to locate this part in my databooks bat based on the ECG Semiconductor Master Replacement Guide, similar devices are normal SCRs - typically 8 A at 400 V which would be suitable since these can pass short very high current (250 A) pulses without damage. There is also a quenchtube. This is fired based on light returning from the scene to turn the SCR *off*. I believe that this is done by discharging a separate capacitor in reverse across the main SCR thus driving it into cutoff long enough for the flashtube to extinguish. Other designs may use a small SCR in place of the quenchtube to apply reverse voltage to the main SCR. Alternatively, a Gate TurnOff (GTO) thyristor may be used in place of the main SCR. GTO devices are designed for this type of application and requires only a modest gate pulse to switch them off. What I surmise is that operation of the Vivitar 292 is basically similar to that of the smaller Vivitar 253 automatic flash unit (see the section: "Vivitar Auto 253 electronic flash circuit" (the circuitry on the photosensor board looks nearly identical) except that instead of the quenchtube dumping the entire charge on the energy storage capacitor, it is used to interrupt the current to the flashtube in mid-stride by turning off the SCR. The Vivitar engineers were probably able to add this energy conserving feature to the simpler 253-type strobe with minimal redesign of other parts of the auto exposure circuit. Electronic flash units which incorporate manually selectable power levels can use a similar design. Instead of the light sensor triggering the quenchtube/thyristor, this would be accomplished with a timing or power measuring circuit. If anyone has one of these or similar energy saving automatic flash units they would be willing to donate to the cause, I would fully reverse engineer the design and add it to this document.
This is what I expect the auto exposure portion of the Vivitar Auto/Thyristor
292 to be like. As noted, I have not fully reverse engineered this design.
Some part values have been estimated. I have assumed that the actual exposure
determining and quench tube firing circuits are identical and have used part
numbers from the Vivitar Auto 253 (which, of course, were also arbitrarily
chosen).
The inverter and main power circuits are not shown but should be understood
to be similar - but of higher energy capacity - to those of the smaller
Vivitar electronic flash units.
|
---------- Power Supply ------- | ----- Automatic exposure control------
|
D2 Flashlamp Quenchtube R6
X o---|>|---+-----------------+--------------------/\/\/\----------+-----+
| FL1 | R11* | |
_|_ +--/\/\--+----+ QT1 | |
Main | | | | 1K | _|_ Quench | |
Trigger || | ||C | | | | Trigger C6 _|_ |
From T2 || | ||C L1 | | o || .047 uF ___ /
---|| | ||C | | ||----+ T3 | \ R7
|| | | | | o || )|| | / 1.2M
|| _ | +_|_ | |_|_| )|| C7 | \
|_|_| ___ C1* | | )|| +----||---+ |
| 4 uF - | 750 uF| | )||( .047 uF | |
| C11* | 350 V | | )||( | |
SCR2 +-----+--||-------|--------+ | )||( | |
CR3DZ-8 | | | | +--+ +--+------)-----+
400 V __|__ / +----+-------------+ | | | |
8 A _\/\_ \ R12* | | +----------+ | |
/ | / 10K | / R10 | | | |
T o--' | \ | \ 1K __|__ SCR1 | | |
| | | / 470K _\/\_ M21C | | /
Y o---------+-----+------+ \ LS1 R8 C9 / | 200 V | | \ R8
| +----+----+-' | .8 A _|_ C8 | / 2.2M
CDS | +++ | | | --- 100 | \
Light | |/| / _|_ | | pF | |
Sensor | |\| \ --- | | | |
--> | |/| / |.05 | | | |
Flash Sensor | +++ | | uF | | | |
Power Opening +---+ +----+----+----------+------+-----+
--------------- | | |
Low 1/8" | _|_ C10 __|__
Med 1/16" | --- 100 _\_/_ D4
High 1/32" | | pF |
Man (Switch) | | |
+--------+---------+
