Nearly all of these designs can be used as the basic blocks for a power supply you can build. Few special parts are required and for the most part, nothing is terribly critical.
The design described below can serve as the basic front-end to a linear or switchmode regulator, inverter, or to a brute force power supply using only an additional ballast resistor (for testing only, right?). It is a simple AC line-connected AC to DC power supply.
Note: Essential safety and protection components not shown. See the section: Requred Safety/Protection Features.
PH-H To Filament Transformer
o
Preheat |
S2 | D5 R5
+---o/ o--+----|>|-------/\/\---+
| |
| OP-H To PS Fan/Igniter |
Main | o |
Power | Operate | |
S1 | S3 | D1 R1 | R2 (or L)
H o---o/ o--+---o/ o--+-+--|>|-----+--/\/\--+-+-----+--/\/\--+-----+--o DC+
: ~| D2 |+ | | | |
: +--|<|--+ | +_|_ / +_|_ /
: D3 | | C1 --- R3 \ C2 --- R4 \
: +--|>|--|--+ - | / - | /
: ~| D4 |- | | | |
N o---o/ o--------------+--|<|--+-------------+-----+--------+-----+--o DC-
Note: S1, S2, and S3 can be switches or relays. Logic controlled relays are
highly desirable to enforce the sequencing requirements on the Ar/Kr ion tube
power. For initial testing, manually operated switches may be used.
S3 also enables the power supply fan and the igniter circuitry. (The fan in the laser head should be powered whenever the laser is running and until the tube has cooled down.)
Note: Additional small ceramic capacitors should be placed in parallel with C1 and C2 to bypass high frequency noise (not shown).
An alternative approach is to use a high current inductor as a smoothing choke. The advantage of this approach is that little power is dissipated in the inductor. The problem is in obtaining a suitable part. An inductance of 2.7 mH is required per ohm of impedance at 60 Hz. A 6 mH inductor requires roughly 50 layers of #14 wire on an EI type laminated steel core (I won't even mention toroids!). The core from a salvaged microwave oven transformer should be suitable. High current inductors are also available commercially. For example, the Stancor C2688 is rated at 10 mH, 12.5 A (and around $45, WOW!).
Using a LARGE Variac (it has to handle the entire tube current and then some!) to power the system is an alternative since it can be set to provide a filtered DC voltage that is equivalent to the use of the resistor. However, without R2, the ripple will be slightly higher so there won't be quite the full benefit of lowered DC to the regulator.
A single bleeder of 1/2 the resistance and twice the power rating could be used. However, mounting R3 and R4 on the filter capacitors themselves assures that they will still be safely discharged even if another part fails or a connection opens up!
A capacitor charge indicator LED can be easily included for additional protection. See the section: Visual Capacitor Bleeder Circuit. An NC relay activated low resistance (say 1K, 20 W) bleeder could also be added to discharge the capacitors more quickly whenever power is removed.
To calculate the required values for R2, and C1 and C2, we can make the following assumptions:
Note: Where an isolation transformer of marginal capacity is used for testing, the peak capacitor recharge current will be limited and ripple will increase. Therefore, don't be surprised if the discharge winks out under these conditions. For this reason, an isolation transformer rated for at least 2X of the laser power supply's maximum power consumption should be used. Of course, such a transformer is HUGE!
Based on this, one approach is to try to equalize the worst case ripple and the drop across R2. This will result in reasonable values for C1 and C2 while still pushing much of the power dissipation to R2. With 35 V between peak and tube voltage (145 - 110) and subtracting out 5 V for headroom, this leaves 30 V to play with. Dividing this in half results in about 15 V of ripple, or from (3), above, about 4,000 uF each for C1 and C2. R2 is then 1.25 ohm, 200 W (!!!). Trading off the values of R2 and C1/C2 may be desirable depending on the specific needs.
However, using just an inductor without some other means of regulation may result in plasma instability or oscillation which may not be obvious without looking at the current waveform on an oscilloscope. This will damage the tube after a few hours of operation.
With R2 = 0, somewhat smaller capacitors can be used as long as adequate current is available for the lowest points on the valleys of the input waveform. Assuming 10 V of headroom is enough, this allows 25 V of ripple resulting in C1 and C2 of 2,400 uF each.
Although the component values will change, the single-phase design is similar to that described above for 115 VAC and the same design approach applies.
However, a simple buck autotransformer may be desirable to reduce the input voltage somewhat to ease the dropping requirements of any series resistor 'heaters' and/or the current regulator pass-bank. A microwave oven high voltage transformer can be modified for this purpose by removing the HV winding and substituting a high current secondary putting out around 50 to 60 VRMS (assuming a 230 VAC input and a 200 VDC tube drop) which is placed in series anti-phase with the power supply input.
Note: 230 VAC is available from most residential wiring by using opposite sides of the incoming power feeds. These come from a centertapped (utility pole) transformer. Electric dryers, hot water heaters, stoves, central airconditioning systems, and other high power appliances will already be wired in this manner. Of course, safety warnings increase exponentially at these higher voltages. There just isn't any room for error!
D1 R1 L1 ::::
H1 o---+---|>|---+-------/\/\----+-----+---^^^^----+-----+---o DC+
| D2 |+ | | | |
+---|<|---|--+ | | | |
D3 | | | / | /
H2 o---+---|>|---+ | C1 +_|_ R2 \ C2 +_|_ R3 \
| D4 | | --- / --- /
+---|<|---|--+ - | \ - | \
D5 | | | | | |
H3 o---+---|>|---+ | | | | |
| D6 |- | | | |
+---|<|------+------------+-----+-----------+-----+---o DC-
Going to three-phase makes sense for high power ion lasers requiring around
200 V across the tube if such power is available since for a given current and
ripple requirement, filter capacitor size (uF rating) is greatly reduced and
loading on the building's power distribution system is more balanced. This
approach makes sense where the Ar/Kr ion tube requires around 200 VDC at 20 A
or more - typical of lasers putting out many W of beam power.
The use of three-phase power greatly reduces the size of the necessary filter components since the rectified pulses from the diodes are at a 60 degree phase angle with respect to each other - 6 per cycle overlapping by 120 degrees. So, raw ripple is down by around 85 percent even without any filter components and the required capacitance on an uF/A basis is greatly reduced. In fact, no capacitors at all may be acceptable assuming the power supply includes a decent regulator. However, due to the generally larger current requirement, an inductor makes more sense than a house heater size resistor in the 'pi' network even though it needs to be a boat-anchor weight chunk of iron wound with #10 AWG wire!
Note that regulator implementation may not change that much compared to those used for single-phase 115 VAC systems with 100 VDC tubes. This is because the voltage drop across a series pass linear or switchmode regulator can be set up to be similar (at most requiring a modest size buck/boost transformer) in both cases. Of course, current levels may be higher and protection devices need to be sized accordingly for the higher total voltages and power levels involved.
