Contents:
Capacitor Testing, Safe Discharging and Other Related Information
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This document describes techniques for the testing of capacitors using a multimeter without a capacitance test mode. Information on safe discharging of high value or high voltage capacitors and a discharge circuit with visual indication of charge and polarity is also included.
WARNING: make sure the capacitor is discharged! This is both for your safety and the continued health of your multimeter. A pair of 1N400x diodes in parallel with opposite polarities may help protect the circuitry of a DMM. Since a DMM doesn't supply more than .6 V generally on ohms ranges, the diodes will not affect the readings but will conduct should you accidentally put the meter across a charged cap or power supply output. They won't do much with a charged 10 F capacitor or high current supply where you forgot to pull the plug but may save your DMM's LSI chip with more modest goof-ups. This approach cannot be used with a typical analog VOM because they usually supply too much voltage on the ohms ranges. However, my 20 year old analog VOM has something like this across the meter movement itself which has saved it more than once.
Some DMMs have modes for capacitor testing. These work fairly well to determine approximate uF rating. However, for most applications, they do not test at anywhere near the normal working voltage or test for leakage. However, a VOM or DMM without capacitance ranges can make certain types of tests. For small caps (like .01 uf or less), about all you can really test is for shorts or leakage. (However, on an analog multimeter on the high ohms scale you may see a momentary deflection when you touch the probes to the capacitor or reverse them. A DMM may not provide any indication at all.) Any capacitor that measures a few ohms or less is bad. Most should test infinite even on the highest resistance range. For electrolytics in the uF range or above, you should be able to see the cap charge when you use a high ohms scale with the proper polarity - the resistance will increase until it goes to (nearly) infinity. If the capacitor is shorted, then it will never charge. If it is open, the resistance will be infinite immediately and won't change. If the polarity of the probes is reversed, it will not charge properly either - determine the polarity of your meter and mark it - they are not all the same. Red is usually **negative** with VOMs, for example. Confirm with a marked diode - a low reading across a good diode (VOM on ohms or DMM on diode test) indicates that the positive lead is on the anode (triangle) and negative lead is on the cathode (bar). If the resistance never goes very high, the capacitor is leaky. The best way to really test a capacitor is to substitute a known good one. A VOM or DMM will not test the cap under normal operating conditions or at its full rated voltage. However, it is a quick way of finding major faults. A simple way of determining the capacitance fairly accurately is to build a 555 oscillator. Substitute the cap in the circuit and then calculate the C value from the frequency. With a few resistor values, this will work over quite a wide range. Alternatively, using a DC power supply and series resistor, capacitance can be calculated by measuring the rise time to 63% of the power supply voltage from T=RC or C=T/R.
(This section from: Raymond Carlsen (rrcc@u.washington.edu) The best technique depends on what the cap is used for. A lot of electrolytics are said to be "leaky" when they are really partially open and just not doing their job. Electrolytics that are actually electrically leaky are not as common. You can take each capacitor out of circuit and test it with a cap checker or even a VOM, but in-circuit testing is faster. I don't like to grab for a soldering iron unless I'm pretty sure the part is bad. Time is money. I first do a visual inspection and see if any electrolytics are bulging (they -are- leaky and usually get hot), or physically leaking (corrosion around terminals). Bulging caps in a switching power supply are a dead giveaway, but can point to leaky diodes as well. Next, if the unit will power up, I look for signs of filter caps open... hum bars in picture, hum in audio, flickering displays, low B+ but nothing gets hot, etc. You can tell quite a lot by just being observent and a makling a few simple checks. Try all controls and switches... you may get other clues. What works and what doesn't? If you have an obvious fault... like a reduced vertical scan on a TV set or monitor for example, to find the cap that is starting to open up, you can bridge each of them with another cap, one at a time and see if it corrects the problem. (Experience has taught me that bad electrolytics will not -usually- kill vertical sweep completely.) In a TV set that is several years old or more, there could be more than one cap dried out (open). Check them all. "Popping" filters (as it used to be called) by bridging the original with a like value is not good practice with solid state electronics. The shock to a live circuit is likely to damage other components, or it could shock the circuit into working again... for awhile. Then you get to sit there like a fool and wait for it to act up again... minutes or weeks later. For small electrolytics, I use a trick of bypassing each one with a small 0.1 to 0.47uF capacitor while the set is running. If I see -any- change in the performance, I KNOW the original is not doing its job (greatly reduced in value or open). Of course if you hit the timing caps, it will upset the vertical oscillator a bit... that's normal. For bigger electrolytics like the one used to feed the yoke or power supply main filters, the only effective way to check them is by substitution with the same or larger capacitance. Turn the set off, connect the new cap into the circuit and power it up again. As I stated before, leaky caps are actually quite rare... but it does happen. They usually upset a circuit a lot more than open ones. Things tend to get hot quickly if the cap is a filter in a power supply. Shorted tantalums and electrolytics in power supplies can literally explode. Obviously, leaky caps must be removed from the circuit to substitute them for test purposes. Most of the other types of small capacitors: mylar, disc ceramic, etc. are pretty rugged. It is rare indeed to find them bad. It happens just often enough to keep a tech humble.