The power inputs, X and Y, may come from the a circuit similar to the Vivitar Auto 253 power supply circuit (but of higher power probably), other battery/AC adapter powered inverter, or other AC line operated power supply. 1. Quench trigger capacitor, C7, charges from energy storage capacitor, C1, through voltage divider formed by R7 and R8. SCR shutoff capacitor, C11 charges from C1 through R11 and R12. 2. When shutter contacts close, a pulse is applied to the trigger of the main SCR (SCR2, or it is already triggered on continuously while the main filter capacitor is charged). Flashlamp is triggered by 4-5 KV pulse from main trigger transformer, T2 (not shown). Note: SCR2 must be turned on before flashlamp is triggered. 3. Because of series inductor, L1, voltage across flashlamp drops abruptly when current starts flowing. 4. This negative step is coupled by C6 to cathode of the quench trigger thyristor, SCR1. The anode-to-cathode voltage does not change but the cathode becomes negative with respect to the energy storage capacitor negative (common) terminal which feeds the gate circuit. 5. CDS light sensor, LS1, R8, and C9 form an RC network with a time constant inversely proportional to the light reflected off of the subject. Voltage on SCR1's gate increases as C9 charges. 6. When enough light has been detected indicating proper exposure, SCR1 is triggered dumping C7 through quench trigger transformer. Resulting 4-5 KV pulse ionizes (xenon) gas in quenchtube. 7. The quenchtube has a discharges C11 applying a voltage pulse in reverse across SCR2. This biases the SCR in cutoff long enough for the flashlamp to extinguish. Very little energy is lost.
1. Cycle time is a function of actual amount of light required for each shot.
It is possible to fire off a half dozen or more low power shots without
any recycle delay.
2. The values of components denoted with * have been estimated (i.e., guessed).
3. Part numbers are consistent with the Vivitar Auto 253 schematic but have
no correlation with actual Vivitar designations.
4.
| | |
---+--- are connected; ---|--- and ------- are NOT connected.
| | |
Here is a schematic for a high power xenon strobe unit suitable for pumping
a small Ruby, YAG, or Neodymium-Glass laser rod. (The term 'small' is used
here in a relative sort of way - well at least compared to those at the Laser
Fusion Facility at the Lawrence Livermore National Laboratory.)
WARNING: This is only an example. We take no responsibility for either
the accuracy or functional correctness of the schematic or any consequences
should you attempt to construct this circuit either in its original form or
modified in any way.
Power D1 R5 L1
+-----+--|>|--/\/\--+-----+------------------CCCCCC----------------+
||( | 5 KV 5K | | 25 uH (est) |
||( |.5 A 25 W +_|_ / |
H --+ ||( | ___ C1 \ R1 C1-C4: Energy storage |
)||( 600 | - | / capacitor bank, Flashlamp |
115 )||( VRMS | | | 3600 uF, 450 V (each!) FL1 |
VAC )||( 200 | +-----+ +|
2A )||( mA | | | R1-R4: Voltage drop _|_
)||( | +_|_ / equalizing resistors, | | |
N --+ ||( | ___ C2 \ R2 200K, 1 W Trigger || |
||( | - | / 30 KV || |
||( | | | R7 + C5 - +--|| |
+-------------------+-----+--/\/\--+-------+-----||---+ ||( || |
T1 | | | 1.8M | | 3.9 uF | ||( || _ |
| +_|_ / 1 W | | 450 V | ||( |_|_|
| ___ C3 \ R3 | | | ||( |
| - | / \ | +-+ ||( -|
| | | / R8 __|__ SCR1 )||( |
| +-----+ \ 1M _\_/_ C107D )||( |
| | | / / | 400 V )||( |
| +_|_ / | | | 4 A )||( |
| ___ C4 \ R4 | | | +-+ +-+ |
| - | / | | | | T2 | |
| D2 R6 | | | | | | | |
+--|<|--/\/\--+-----+--------+-------+-----+----+--------+---+
5 KV 5K R9 | R10 |
.5 A 25 W Fire o--/\/\--+--/\/\--+
(+5) 100 100
1. Power transformer, T1, in conjunction with D1, D2, and C1-C4, provides 1.7 KV DC. The power supply doubler capacitors are also used as the energy storage capacitors. Resistors, R1-R4, equalize the voltage drops across the series capacitors to compensate for slight differences in leakage resistance. R5 and R6 limit inrush current and charge rate. 2. The trigger capacitor, C5, charges through T2 from the voltage divider formed by R7 and R8. 3. Ready light and capacitor bank voltage monitoring circuits are not shown. 4. Applying a 5 V signal to the Fire input turns on SCR1 dumping C5 into the primary of the trigger transformer, T2. This generates a 30 KV pulse which ionizes the xenon gas in the flashlamp, FL1. 5. The energy storage capacitor bank discharges through L1 and FL1.