If you really need these sorts of power levels, I am confident you will be able to come up with the obvious extensions to these schemes as well as all of the remaining nitty-gritty details. :-) However, here are some comments and cautions on large-frame three-phase power:
(From: Dean Glassburn (Dean@niteliteproducts.com).)
If you do not have sufficient current and balanced voltage, specifically on a large frame laser from (Coherent, Inc.) with indium seals on the window stubs, the tube will see an undue amount of ripple which in turn will heat up the anode to the point of melting the indium seals. This is a common fault on large lasers of this type when one phase drops out. Then you have a 500 pound boat anchor, no matter what you paid.
Most, if not all of the power supplies on three-phase units do not use the Neutral as a load bearing wire. In fact, most Neutral wires are the same as the grounding conductor (Safety/Earth Ground) at the service panel except the latter is usually of a smaller gauge. The low voltages required for logic and control in these supplies are usually derived off a common winding of the three phases or off a separate transformer isolating the low voltage AC from the mains.
Except for testing (where just a low value high power ballast resistor and large Variac will suffice as long as current is monitored closely, this current control is essential and sets the operating point providing immunity from line voltage fluctuations and slight changes in tube operating voltage (e.g., due to tube warmup).
Small series resistors or hall-effect devices are typically used for the current sensors.
The DC coupled 'light control mode' is desirable to stabilize beam power and maximize Ar/Kr ion tube life and adjusts the current around the operating point.
An AC coupled 'noise signal' is also usually provided to reduce random high frequency variations in beam power. These may be the result of plasma oscillations or other instabilities possible with some laser head and/or ion tubes.
In all cases, the implementation usually uses some common op-amps and will have loop compensation in the form of integral/proportional/differential terms in the feedback network equations. Stability must be guaranteed for under all conditions.
A linear regulator controls the current going to the Ar/Kr ion tube in many ways analogous to that of a common IC like an LM317. However, there are some notable differences:
To reduce the thermal stress on the transistor pass-bank, its input voltage can be reduced in a number of ways. There include the use of a high power resistor (e.g, a heating element), a buck transformer (which could be a fat winding added to the filament transformer), or a switchmode pre-regulator (especially when driving an 80 to 110 V ion tube from a 230 VAC line). However, in all cases, ripple on the main filter capacitor(s) and line voltage fluctuations must be taken into account to assure enough headroom under worst case conditions. With care, this would permit fewer and/or lower cost transistors to be used. The latter two approaches would also reduce total power dissipation.
WARNING: The entire heat sink will be line connected! DO NOT even think about touching it (or anything else) until the unit is unplugged AND the main filter capacitors have discharged!
Note: For testing and troubleshooting, mounting the transistors on the heat sink with mica insulators (and silicone heat sink compound) may be desirable to permit faulty or suspect devices to be easily isolated if necessary. However, the mica will increase the thermal resistance a bit - the power transistors will run hotter and/or more of them will be needed. On the plus side, the heat sink can be electrically isolated as well - just don't touch those TO-3 cans!
A plenum must be provided to guide the gale-forced-air from the power supply cooling fan over the heat sink fins. This can be made of an insulator like Plexiglas to also serve as protection from accidental contact with the electrically and thermally hot heat sink and/or transistors.
NEC uses a pair of MJ11032s (ECG2349) - one as a preregulator chopper to drop the voltage from the rectified AC line and the second as a linear regulator for up to 9 A of tube current. The MJ11032 is rated at 120 V, 50 A (!!), 300 W (!!), and it's a Darlington so the minimum Hfe is 1,000 (!!). This could be the ideal pass-transistor since so few may be required - if you can afford it without taking out a second mortgage! I heard a price quote of $17 in singles. :)
However, with care, it may be possible to use relatively inexpensive power transistors like the MJE15015 (120 V, 15 A, 180 W) or possibly even the ubiquitous 2N3055 (60 V, 15 A, 110 W) for the pass-bank. There are also many types with specifications that are just slightly lower than the 2N6259 (mostly with respect to maximum power dissipation) so these may be worth checking out (starting with a semiconductor cross-reference followed by a search of your junk drawer).
Whatever route you choose, it is very important that the transistors in the pass-bank be well matched in terms of Hfe. In addition, minimum Hfe at the highest possible collector current be much more than adequate (say double) to guarantee sufficient base drive. Otherwise, depending on the design of the driver circuits, the collector currents could end up being seriously unequal despite any emitter current balancing resistors. This may result in significantly unequal stress (current and thus power dissipation) leading to spectacular failures!
A sophisticated transfer function for the feedback network, F(s), may be used comprising separate loops for Current Control, Light Control (DC and AC - not shown), Standby mode, and an external Modulation Input. See the section: Multiple Loop Controller Organization for further details.
This regulator connects between the negative output of the main AC line rectifier/filter (DC-, which is also the analog common) and the centertap of the filament transformer (Tube-).
Control Amplifier
+------+ <---------- Pass-Bank --------->
+--| F(s) |--+ Tube- o---+------------+-------------+
| +------+ | | | |
| |\ | |\ Qx1 | Qx2 | ...... Qxn |
Vcl o---/\/\---+-|---|+ \ | | \ |/ C |/ C |/ C
(Current Level) | | | >---+---|Buf >----|------------|-------------|
+---/\/\---+----|-+---|- / | / |\ E |\ E |\ E
| | Rcl | | |/ Op-Amp |/ | | |
+----+ / / +-----+ +-----+ +-----+
| Current \ \ | | | | | |
| Limit / / / / / / / /
| \ \ Rs1 \ Re1 \ Rs2 \ Re2 \ Rsn \ Rsn \
| | | / / / / ..... / /
| DC- o---+----+-------------+ \ \ \ \ \ \
| V | | | | | | |
| +----------|-----+------|-----+-------|-----+
| Vcs = V(Current Sense) | | |
+----------------------------------------+------------+-------------+
This regulator also has an input for a light control feedback signal but additional circuitry would be needed to interface it to the typical laser head light preamp output. (If a sensed light intensity signal is used directly, it will only provide proportional control which is better than nothing but not the integral/differential loop response required for best performance and to suppress plasma oscillations. For that, the head preamp can be modified or an op-amp circuit similar to the one shown on the SG-IL1 - Control and Interlocks subsystem can be added in the signal path.)
It should be possible to simplify the regulator portion of the design even further for use as an ion tube testing power supply by just implementing the pass-bank and a massiveemitter follower. While this won't have the same tiffness as a differential amplifier based control circuit, it should be quite adequate for many purposes. This approach is shown in the schematic for the SG-IY1 - Power Unit. This requires no floating power supplies as the needed power is derived from DC+ and the voltage across the pass-bank itself.
If you have a bucket load of high voltage regulator ICs (e.g., LM317H), it should be possible (if not entirely practical) to use several (like 10 or 20!) in parallel on a BIG heat sink with current balancing resistors but no additional pass-bank transistors. Details on this as well as adding light feedback capability are left as an exercise for the student! :) (Hint: replace R4 with some form of the light feedback signal.)