(From: Gary Collins (collgra@preferred.com)). All an ohm-meter tells you is if the cap is shorted or not if it is an electrolytic of fairly large value it can tell you if a cap is open. I am a tech in a large industrial controls company in the factory service center. We consider any electrolytic cap to be suspect if it's code date is over five years old. We have a Fluke 97 and it is useless for in circuit tests. All a meter like a Fluke 97 can tell you is if the Cap is on the way to being open from electrolyte loss or if it is shorted. Actually not all you need to know. Several other facts you need to know are what is the conductance (internal leakage resistance), it sometimes varies with voltage. You also need to know what a caps power factor is in some cases. That is its ability to pass A.C. This is especially important in computer equipment that has to pass harmonics and noise to ground. Switching power supplies like are found in almost all PC's these days use high frequency voltage converters to regulate voltage. The harmonics and noise produced by this rapid switching heats DC filter caps and causes them to loose moisture from their imperfect seals. This effect causes the capacitor to gradually open or drop in capacitive value. If you are talking about other types of capacitors you can test their value with a meter but I have seen caps that look good with a meter but break down under voltage. Special cap meters exist that test all these parameters and let you judge whether the cap is good or not but the best test short of that is to replace the cap and see if it works or not. Feel free to ask if that isn't what you wanted to know. Actually sometimes the best test is to use a oscilloscope to look at what the cap is doing in the circuit.
Simple capacitance scales on DMMs just measure the capacitance in uF and do not test for leakage, ESR (Equivalent Series Resistance), or breakdown voltage. If the measurement comes up within a reasonable percentage of the marked value (some capacitors have tolerances that may be as much as +100%/-20% or more), then in many cases, this is all you need to know. However, leakage and ESR frequently change on electrolytics as they age and dry out. Many capacitance meters don't test anything else but are probably more accurate than a cheap DMM for this purpose. A meter of this type will not guarantee that your capacitor meets all specifications but if it tests bad - very low - the capacitor is bad. This assumes that the test was made with the capacitor removed (at least one lead from the circuit - otherwise other components in parallel can affect the readings. To more completely characterize a capacitor, you need to test capacitance, leakage, ESR, and breakdown voltage. Other parameters like inductance aren't likely to change on you. ESR testers, which are for good for quick troubleshooting, are designed to just measure the Equivalent Series Resistance since this is an excellent indicator of the health of an electrolytic capacitor. Some provide only a go/no go indication which other actually display a reading (usually between .01 and 100 ohms so they can also be used as low-ohms meters for resistors in non-inductive circuits). See the section: "What is ESR and how can it be tested?". Note: always place the test probes on the capacitor terminals themselves if possible. Any wiring between your meter and the capacitor may affect the readings. Although your user manual may state that you can test capacitors in-circuit, other components in parallel with the capacitor can screw up the readings - usually resulting in an indication of a shorted capacitor or excessively large uF value. Removal is best. Unsoldering only one of the pins is adequate if you can isolate it from the circuit. Substitution is really the best approach for repair unless you have a very sophisticated capacitance meterd. If you are into building things, the March 1998 issue of "Popular Electronics" has plans for a digital capacitance tester with a range from 1 pF to 99 uF.