1. WARNING: If you thought line operated equipment was dangerous, this is
much much worse. The power transformer output is enough to kill. Once
doubled and stored in the capacitor bank, it is LETHAL. The total energy
storage is about 1300 W-s (this is not a typo!). Based on one estimate,
this is enough energy to KILL 20 adult humans simultaneously with the
power supply unplugged from the AC line - and still have some juice left
over. TAKE EXTREME CARE!
2. Fuse, power switch, power-on light, and all other absolutely essential
safety interlocks and indicators are not shown. R1-R4 do act as bleeder
resistors and will discharge the capacitor bank to safe levels in about
10 MINUTES. However, don't depend on these. Resistors can fail. Use
the capacitor discharge tool and indicator.
3. The power transformer from a tube type (old) TV set would probably be
suitable for T1. Microwave oven high voltage rectifiers may be used
for D1 and D2. A high power xenon tube like this requires a 30+ KV
trigger pulse. Those little tiny trigger transformers will NOT work.
Capacitors, C1-C4, must be rated for photoflash rapid discharge.
4. High power strobes require special flashlamps - anything from a pocket
camera or electronic flash will explode into a mass of molten bits of
glass and metal. This design is derived from one using a tube from
EG&G, Electro-Optics Division (Salem, Massachusettes, 35 Congress St.,
Salem, MA 01970, (508)745-3200. They produce a complete line of xenon
strobe and continuous output lamps. Another division of EG&G produces
most of the xenon flashtubes used in disposable cameras in the world.)
Even a properly specified flashlamp may explode - operate only behind
protective shielding. Flashlamp cooling must be adequate for desired
cycle time.
5. L1 helps to shape the discharge current pulse. For some high power
strobe designs, a series inductor is essential to optimize power output
and prevent damage to the flashlamp due to excessively high current and
negative voltage (undershoot resulting in reverse current). A damping
factor of .8 is generally recommended. The 25 uH value is just an
estimate - L1 must be calculated for each combination of energy storage
capacitor value, voltage, and the impedance characteristics of the
specific flashlamp to be used.
6. If you are serious about constructing a high energy strobe system (and
your life and accident insurance is fully paid), consider some advanced
reading first. The flashlamp manufacturer's datasheets and application
notes will prove essential. The following is one example of possible
sources of general design information:
(From: Bill Reuber (breuber@mail.wcl.on.ca)).
I have found this to be useful:
Solid State Laser Engineering (revised)
Springer-Verlag Heidelberg, New York
ISBN 3-540-18747-2 or 0-387-18747-2
They have chapters on many aspects of laser system design including pulse
forming networks for flashlamp systems.
7. Flash energy is about 1300 W-s. For a typical flash duration of 250 uS,
this is an equivalent power input to the flashlamp of 5.2 MW! Adjust
component values for the desired application.
8. DO NOT even think about staring at the flashlamp when fired. The peak
light output is equivalent to at least 500,000 - 100 W light bulbs!
Even when averaged over the 1/40th of a second typical response of the
human eye, this is still more than 5,000 - 100 W light bulbs. (Note:
this estimate takes into account the increased luminous efficiency of
xenon flashlamps compared to incandescent light bulbs.)
9. Make sure all optical components - especially the flashlamp - are cleaned
with isopropyl alcohol and a lint free cloth to remove all traces of
contaminants.
10.
| | |
---+--- are connected; ---|--- and ------- are NOT connected.
| | |
This schematic was taken from a cheap commercial automotive timing light.