One thought would be to use an opto-coupler. However, common opto-couplers are not linear devices so using one will result in a non-linear transfer function from the D/A or whatever and the output current. This probably doesn't matter - a lookup table can take care of it if there is minimal drift, but that isn't something that one can take for granted. One way to deal with drift is to use two identical opto-couplers, one in a feedback loop to linearize the overall transfer response.
Another way to provide isolation is to use a Pulse Width Modulated (PWM, digital) signal via an opto-coupler or transformer. A simple low pass filter and buffer amp will then produce an output which is proportional to pulse width. Generating a PWM signal is a simple matter of comparing a linear ramp with the input voltage - any of the PWM SMPS controller chips will do this easily. For example, (though it would appear to be unnecessary), the Lexel-88 ion laser power supply couples the light feedback signal to its linear pass-bank in this manner. See the chapter: Complete Ar/Kr Ion Laser Power Supply Schematics for the circuit details.
Rather than using the pass-bank essentially as a controlled variable resistor, it is switched on and off at at high frequency - up to several hundred kHz.
A multiple L-C smoothing filter following this chopper removes ripple from the resulting voltage to the tube. This is called a 'buck converter' because it can only reduce the available voltage.
You might ask: "Can large SCRs be used instead of transistors for the chopper since they may be cheaper?" Good question. However, the answer is: Not easily. The problem with SCRs is that you can't turn them off once they are triggered whereas transistors go on and off at will. Without being able to turn them off, you can't control the duty cycle and thus the power output. There are some power supplies that do use SCRs in the front end but they operate on the rectified AC - somewhat like a phase controlled light dimmer. The switch-on point on the AC cycle can be controlled in that case.
Forced-air cooling is still probably required but at least the jet-engine power fans are unnecessary (for the power supply - the laser head still requires a hurricane to survive)!
The block labeled 'CS' is the current sensor used to provide regulator feedback and is described in the section: Hall-effect current sensor (CS).
The chopper uses one or more power MOSFETs. Since high current devices of this type are readily available, a single part may be adequate for a 10 to 12 A regulator. However, they may easily be connected in parallel if needed. These devices have internal reverse protection (Dx) and input clamp diodes but additional protection is critical to prevent them from turning into short circuits at inconvenient times (including switching spikes, power on/off transients in addition to overload/fault conditions).
R1 limits peak current through the Qx bank into C2. Filtering is provided by C2 through C4 and the associated L1 and L2. Since a high switching frequency is used (e.g., 200 kHz), all of these components are quite small and compact (at least relative to those required for 60 Hz filtering!). However, the inductors need to pass the entire 10 A or more of tube current and the capacitors need to be able to handle the high frequency high ripple current. D1 is a 1000 V (typical) high current diode to isolate the igniter boost voltage from this circuitry.
Y1 +----+ Y2 :::: D1
DC+ o---+--------+---------------------+----| CS |-----^^^^---+---|>|---o To
| | R1 | +=---+ L2 | Anode
| +---/\/\---+ | |
| | | |
_|_ C1 C2 _|_ _|_ C3 _|_ C4
--- --- --- ---
| | | |
| +---|>|---+ | | |
| | | | :::: | |
DC- o---+-+--+-+--+ +-+--+-+---^^^^---+----------------------+
| | | | | L1 |
| S _|_ v _|_ D | |
| --------- | PH-H o---------+ T1 |
| Qx1 | | )|| +-|--o F1
| +-------|-----+ Filament )||( |
| | | Supply )|| +-+ Tube-
| +---|>|---+ | | )||(
| | | | | )|| +----o F2
+--+-+--+ +-+--+ | N o---------+
| | | | |
| S _|_ v _|_ D |
| --------- |
| Qxn | | o o
| +-------------+----||------+ T2 +--------o MD1
| D1 | C5 )::(
+--|>|--+ )::( PWM Drive
| )::(
+----------------------------------+ +--------o MD2
Isolation Transformer
Coupling of the pulse width drive signal to the Qx bank is done via the
transformer, T2 (most MOSFET gate protection and balancing components NOT
shown).
The Buffer converts the output signals to drive the isolation transformer, T2. Since the circuit is AC coupled, a stuck-at failure will result in the chopper being disabled rather than full-on.
Rt Ct Voltage
+--/\/\--+---||---+ Comparator Buffer
| | | Vosc
+---------------------+ |/|/|/ |\ |\ | |
| Sawtooth Oscillator |--------|- \ _|_|_ | \ _|_|_
+---------------------+ Verr | >------| A >--------o MD1
+---|+ / | /
+------+ | |/ |/ Chopper Drive
+---| F(s) |--+
| +------+ | +------o MD2
| |\ | _|_
Vcs (+) o-----/\/\---+----|- \ | -
| | >---+
Vcl (-) o---+-/\/\---+ +--|+ / Typical PWM Drive (Expanded)
| | Rcl _|_ |/ _ _
+---+ - Verr Low ____| |___________| |______
Current ______ ______
Limit Verr Med. ____| |______| |_
___________ ________
Verr High ____| |_|
Note that while the simplified diagram, above, shows a single op-amp (and
single control loop for current feedback only), actual implementation may have
several. Since current is the actual controlled variable, this will be the
'inner' or 'primary' loop which is active as long as the tube is on in Standby
mode. The 'outer' or 'secondary' loops are responsible for user adjustable
Current Control, Light Control, and external modulation inputs. The Vcs
(Current Sense) signal is proportional to the Ar/Kr ion tube current. Vcl
(Current Level) is a voltage (negative in this case) from the front panel tube
current pot. See the section: Multiple Loop
Controller Organization for further details.
Typical oscillator frequency is 200 kHz. To analyze this circuit precisely would require digital signal processing (DSP) techniques. However, where the loop response is limited (by the Control Amplifier feedback) to much less than the switching frequency, analog techniques can be used.
The core has a 1 turn winding for the full tube current and a 1000 turn winding (typical) for the feedback. The Hall Device (HD) is placed in a gap in the core so that it intercepts the magnetic flux. The idea is to null out the sum of the magnetic flux provided by the two windings to maintain tube current at the selected level.
+---/\/\-------------+
| |
Gapped Core | +-----|--+---/\/\---o +V
_____ _ _____ Op-Amp | HD1 | | |
| __||H||__ | /| | + +------+ | /
| | ^ | |o / +|---+---/\/\---| Hall | +->\ Offset
| | | ---------< | | Chip | /
o| | HD1 ----- \ -|-------/\/\---| |H| | \
Y1 o-------- ----- \| - +------+ |
Y2 o---| | 1T NT ----- | |
| | ----- Rcs +--------+---/\/\---o -V
| |________| |---+---/\/\---+
|______________| | _|_
| -
+---o Vcs to control amplifier
(+V and -V are the power supply voltages for the analog circuitry - see the
section: Low Voltage Power Supplies.)
The voltage, Vcs (Current Sense), is proportional to the current required to zero the magnetic flux and is thus proportional to the tube current which equals (N * V)/R7 (where N is the number of turns in the feedback winding). For example, with N = 1000 and Rcs = 100 ohms, the sensitivity is .1 V/A.
There are a variety of approaches one can take to handle these situations without a lot of smoke and flames. However, they all consist basically of three parts:
The smoothing inductor in the chopper will limit the rate of rise of current/voltage in a switchmode design. However, the transistors in a linear pass-bank are often not able to hold off the full rectified line voltage even for an instant. Thus, some means of limiting voltage across the pass-bank long enough for shutdown to take place is essential. Of course, this may mean that output current DOES climb above continuous safe limits for a short time.
Disabling drive to a switchmode chopper may be all that is needed if it is fast enough. However, for linear pass-bank, input voltage will have to be removed.
For example, the Omni-150R, a switcher, includes an overcurrent shutdown circuit which will shut off drive to its chopper MOSFETs, However, this may not be soon enough to prevent their failure and that of several other nearby components. The linear pass-bank in SG-IL1 is protected by limiting its maximum voltage to about 60 V. A separate overcurrent shutdown circuit should shut off main power - hopefully in time! But during a fault, lots of unexpected things can happen and as they say: "This best laid plans of mice and men....". :) In other words, the best advice even with fancy protection is to make every effort to avoid major faults! See the chapter: Complete Ar/Kr Ion Laser Power Supply Schematics for circuit details.
Inverter
Line Rectifier/Filter Transformer Tube Rectifier/Filter
D1 DC+ o D5 :::: D6 Anode
H o----+--|>|-----+-----+------+-----+ T1 +--|>|--+--^^^^--+----|>|----o
~| D2 | | | )::( | L1 |
+--|<|--+ | +_|_ _|_ 25T )::( 22T _|_+ _|_+
115 VAC D3 | | C1 --- C2 --- #12 )::( #12 --- C3 --- C4
+--|>|--|--+ - | | )::( | - | -
~| D4 | DC- | | )::( o | | R1 Tube-
N o----+--|<|--+--------+ +-----+ +-------+--------+--/\/\--+--o
| | |
Drive Transformer | | Vcs o
o T2 o | |/ C
X1 o-----+ +---|----| Q1 Both primary and secondary are wound
)::( | |\ E with Litz wire to minimize losses.
PWM Drive )::( | |
)::( | |
X2 o-----+ +---+------+ Drawn assuming a flyback converter.
The AC line front-end is similar to that described in the section: Single-Phase 115 VAC Line
Front-End. The filter capacitor, C1, should be selected to provide
acceptable ripple at full load but this is less stringent than for a linear
power supply or one without a regulator at all!
The chopper transistor, Q1, is a high power high voltage NPN power transistor. Snubber/protection components are not shown. The drive signal must be passed via an isolated interface since the emitter of the transistor is the line connected DC-.
The rectifier on the secondary side, D5, must be a fast recovery type suitable for the switching frequency used. The filter components can be relatively small.
D6 must pass the full tube current and is used to allow the Boost voltage to build up on the igniter circuit.
The PWM controller can use the same basic organization as that described in the section: Switchmode Regulator Controller. However, details will differ including the transfer function(s) of the feedback network(s). These details are, as usual, left as an exciting exercise for the student. :-)
A typical circuit (from a Sharp microwave oven) uses full wave rectified but mostly unfiltered pulsating DC as the power to a large ferrite inverter transformer which sort of looks like a flyback on steroids. This means that the microwave output is pulsing at both 60 Hz and the frequency of the inverter!
Bridge Rectifier Inverter Transformer Magnetron
o
H o----+---|>|------+--------+-------+ +--------------------------+
~| |+ _|_ Drive )::( Filament 1T #18 |
+---|<|---+ | --- 25T ):: +--------------+------+ |
115 VAC | | | #12 ):: HV Cap | +-|----|-+
+---|>|---|--+ +-------+ :: +-------||-----+ | |_ _| |
| | | ::( .018uF | | \/ |
N o----+---|<|---+ Drive |/ C ::( 2,400V __|__ | ___ |
~ |- o---| Chopper ::( HV _\_/_ +----|:--+
(Interlocks and | |\ E ::( 250T | HV |'-->
fuses/protectors | | ::( #26 Sense | diode | uWaves
not shown) +-----------+ +--+---/\/\----+---------+
o | 1.2 _|_
(Except for filament, # turns estimated) o H1 - Chassis Ground
The chopper transistor is marked: Mitsubishi, QM50HJ-H, 01AA2. It is a LARGE
NPN type on a LARGE heatsink. :-)
Note the similarity between the normal half wave doubler circuit and this output configuration! Base drive to the chopper transistor is provided by some relatively complex control circuitry using two additional sets of windings on the inverter transformer (not shown) for feedback and other functions in addition to current monitoring via the 'Sense' resistor in the transformer return.
It is not known whether power levels in this over were set by the normal long cycle pulse width modulation or by control over a much shorter time scale. However, since the filament of the magnetron is powered from the same transformer as the HV - just as in a 'normal' microwave oven, this may not be very effective.
Compared to the simplicity of the common half wave doubler, it isn't at all surprising why these never caught on (what is diagramed above includes perhaps 1/10th the actual number of components in a typical inverter module). Except for obvious problems like a tired fuse, component level troubleshooting and repair would be too time consuming. Furthermore, as with a switchmode power supply (which is what these really are) there could be multiple faults which would result in immediate failure or long term reliability problems if all bad parts were not located. Schematics are not likely available either. And, a replacement module would likely cost as much as a new oven!
This is simply a situation where a high tech solution was doomed from the start. The high frequency inverter approach would not seem to provide any important benefits in terms of functionality or efficiency yet created many more possibly opportunities for failure. The one major advantage - reduced weight - is irrelevant in a microwave oven. Perhaps, this was yet another situation where the Marketing department needed something new and improved!
+---/\/\---+ +---o/ o---o -V
S3 | |\ | | S4 Standby
Vmi o--o/ o---/\/\---+--|- \ | /
Modulation | -1 >--+--------/\/\---+--/\/\-->\ Rsb
Enable +--|+ / | / Standby
| |/ IC1A | \ Adjust
V | |
Vcs o--+-----------------------------/\/\---+ V
| S1 CC Disable | Rdc
| +-----o/ o---+ | +----/\/\----+
| | +------+ | | | +------+ |
| +--| C(s) |--+ +---+-+--| P(s) |--+
| | +------+ | | | +------+ |
Vls | | |\ IC1B | / | |\ IC1C |
o Vcl o--|--/\/\--+---|- \ | D1 Verr \ +-----|- \ |
| | | >---+--|<|--+--+ / | | >---+
/ +------------|+ / | | \ | +--|+ / |
\ |/ | / | | | |/ o
/ S2 LC Disable | \<--+ / V Verr (to
\ Vll o----/\/\---+------o/ o--+--|<|--+ / Rcl \ regulator)
| | +------+ | D2 \ Cur. /
+---/\/\---+ +--| L(s) |--+ | Lim. \ Rng
| |\ IC1D | | +------+ | V | Noise Gain
+--|- \ | | |\ IC2A | +---------/\/\----+
| -1 >--+--/\/\---+---|- \ | C1 | D3 |
+--|+ / | >---+--||--+--+--|>|--+ V
| |/ +--|+ / | D4 |
V | |/ +--|<|--+
V |
V
The Primary (inner) loop feedback network (IC1C) consists of Rdc to provides a
DC set-point (proportional) for the tube current based on the Vcs (current
Sense) feedback signal and P(s) which is in the form of one or more series R-C
networks in parallel with Rdc to control loop frequency response. This
results in a proportional-integral loop response.