R1 150 ohms D2 1N4007
H o--------/\/\/---+-----|>|------+------------+
_ 10 W | | | FL1
_|_ | D1 1N4007 |+ +-|-+
N o---- ----+ +--|<|--+ _|_ 16 uF | | || HV Wire
Power |+ | C2 ___ 420 V | ||--------o From #1 Sparkplug
_|_ 16 uF | |- | _ ||
C1 ___ 420 V | | +-|-+
|- | | | Flashlamp
+------------+------+------------+
1. D1 and C1 form a half wave doubler which produces a waveform across D1 which is approximately a sinusoid with a p-p voltage of 2*1.414*VRMS of the line or about 320 V. (The peaks will get squashed with a significant load). 2. C2 charges from this through D2 to about 300 V. The flashlamp fires when triggered by the HV pulse from the #1 sparkplug connection. Note: this requires a direct connection, not an inductive pickup. 3. I would think that there will be some beating of the charging and flash for high rpms but the timing will be accurate. In other words, it will not fire for every rotation of the crankshaft since C2 cannot recover quickly enough but will flash at the proper instant when C2 has charged to a sufficient voltage. (This is probably by design - otherwise, the flashlamp would overheat very quickly at high rpms.)
Here is a schematic for a typical line operated variable rate stroboscope.
This unit is from Welch Scientific - Model 2153C Stroboscope.
Its uses include the visualization of moving parts as well as rotation speed or
frequency determination of rotating or vibrating machinery.
Specifications:
Flash energy - .1 W-s.
Low range - 1.6 to 20 pps (96 to 1200 rpm).
High range - 8 to 120 pps (480 to 6000 rpm).
Flash duration - approximately 10 uS.
D1 D2 L1 R1 Flashlamp
+--|>|--|>|--+------CCCCCC--------/\/\/-----------------+----+
| 1KV 1KV | 2.5H 25K 25W FL1 | |
| | | |
Power | | 200K 2.5M | |
| +---+--/\/\--/\/\--+--+-----+ +| |
T1 | | R2 ^ R3 ^ | | | Trigger _|_ |
+--+ | Trim | Adj| | o | | | | |
||( | / +---+ | S2 | T2 || | |
H o--+ ||( | R4 \ | o X5 | || | _|_
)||( | 100K / | | +-------+ || +--|| | --- C3
115 )||( | \ | / R7 )||( || | | .5
VAC )||( | R5 | | \ 100 R8 )||( || _ | | uF
)||( +--/\/\--+ C4 | / 220 )||( |_|_| |
N o--+ ||( | 330K | .05_|_ \ +--/\/\--+ ||( | |
||( C1 __|__ / uF--- C5 | | ||( | |
+--+ 16 _____ R6 \ | .20_|_ _|_ D3 ||( -| |
| uF | 680K / | uF--- \/\ Diac || +-+ | |
| 700V | \ | | | 175 V | | |
| | | | | | | | |
G o----------+--------+--------+-----+------+----+---------------+---+----+
1. The power supply consisting of T1, D1, D2, C1, and L1, develops about 650 V. This charges the energy storage capacitor, C3 through R1. 2. The trigger capacitor, C4 (X1 range) or C4||C5 (X5 range), charges through the frequency (speed) control network consisting of R2, R3, R4, R5, and R6. The main (user) control is R3 which provides roughly a 12:1 range. 3. When the voltage on the Diac, D3, exceeds about 175 V, D3 changes to the conductive state - it becomes a short circuit. This discharges the trigger capacitor(s) through T2 generating a high voltage pulse (4-5KV) which ionizes the xenon gas in the flashlamp, FL1. 4. The energy storage capacitor discharges through the flashlamp. 5. The energy storage capacitor and trigger capacitor(s) recharge and the cycle repeats. The inductor, L1, prevents the voltage across the energy storage capacitor from increasing too quickly for a few critical milliseconds just after the tube fires to enable the arc to reliably extinguish at high repetition rates.
1. Power switch, S1, and fuse, F1, not shown.
2. Flash duration is minimized by using the small (.5 uF) but high voltage
energy storage capacitor.