The Secondary (outer) loop feedback networks, C(s) (Current, IC1B) and L(s) (Light, IC2A), typically consist of one or more series R-C networks in parallel to produce an integral response characteristic. Stability must be assured for any combination of Current and Light front panel control settings. (With some designs, it is possible to destabilize the loop by turning up the Noise Gain pot (Rng) too high.) In the circuit, above, D3 and D4 are included to clip the AC component of the light signal to prevent this from happening.
Possible modes are as follows:
The Noise Gain pot (Rng) adjusts the extent to which this AC signal is used in reducing optical noise while D3 and D4 clip the noise signal to prevent loop instability.
+--+ Sensitivity Adjust
| | Rsa
+-------------/\/\-+------------+
| |
| C1 +V |
+-----||-----+ o |
| | | |
| |\ | | |
+------+---|- \ | Q1 |/ C |
| | >---+----/\/\----| NPN |
_|_ +---|+ / |\ E |
--> /_\ | |/ Op-Amp | |
SC1 | | +---+-+---o Vls (Light Sense)
| | | | to control amplifier
+------+ | / light control input
Solar Cell | / \
| \ /
| / \
| \ |
| | +---o +
| | Laser Power Test Point
+---------------------------+---------o -
V
The Solar Cell, SC1, generates a current which is proportional to incident
light from a portion of the laser beam. The Op-Amp, IC1, converts this to
a voltage which is buffered by Q1. C1 limits the frequency response to
assure loop stability. The Op-Amp's output is applied to the control
amplifier to maintain beam power stable based on its actual intensity.
A PIN photodiode could be used instead, appropriately biased to inject current into the amplifier input.
A high frequency signal (AC coupled) may also be derived from the light sensor to be used for noise reduction (see the section: Multiple Loop Controller Organization).
Trip Set
Rts
+----/\/\----/\/\---o +Vcc
| | |\ Comparator
V +-----------------------|- \
| >---+------o OVERCUR
Vcs o----/\/\---+------+---|>|---+------+---+---|+ / |
| | D1 | | | |/ |
/ | | / | |
\ _|_ _|_ \ +----/\/\----+
/ --- --- / |
\ | | \ |- Reset
| | | | |
Com o-----------+------+----+----+------+---+
|
V
The first part of the circuit is used to generate an approximately 400 VDC 'boost' source from the AC line. This is a classic voltage multiplier. The Boost output is used to charge the energy storage capacitor (C6) for the pulse circuit and power the relaxation oscillator that triggers it repeatedly until the Ar/Kr ion tube starts. The supply voltage to the relaxation oscillator (across D6) is then automatically removed and triggering ceases.
See the chapter: HeNe Laser Power Supply Design for descriptions of HeNe pulse starters and operation of a voltage multiplier.
The primary reason to use the boost voltage rather than the 150 VDC available from the line rectifier/filter at DC+ is to dump some additional energy into the tube at the instant of startup (from C8) to aid in transition from a glow discharge to the high current arc required during normal operation.
Another reason for using the higher boost voltage is to reduce the number of fat wire turns on the pulse transformer toroid. Increasing the voltage on the energy storage capacitor from 150 to 400 V reduces the turns ratio by better than 2.5:1 requiring only 30 instead of 80 turns on the secondary. Once you have wound one such transformer, you will appreciate this savings!
I've used the ferrite core of a deceased flyback transformer for T1 with a 2 turn primary and 30 turn secondary. This worked fine for my home-built Cyonics tube based laser head. The large ferrite cores from PC (or other) switchmode power supplies should be fine as well. Make sure the secondary can handle the full ion tube current and is adequately insulated for the several kV or more that is produced.
The bypass capacitors, C9 and C10, complete the return circuit from the bottom of the secondary of the igniter transformer to F1 (of the ion tube filament/cathode). Without these, in addition to interfering with starting, this pulse could find its way back into the power supply itself resulting in cascade failures of regulator or other components.
- C1 + - C2 +
H o----------------)|----+--------)|---------+ C1-C4: 10uF, 350V
(OP) D1 | D2 D3 | D4
+---|>|---+---|>|---+---|>|---+---|>|---+
R1 | - C3 + | - C4 + |
N o----/\/\----+---+----)|----+----+---+----)|----+----+---+---o Boost
| R2 | | R3 | | (>400 V)
+---/\/\---+ +---/\/\---+ |
|
+--------------------------+-------------------------+
| |
/ / Igniter pulse transformer
R4 \ R8 \ Typical stepup ratio 20:1
100K / 100K / o
\ \ T1 +-----+--o HV+
| | ::( |
+--------------------------|---------------+ ::( |
| | C6 1uF | ::( 30T _|_ C7
R5 / +---||----+------------+ ::( #14 --- 500pF
8M \ DL1 SCR1 __|__ | | 1.5T )::( |
/ NE2H 600V _\_/_ / | #14 )::( |
| +--+ 25A / | R9 \ | +--+ +-----+
+--------+---|oo|---+---' | .1 / | | o |
| | +--+ | | \ | | |
/ | / | | +---|-------------+
R6 \ C5 _|_ R7 \ | D5 __|__ | | |
3M / .1uF --- 180 / | MR826 _\_/_ _|_+ | D6 _|_
\ 250V | \ | | --- | 1N1190AR /_\
| | | | | | | 600V,40A |
+--------+--------+-+------+-+-------+-----+---+-------------+
| | C8 10uF |
C9 _|_+ C10 _|_ 600V |
10uF --- .1uF --- R10 .1 50W |
450V | 500V | DC+ o---+---/\/\---+--+
F1 o----------+----------+ | |
o + Test - o
.1 V/A
This igniter is similar to the one used in the Omnichrome 150R power supply
and 532 laser head (see the chapter: Complete
Ar/Kr Ion Laser Power Supply Schematics for details) and is intended to be
placed in the high-side (anode circuit). However, some designs may put the
igniter in the low-side (cathode circuit) instead.
An ALC-60X/Omni-532 or other large tube (e.g., a Lexel-88) needs considerable energy to form the cathode spot. And, over time as the pressure goes down it WILL need the high power resonant ignition approach described in the section: Ar/Kr ion tube pulse type igniter.
However, a small, modern, tube like the Cyonics starts rather easily with its short bore and oversized cathode. Therefore, it may be possible to use a simpler approach for its igniter using a low current high voltage (say 2 kV) supply feeding onto the anode side of a HV bypass diode as shown below:
R1 R2
+2 kVDC o----/\/\------+--------/\/\-------+----------+
100K | 100 | |Tube+
C1 _|_+ 10W | .-|-.
1uF --- | | | |
3kV | - | | |
| D1 | | | LT1
DC+ o--------------|--------|>|--------+ | |
| 3kV | |
| 20A ||Z.|
| '+-+'
DC RET o--------------+------------------------+ F1| |F2
| | |
AC o--------+ T1 | | |
)|| +--------|----+ |
Filament )||( Tube- | |
Supply )|| +--------+ |
)||( |
)|| +---------------+
AC o--------+
R1 limits current from the HV supply while R2 limits current from C1 at the
instant the tube starts. D1 allows the HV to build up across the tube. Of
course, high current diodes with 3 kV ratings aren't cheap either! But, see
the section: Construction of HV High Current
Blocking Diodes.
I have not tested this circuit but promise to do so in the future. While this general approach works well for starting HeNe tubes, they are not quite the same animal!
The ALC-60X/Omni-532 design does use a diode switch like this for the boost voltage but I have only seen xenon arcs using it for starting HV. I suspect there is a impedance problem - i.e., you get a glow that doesn't progress to a arc and the cathode spot doesn't form. Arc lamp supplies that use this method often overcharge the caps in the power supply to insure they start as well as use a trickle supply of about 1 kV at several mA while running to keep things ignited.
However, for an older or larger tube, you really have to hit it with the pulse igniter.
Also see the section: Pulsed Operation of an Ar/Kr Ion Tube since the approaches described there may be useful for igniters as well.
To balance the current feed through the Ar/Kr ion tube cathode, the negative from the power supply is applied to the center tap of this winding. Since the low voltage secondary has only a few turns of fat wire and is an outer winding if not on its own bobbin, it is a simple matter to add a centertap if you are modifying an existing transformer and one doesn't already exist.
Since this voltage must be relatively accurate, a Variac or some other means of adjusting it needs to be provided. Commercial designs typically provide multiple taps on the transformer primary to set up the proper current. With a semi-homemade secondary, partially unwinding one turn may be all you need to tweak the current.
Note that while some commercial ion laser power supplies claim to use DC for the filament to reduce ripple and noise in the laser output (Melles Griot 176B, for example), this is not recommended even if you have a suitable low voltage high current DC power supply available:
(From Steve Roberts (osteven@akrobiz.com).)
DC requires some changes to the cathode-to-bore spacing (longer), or a slowly wandering DC offset from inside the switching PSU, or you end up with a hot spot. The only DC cathode tubes I know of go into very precise semiconductor wafer measurement stuff. I got a call from one of the major tube makers about two years ago asking for advice, it seems their main plasma guy had left and they wanted to find another way after X heads on DC test killed their cathodes, periodic polarity flipping becomes a option too. For long Life, AC rules.
The only major consideration is that one or more supplies of this type may be needed that are electrically floating to power the regulator controller and/or light feedback circuitry if they are direct coupled to the regulator (which is on the line connected DC+ or DC- feed). However, this is easily solved since any decent power transformer will be rated for at least 2,500 V isolation.
28VCT,1A
H o--+ T1
)|| D1 V+ In +------+ Out
)|| +--+--|>|-----+--------------+----| 7815 |---------+----o +15 VDC
)||( ~| D2 | C1 +_|_ +------+ C3 +_|_
)||( +--|<|--+ | 10,000uF --- Com | 10uF ---
)||( L1 | | 25V - | | 25V - |
115 VAC )|| +----------------------------+--------+------------+--+-o Analog
)||( L2 D3 | | C2 +_|_ | C4 +_|_ V Common
)||( +--|>|--|--+ 5,000uF --- Com | 10uF ---
)||( ~| D4 | V- 25V - | +------+ 25V - |
)|| +--+--|<|--+-----------------+----| 7915 |---------+---o -15 VDC
)|| In +------+ Out
N o--+ D1-D4: 1N4007 or 2 A bridge
Note: Pinouts for 78 and 79 series parts are NOT the same!
And, for the logic supply (if needed) AND with its common at the same potential (floating or grounded):
R1 In +------+ Out
V+ o-----/\/\-------+----| 7805 |---------+-----o +5 VDC (Vcc)
C5 +_|_ +------+ C6 +_|_
1,000uF --- Com | 10uF ---
25V - | | 16V - | X
+--------+------------+--+--o Digital Common
_|_
-
Single point connection between analog and digital commons is made at point X.
In addition, place .1 uF ceramic capacitors across each of the electrolytics to bypass high frequency noise.
WARNING: If these are floating - not at earth ground - there will be blown parts and vaporized wiring if connected there!
Depending on the current requirements, the regulator ICs will likely need to be mounted on heat sinks (isolated from each other using mica spacers and silicone heat sink compound if on the same one).
The circuit as shown above is rated at about .5 A for each of the 15 V outputs and an additional .5 A for the +5 V output (using a nice heat sink for the IC regulators!). R1 should be selected to leave about 2 or 3 V of headroom at maximum logic current to reduce power dissipation in the 7805 chip.
An isolated 60 Hz clock can be easily extracted from the secondary winding of any of these low voltage power transformers:
R2 R3
L1 o----/\/\-----+-------+ +----+--/\/\-----o +5 (Vcc)
1K | | | | 1K
D6 _|_ __|__ |/ C +-----------o CK60-P
1N4002 /_\ _\_/_-->|
| | |\ E
| | |
+-------+ OC1 +----------------o Common
| 4N35
V
The opto-isolator, OC1, can be either a photodiode or phototransistor (shown)
type although the value of R1 may need to be adjusted based on this and the
transformer's output voltage. Using a bridge or full wave rectifier in series
with OC1 instead of D1 across it will result in a 120 Hz clock. However, the
lower frequency clock is probably better for the most likely use - a timer for
the filament preheat delay.
A simpler circuit can be used where isolation isn't needed:
R5
+----/\/\-----o Vcc
| 1K
+-------------o CK60-P
|
R4 D7 D8 |/
L1 o----/\/\----|>|------|>|----| Q1 2N3904
10K 1N4148 1N4148 |\
_|_
-
The schematic in the U.S. Patent #4,504,951: High Speed SMPS for a Light Controlled Laser System (ALC), is a reasonably simple design. (The U.S. Patent & Trademark Office currently has a search facility with free access to the text and graphics.) Actual production units have about twice the parts so as to get a reprographic or instrument grade beam. See the chapter: Complete Ar/Kr Ion Laser Power Supply Schematics.
For a light show, an ion laser power supply just has to do the following (assuming an ALC-60X/Omni-532 class tube - others will use different values):
The fan power should be maintained until the relevant assembly's temperature drops to guaranteed safe value.