3.
| | |
---+--- are connected; ---|--- and ------- are NOT connected.
| | |
The circuit in the companion document strobex.ps is designed to provide a variety of options in terms of repetition rate, flash intensity, and various repeat and triggering modes. Due to its complexity, it was drafted using OrCad SDT (Schematic Capture), output in postscript, and converted to PDF and GIF formats:
Roulette wheels and wagon wheels spinning backwards represents a form of aliasing due to sampling. OK, the technical jargon aside this effect will only take place in a situation where images are captured at discrete intervals as they are in a motion picture or video - or when illuminated with a repeating strobe. For motion pictures, something like 24 frames per second are recorded; for video there are 30 frames per second (in the US, 25 in many other countries). However, the use of interlacing where a complete frame is scanned in two parts - the even and the odd lines - complicates the explanation so I will restrict the remainder of this discussion to motion picture film). If the rotation rate of the wheel is such that one spoke or slot goes by a given position in exactly 1/24th of a second, the wheel will appear stationary since successive images will be identical. If it is moving a bit faster than this it will appear to be moving forward slowly. However, if it is going a bit slower, then it will be appear to be turning backwards slowly. The shorter the exposure with respect to the total frame time, the sharper will be the apparent effect. The number of slots per second of perceived motion will be equal to the difference in frame rate and number of slots per second passing a given point. So, a roulette wheel rotating such that 23 slots are passing by per second captured on a 24 frame per second camera will appear to be moving backwards at 1 slot per second. The same applies to the use of a strobe light to freeze repetitive motion like the rotation of a shaft. It is all a matter of the relative speed of the sampling (the movie, video or strobe) with respect to an object which is periodic like a roulette or wagon wheel. You can perform a simple experiment: run an electric fan under a fluorescent lamp (one with an ordinary magnetic ballest). The light from such a lamp is not continuous but pulses 120 times per second. Watch for stationary or slowly rotating blade patterns as the fan speeds up and slows down. See if you can compute the speed of the fan from this behavior.
(This from: Kevin 'Destroyer of Worlds' Horton (khorton@tech.iupui.edu)) Just for funsies, I decided to see how much torture I could inflict on the flashlamp and energy storage capacitor from one of those little Kodak cameras. The tube was 1.2" long, in a metalized plastic reflector, with a thin metal backing to hold it in. The capacitor was 120 uf, 330 V. I hooked it up to my inverter (12 V->300 V at high current) and fired 'er up! Pop, pop, pop, pop, pop, pop, pop, (turn up trigger oscillator frequency) popopopopopopopopopopopop! It was firing about 30 or 40 times a second; it appeared as it was constantly on! I turned it down to about 15 flashes a second, and let it run. First thing I noticed was that wonderful scent of melting acrylic. Then, I noticed that the tube was kind of skewed in the reflector. The plastic was in full smoke-mode by this point. Still, the tube kept firing! (Let's see: 5 W-s times 15 flashes per second is 75 W average power, not bad for an itty bitty tube --- sam). I left it on a bit more, and the plastic really started the smoke-signals! I noticed that one electrode was glowing cherry red. Even after all this torture, it kept going! The smoke was getting too much, so I hit the 'off' on my inverter. A few more gouts of smoke, and the little fire I created was extinguished. I let it cool down and then I examined the damage. The reflector was totaled; the tube had all but melted clean through. When I touched it, the little metal plate popped off. On closer examination, the tube appeared to be in good shape. I couldn't see any visible damage to either the electrodes, or the glass seals. A quick test reveals that the tube still functions. As a side note, the storage capacitor got quite hot; probably around 35 degrees C. All in all, an interesting test, I must say. The next will involve connecting up a normal NE2 neon bulb and observing the results of high voltage and high current on it. I suspect it will be quite spectacular, so I'm taking precautions - It will be performed in a proper enclosure, so if the neon decides to really go 'pop', it won't do any damage.