Cheaters can be provided for testing - but only for testing!
Head Fan or Cover Head Limit
+-+ +-+
+---------|I|---------+ +---|I|---+
| +-+ | | +-+ |
| P1 S1 | | TP1 |
Interlock o---+-<<--->>------_|_----+--+---_|_---+---+
(to relay Part of LH |
or logic) fan plug PS Cover PS Limit |
+-+ +-+ |
+---|O|----+ +---|I|---+ |
Keyswitch | +-+ | | +-+ | |
S2 | S3 | | TP2 | |
Return o-----o/ o-----+---_|_----+--+---_|_---+---+
If ANY switch or interlock opens during operation or is open before powering
up, power to the Ar/Kr ion tube should be shut off or be prevented from
coming on. Control power should remain active and the relevant interlock
status indicators (if present) should light up.
While everything below isn't essential for a bare-bones hobbyist supply, these features will add a professional touch to your system! The functions that need to be provided are as follows (these are all outputs or states):
Function Description Type Conditions/equations
----------------------------------------------------------------------------
MAIN Main Power Breaker Manual, trip on system overload.
IDLE System idle FF State entered when system powered up or
RESET PB pressed.
PREHEAT Filament Pwr. SW or FF Manual, reset by abort.
Precharge Initiates 30 to 60 second delay before
enabling SB or OP modes.
FILHOT Filament Hot FF Goes active after Preheat delay as long
Preheat still enabled and Safe.
SBREQ Standby Req. SW or FF User activated to request Standby mode.
SBMODE Standby Mode FF Enabled after Preheat delay if SBREQ is
set and all interlocks and protectors
are 'go' (closed).
SBMODE = SBREQ * FILHOT * SAFE * ~ABORT
OPREQ Operate Req. SW or FF User activated to enter Operate mode.
OPMODE Operate Mode FF Enabled after Preheat delay if OPREQ is
set and all interlocks and protectors
are 'go' (closed).
OPMODE = OPREQ * FILHOT * SAFE * ~ABORT
DCPWR Tube on Relay SBMODE + OPMODE
LH Fan Tube cooling Inter/Pro SBMODE + OPMODE + LHWARM
PS Fan PS cooling Inter/Pro SBMODE + OPMODE + PSWARM
SAFE System safe SWs, Prots ~(LHHLIM + PSHLIM + PSOPEN + LHOPEN)
FAULT OVERCUR Inter/Pro Disables everything except main power
if: OVERCUR.
OVERCUR Over current I-Sense Pulse to kill power on extended excess
tube current.
Switch Indicator
-----------------------------------------------------------------------------
Main Breaker Line live/control power on (neon)
Key Switch
System idle - powerup or reset (green)
Preheat PB SW Blink while heating, solid when hot (green)
Standby PB SW Blink until tube starts, solid when running (green)
Operate PB SW Blink until tube starts, solid when running (green)
Fault Reset PB SW Fault on until reset (red)
Meter select I/V
All except the main breaker may be pushbuttons to set appropriate flip flops
or toggle switches if less sophistication is good enough!
For a fabulous ASCII rendition of a possible front panel layout, check out the one I intend to use for "Sam's linear Ar/Kr laser power supply (SG-IL1). See the section: SG-IL1 front panel layout.
Switch Description Indicator
------------------------------------------------------------------------
PS Interlock Cover missing PS Unsafe (red)
PS High Temp Heat sink > 180 degrees F PS High Temp (red)
Head Interlock Fan or cover missing LH Unsafe (red)
Head High Temp Tube > 180 degrees F LH High Temp (red)
PS Warm Interior > 120 degrees F PS Warm (yellow)
Head Warm Interior > 120 degrees F LH Warm (yellow)
Note: some tubes may be happier if their cooling fan is shut off at the same
time as power or shortly thereafter rather than waiting for full cool-down.
Thus the 'Head Warm' switch (thermostat) may not be needed (or can provide
reduced airflow after the tube has been shut off).
(From: Steve Roberts (osteven@akrobiz.com).)
The 60X generally will withstand a sudden shutdown, in fact rapid cool-down with the fan on is more of a lifetime shortener. This is not true of all air-cooled lasers, some of which store too much heat and need the fan. I generally just shut everything down at once with no problems in years of operation. More modern supplies run the fan about 1 minute after shutdown.
For the power handling components, derate resistors, capacitors, and semiconductors by 30 to 50 percent. For example, where the main bridge needs to be rated for 10 A, use a 20 to 25 A device - the increase in cost will be minimal and well worth it. Electrolytic capacitors should be 200 V minimum, 250 desirable for the line filter. Be particularly conservative with the regulator power transistor ratings. Figure on at least 5, 150 W power transistors in parallel for a linear pass-bank handling 10 A even though the individual devices may be rated for 15 A.
For the logic and analog circuits, use high quality name brand components (no unmarked op-amps you happened to have in the junk bin!). Make sure you follow the recommended practices of providing bypass capacitors where needed (and on all logic devices), tie unused inputs to a legal state, etc. Separate digital and analog grounds except at a single point. Route digital and analog wiring separately.
For the power supply itself:
CAUTION: Keep in mind that portions of this circuitry will very likely need to be floated electrically since it will be line connected (via DC+ or DC-). Therefore, make sure any copper cladding is peeled away from screw locations and/or non-conductive standoffs are used for mounting.
Incandescent light bulbs are the usual low cost solution for troubleshooting of electronic equipment to provide current limiting. However, the resistance of their filament can vary by a factor of 10 from cold to hot - not good for our purposes. However, there is a nearly as readily available alternative:
The elements from space heaters, electric dryers, and other heating appliances with exposed coils are ideal for the fabrication of home-brew but perfectly usable power resistors. These are made of NiChrome, an alloy of nickel and chromium which is resistant to oxidation even when yellow/orange-hot and its resistance is relatively stable with respect to temperature (which cannot be said for tungsten light bulbs).
The original mounting can be used or portions of the element(s) can be transferred to a suitable non-conductive and NON-FLAMMABLE support. Allowing the wire to come in contact with this material at as few locations as possible will minimize heat transfer and make the most effective use of air cooling.
Taps can be provided for easy adjustment. Initially, these can make use of heavy duty crocodile clips later made permanent with nuts and bolts or crimp connections (soldering won't be reliable, surprise, surprise!).
For current capability approaching the original application, paralleling multiple heating element wires will permit them to run cooler.
Sealed heating elements of the types used in electric stove tops, ovens, broilers, electric hot water heaters, etc., may also be used but there is no way to adjust their value except by switching in various series and parallel combinations. However, this is convenient with dual-element stove top 'burners' with pushbutton selector switches (not thermostats or 'infinite' controls - your 1950s vintage GE range probably has what is needed!).
CAUTION: Any element designed for direct immersion in water or another liquid may burn out even at far less than its rated current if run in air despite a cyclone of cooling!)