(From: Ruben (ruben@worldaccess.nl)). I used to design stage-effects, and played some time with strobes. Built a number, from 750 W-s at high rates to 22,500 W-s single flash. Philips makes xenon lamps, designed for photographic use - they are not flashtubes but burn continually - so using them as flashtube shortens their life span (assuming you increase power). They are expensive, from $250 for the smallest to $1,100 for the biggest. For caps for the smaller (20 to 100 W-s) strobes, I used a huge array of MKT motor-caps. 10 uF at 630V is cheap, a few dollars, and building an array of these is not too hard. These caps are screw-mount, and you can just fill a board with them, and switch them in parallel. Keeps the ESR low, which is a requirement. The monsters used larger caps, 680 uF each. My boss often visited executory sales and bought components and machinery. These caps (he had a number of crates full of them) sat on the shelves for a year before I decided to do something with them. Beautiful Siemens stuff. Very low ESR, large cap. Nevertheless, I did say "huge array" which was exactly that. Two boards (one for each set of lamps) filled with them, each board 1 by 1.5 meters, which dictated the size of the case. As I remember these caps were about 5 centimeters diameter, and something like 12 centimeters high, so - guessing - you could stack around 280 of them on a board, which sounds right. All problems I had related to heat. The 750 and 1500 W-s models had a habbit of melting their main wire. On typical stages one uses a lot of extension-wires, and its power consumption could be high enough to heat the extension to the point the insulation came dripping off, without blowing a fuse. Had to lower the amount of energy per flash at higher rates. The protective window in front was another problem area. Poly-carbonate covers work fine, but a single fingerprint absorbes enough IR to melt a hole in the cover. Glass won't melt, but shatters if dirty. Don't allow anything near it. Colored paper will catch fire within seconds, at max rate. Always use it to flash at a wall, never let the public look into such a bright flash. The biggest used four larger tubes, flashing two by two, but charge-times were too long to make it useable as a real strobe. It was used to flash the ceiling of a large stadium. I considered it to be useless. I never solved the problems it had, like eating its eight, expensive, diodes (>$40 each) for lunch. It sucked dips in the mains, big enough to cause digital equipment to fail. Imagine all the effects of the audio-boys resseting after each flash. I kept it running for a few months, but when the edges of the window caught fire I scrapped it. I modified one off the triggers for the small strobe (750 and 1500 w-S) to allow multiple units. I found a note in the same binder with a capacitor-free design for a strobe. The smallest Xenon tube made by Philips has a burning voltage low enough to start it on 220V. With a suitable choke and a diode in series it will burn - after ignition - for the rest of the half-cycle. The diode makes sure it dies when the polarity reverses. (Residual ionization will make it re-ignite without the diode) The choke will keep it from aggravating the utility-companies. I wonder if anyone knows a trick to enhance ionization. Fully ionized it has a burning-voltage of 50 Volts, but even after the 4KV pulse it needs 200 Volts to get started. Only tricks I know are the 'normal' starter-pulse, microwave pulses, radioactivity and laser-pulses. Only the first one is acceptable with audiences around. It probably won't work with 110 V mains.
Common electronic components can be obtained from any large distributor.
Even Radio Shack may have what you are looking for. However, many do not
list any xenon flashlamps or trigger transformers.
1. Garage sales (and the like), flea markets, thrift stores. These are often
good sources for cameras with a built-in electronic flash, photographic
strobes, and other similar equipment. Your (or your Aunt Minnie's) attic
or basement may even hold some of these treasures!
The going rate for a typical cheap flash camera is generally $.50 to $1
at a garage sale or flea market. While these may in fact still work, they
often use 110 size film so you won't feel too badly about gutting them for
the flash unit or its parts.
Although in principle the capacitor may deform after a long period of
non-use, I have yet to see any real trouble having picked up over 2 dozen
cameras and strobes from these sources. None of these have had any actual
defective components (though a couple had bad connections or broken wires).
My last acquisition was a completely functional variable rate stroboscope
for $2.
2. Photo processing labs who accept disposable flash cameras may just throw the
carcasses away after extracting the film. These may be available for the
asking. Unfortunately for our needs, I have heard that the reusable parts
are now being recycled. :-(
(From: Scott Johnston (sj@thor.iac.net)).
Complete working strobe circuits are available for *free* at photo
developing places (not K-mart, but the expensive places that actually do
the developing in-house). When they develop film from those cheap weekend
disposable cameras (you know, the kind that are made out of plastic and
cardboard?), they rip out the film and throw away the camera housing. The
disposables with flashes have a complete xenon strobe circuit (triggered by
a tiny little switch on wire leads) powered by a single AA (1.5v) alkaline
battery. Recently, I called the local photo developer, asked if they could
save some of the kind with flashes, and a few days later I picked up a pile
of 12 complete flash units, with almost unused AA batteries in all of them!
Really fun, although I discovered quickly that the capacitors in those
things don't have bleeder resistors...
(From: Alfred C. Erpel (aerpel@ptd.net)).
I was picking up my Halloween party photos from the 1 Hour Photo place at
my local drug store and I noticed a trash box full of thrown out single
use cameras, empty 35mm spools and plastic containers. I asked if I could
have the entire box.
When I got home I found I had 27 cameras with usable flash units and
most of the AA batteries were still good. My wife got the plastic
containers for her Girl Scouts crafts. The inside of used Kodak film
canisters contains a nifty spool which may make a useful bobbin for some
types of coils.
Watch out for residual charge on the flash capacitor when you disassemble
these! Also, observe the mechanics carefully because, although I'm not
certain about this yet, it seems that some of the cameras are designed to
purposely disable the flash circuitry by mechanically breaking an existing
connection when the board is removed. Obviously this could be restored if
you see where it is.
No doubt some places won't give you their trash (afraid of the potential
for liability), but it can't hurt to ask.
3. Mouser Electronics (General electronics parts including trigger
U.S. Voice: 1-800-346-6873 transformers, magnet wire, rechargeable
U.S. Cat. Sub: 1-800-992-9943 batteries, laser diodes, photodiodes.)
Sales/Service: (800-34-MOUSER)
Web: http://www.mouser.com/
Mouser stocks a few xenon flashlamps and trigger transformers suitable
for both small and medium power strobes.
4. Dalbani (Excellent Japanese semiconductor source,
U.S. Voice: 1-800-325-2264. VCR parts, other consumer electronics,
U.S. Fax: 1-305-594-6588. car stereo, CATV).
Int. Voice: 1-305-716-0947.
Int. Fax: 1-305-716-9719.
Web: http://www.dalbani.com/
Dalbani's current catalog lists a few types of xenon flashtubes - some of
relatively high capacity (100s of W-s). Their selection used to be much
larger. My guess is that they acquired someone else's inventory and have
been selling this off without replenishing it.
5. Radio Shack stocks a couple of pricy strobe lights as well as a small
xenon flash lamp. At least you can just walk over to your neighborhood
store!
6. All Electronics (Large selection of surplus and new
U.S. Voice: 1-818-904-0524 electronics and hobbyist items.)
A flashtube, trigger coil, and a more complete camera flash assembly are
listed in their catalog.
7. Electronic Goldmine (1-602-451-9495 or 1-800-445-0697) has some strobe
kits, flashtubes, reflectors, flashtube-reflector combos, a trigger
coil, a quench tube (!!), two different inverter transformers, and two
complete strobe schematics, one of which is a 12 volt strobe using one
of these transformers.
The also sell small flashtubes by the bushel :-) about 1.2 inches long
(~30 mm) by .15 inch (~3.5 mm) diameter. These cost 49 cents each, or
100 for $25. So, if you are planning on building your own New Year's
Times Square celebration sphere, these may be ideal! These were offered
in 1996 and may no longer be available but should be worth an inquiry.
8. (From: Gary M. Reese (greese@comteck.com)).
High power capacitors (like 450 uf at 500 volts) and other strobe parts
may be had though the list of strobe service centers at the following site:
http://www.lumedyne.com/service.htm
However, it should be remembered that they are repair centers and do not
normally sell parts at retail. I have ordered a capacitor like the one
mentioned above from one them at a cost of $26.00 plus $3.50 S&H.
(What this means is that (1) their prices may be quite high and (2) they
may not be eager to sell to the public. --- sam)
9. (From: Scott Tilton (stilton@execpc.com)).
The original Strobotac flashtubes were made by EG&G Optoelectronics. They
used a FX6-A. I believe they now supply a FX7-A as a replacement. You can
reach them at 1-800-950-3441.
You might also try Quad Tech, which still manufactures the General Radio
1531AB, and other General Radio stroboscopes. They can supply spare parts.
You can reach them at 1-800-253-1230.
Additional information including part numbers and minimum order amounts is
available at: http://www.misty.com/~don/flashsrc.html.