For example, for a 10 A power supply, use 2 series strings of four 1000 V, 6 A diodes with a .05 ohm, 5 W resistor in each string as shown below:
Anode o----+----|>|--|>|--|>|--|>|---/\/\----+
| |
+----|>|--|>|--|>|--|>|---/\/\----+----o Cathode
The resistors can even be made from suitable lengths of wire. For example, #24
copper wire has a resistance of about .025 ohms per foot.
You can also make your own solid state relay. See the section: Driving Relays with AC Coils.
Since solid state relays are so easy to use, no further discussion of this type is needed!
o--o NC
C o--o/ (typical)
o--o NO
AC line Hot o------------+
)||
Relay with )||
115 VAC coil )||
500 )||
Vcc o---/\/\-----+ +----------+
| |
__|__ __|__
_\_/_-->_\/\_
1/6 7404 | |
|\ | |
ON-H o---| >o-------+ +--------------o AC line Neutral
|/ MOC3033
Optoisolated Triac
(Zero-crossing)
CAUTION: Snubber components (a series RC network, not shown) may be required
across the triac to limit voltage spike amplitude when the device switches
off if a non-zero-crossing type device like the MOC3012 is used.
As always, when driving an DC inductive load, a back biased 'free wheeling' diode is placed across the coil to provide a path for the coil current to continue flowing (and ramp down to zero relatively slowly) when the driver switches off. This prevents any inductive voltage spike which would result in stress on the driver and/or logic circuits and may generate excessive electrical noise.
The following are several alternatives for driving these relays (12 VDC, 10 mA coil assumed; typical contact configuration shown):
o--o NC o--o NC o--o NC
C o--o/ C o--o/ C o--o/
o--o NO o--o NO o--o NO
V+ o---+---+ V+ o---+---+ V+ o---+---+
_|_ )|| _|_ )|| _|_ )||
1N4002 /_\ )|| 1N4002 /_\ )|| 1N4002 /_\ )||
| )|| | )|| | )||
1/6 7406 +---+ +---+ +---+
|\ | | D |
ON-H o---| >o---+ R1 |/ C .|---+
|/ ON-H o---/\/\---| 2N2222 G||<--. 2N7000
1K |\ E ON-H o----'|---+
TTL Open Collector Driver _|_ S _|_
(HV output if V+ > +5 VDC) - -
Bipolar Transistor MOSFET
Depending on specific coil voltage and current, devices other than those shown
may have to be used as the drivers.
Or, to control a low current DC relay on a separate power supply:
o--o NC
C o--o/
o--o NO
V+ o----+---+
_|_ )||
1N4002 /_\ )|| Relay coil
500 | )||
Vcc o----/\/\----+ +-----+---+---+
1| | |5
__|__ |/ C |
_\_/_ -> | |
| |\ E |
|\ 2| | |/ C
ON-H o---| >o----+ +---|
|/ |\ E
4N33 |4
Opto-Darlington +----o V-
For the 4N33, the current transfer ratio is 500 percent so to drive a relay
with a 20 mA coil requires a minimum of 4 mA through the LED. Add another
buffer transistor to drive a higher current relay coil.
In the good old days, everyone used neon indicators. The problem with neon indicators aside from the fact that they invariable start to flicker after a few years of constant operation (which really shouldn't be a problem here) is that you have any choice of color as long as it is orange. :-) However, these really are the cheapest and easiest solution for putting an indicator on a power line:
IL1
R1 +--+
115 VAC or DC o-----/\/\-----|oo|-----o Return
47K +--+
NE2H
Neons come bare (you add resistor), as part of fancy (and expensive) indicator
assemblies - and everything in between.
To use an LED on a 115 VAC circuit, it is better to use a capacitor to limit the current than a resistor as power dissipation is greatly reduced. The following circuit will work with minor modifications in component values for most LEDs (4 or 5 mA assumed as drawn):
C1 R1 IL1
115 VAC o------||------/\/\-----+---|>|---+------o AC Return
.2uF 3K | LED |
250V | D1 |
+---|<|---+
1N4002
D1 bypasses reverse current and R1 is for surge limiting to prevent any
possible harm to the poor little low voltage LED if you apply power when the
AC input is near its peak.
Using a small bridge rectifier (almost any voltage rating) would double the brightness for the same value of C1 (assuming the LED can handle the current) but this hardly seems worth the effort for a simple indicator!
D1
C1 R1 +--------+ IL1
115 VAC o-------||---------/\/\----|~ +|-------|>|----+
.1uF, 250 V 3K | Little | LED |
| Bridge | |
AC Return o--------------------------|~ -|--------------+
+--------+
Note: For both these circuits, a high value resistor - say 1 to 10 M ohm - is
recommended across C1 or the input (not shown). This will quickly discharge C1
when power is removed. The energy C1 can hold isn't going to kill you, but
all those little shocks can add up to a lot of @#$% words!
Even with several (e.g., up to 4) of these devices in series), enough current should flow with any combination of them open to light their respective LEDs at nearly the same brightness. However, maximum bypass current will well below the minimum needed to activate the controlled relay.
+-+
On the interlock wiring diagrams, this type of indicator is shown as: --|I|--
+-+
C1.1 uF
| 250 V
+----<<----||-----+-------+ +-------+-----------------o +5 (Vcc)
| | | | |
| D1 _|_ __|__ |/ C |
PR1 \ 1N4002 /_\ _\_/_-->| | Isolated
| | | |\ E | Low Voltage
| R1 | | | |/ C Power
+----<<---/\/\----+-------+ OC1 +-----| Q1 2N3904
| 5K 4N35 |\ E
| R2 IL1
PR1: NC Interlock Phototransistor +---/\/\----|>|----o Common
or Protector Opto-isolator 100 Red LED
Note: Check the specs for the coil of the relay you intend to use. If its holding or activation current is close to 5 mA or less, this circuit will need to be modified. Or, provide a load resistor from V+ (relay power) to the top of the interlock chain to increase the available current (bypassing the relay coil).
IL1
+-------|>|--------+
| Red LED |
(+) | | R1 |/ C
+----<<---------+----/\/\----+---| Q1 2N3904
| 10K | |\ E
| | |
| D1 D2 | /
PR1 \ +---|<|-----|<|---+ \ R2
| | 1N4148 1N4148 / 140
| | \
| | |
+----<<----+-----------------------+
(-) |
PR1: NC Interlock or Protector
Special LEDs with constant current drive circuitry built-in may also be
available and may be used instead if their maximum voltage ratings are
greater than your worst-case relay power supply voltage.
5K, 10 W D1 D2 D3 D4
+ o-------/\/\----+----|>|----|>|----|>|----|>|----+------o -
| |
| 100 LED |
+--------/\/\---------|>|--------+
D1 to D4 can be any general purpose diodes (e.g., 1N4002s).
For permanent installation, a yellow LED is probably best meaning: CAUTION.
An alternative to this circuit which would have a quainter look is to use a 4 W night light bulb in series with a 1K, 2 W resistor (since the bulbs are designed for 115 VAC - not 150 VDC!