It is essential - for your safety and to prevent damage to the device under test as well as your test equipment - that large or high voltage capacitors be fully discharged before measurements are made, soldering is attempted, or the circuitry is touched in any way. Some of the large filter capacitors commonly found in line operated equipment store a potentially lethal charge. This doesn't mean that every one of the 250 capacitors in your TV need to be discharged every time you power off and want to make a measurement. However, the large main filter capacitors and other capacitors in the power supplies should be checked and discharged if any significant voltage is found after powering off (or before any testing - some capacitors (like the high voltage of the CRT in a TV or video monitor) will retain a dangerous or at least painful charge for days or longer!) A working TV or monitor may discharge its caps fairly completely when it is shut off as there is a significant load on both the low and high voltage power supplies. However, a TV or monitor that appears dead may hold a charge on both the LV and HV supplies for quite a while - hours in the case of the LV, days or more in the case of the HV as there may be no load on these supplies. The main filter capacitors in the low voltage power supply should have bleeder resistors to drain their charge relatively quickly - but resistors can fail. Don't depend on them. There is no discharge path for the high voltage stored on the capacitance of the CRT other than the CRT beam current and reverse leakage through the high voltage rectifiers - which is quite small. In the case of old TV sets using vacuum tube HV rectifiers, the leakage was essentially zero. They would hold their charge almost indefinitely. 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 (monitoring is not needed for the CRT - discharge is nearly instantaneous even with multi-M ohm resistor). Obviously, make sure that you are well insulated! * For the main capacitors in a switching power supply, TV, or monitor, 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. * For the CRT, use a high wattage (not for power but to hold off the high voltage which could jump across a tiny 1/4 watt job) resistor of a 1 to 10 M ohms discharged to the chassis ground connected to the outside of the CRT - NOT SIGNAL GROUND ON THE MAIN BOARD as you may damage sensitive circuitry. The time constant is very short - a ms or so. However, repeat a few times to be sure. (Using a shorting clip lead may not be a bad idea as well while working on the equipment - there have been too many stories of painful experiences from charge developing for whatever reasons ready to bite when the HV lead is reconnected.) Note that if you are touching the little board on the neck of the CRT, you may want to discharge the HV even if you are not disconnecting the fat red wire - the focus and screen (G2) voltages on that board are derived from the CRT HV. * For the high voltage capacitor in a microwave oven, use a 100 K ohm 25 W (or larger resistor with a clip lead to the metal chassis. The reason to use a large (high wattage) resistor is again not so much power dissipation as voltage holdoff. You don't want the HV zapping across the terminals of the resistor. Clip the ground wire to an unpainted spot on the chassis. Use the discharge probe on each side of the capacitor in turn for a second or two. Since the time constant RC is about .1 second, this should drain the charge quickly and safely. Then, confirm with a WELL INSULATED screwdriver across the capacitor terminals. If there is a big spark, you will know that somehow, your original attempt was less than entirely successful. At least there will be no danger. DO NOT use a DMM for this unless you have a proper high voltage probe. If your discharging did not work, you may blow everything - including yourself. 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 muli-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 value main filter
capacitors in TVs, video monitors, switchmode power supplies, microwave
oven capacitors, and other similar devices quickly and safely. This circuit
can be built into the discharge tool described above (Note: different value
resistors are needed for LV, HV, and EHV applications.)
A visual indication of charge and polarity is provided from maximum input
down to a few volts.
The total discharge time is approximately:
* LV (TV and monitor power supplies, SMPSs, electronic flash units) - up
to 1000 uF, 400 V. Discharge time of 1 second per 100 uF of capacitance
(5RC with R = 2 K ohms).
* HV (microwave oven HV capacitors) - up to 5,000 V, 2 uF. Discharge time
of .5 second per 1 uF of capacitance (5RC with R = 100 K ohms)
* EHV (CRT second anodes) - up to 50,000 V, 2 nF. Discharge time of .01
second per 1 nF of capacitance (5RC with R = 1 M ohm). Note: discharge
time is so short that flash of LED may not be noticed.
Adjust the component values for your particular application.
(Probe)
<-------+
In 1 |
/
\ 2 K 25 W (LV) Unmarked diodes are 1N400X (where X is 1-7)
/ 100 K 25 W (HV) or other general purpose silicon rectifiers.
\ 1 M 10 W (EHV) Resistors must be rated for maximum expected
| voltage.
+-------+--------+
__|__ __|__ |
_\_/_ _/_\_ /
| | \ 100 ohms
__|__ __|__ /
_\_/_ _/_\_ |
| | +----------+
__|__ __|__ __|__ __|__ Any general purpose LED type
_\_/_ _/_\_ _\_/_ LED _/_\_ LED without an internal resistor.
| | | + | - Use different colors to indicate
__|__ __|__ +----------+ polarity if desired.
_\_/_ _/_\_ |
In 2 | | |
>-------+-------+--------+
(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!
For the specific case of the main filter caps of switchmode power supplies,
TVs, and monitors, the following is quick and effective.
(From: Paul Grohe (grohe@galaxy.nsc.com)).
I've found that a 4 watt 'night light' bulb is better than a simple resistor
as it gives an immediate visual indication of remaining charge - well down to
below 10 V.
Once it stops glowing, the voltage is down to non-deadly levels. Then leave
it connected for a little while longer, and finish it off with the `ole
screwdriver.
They're cheap and readily available. You can make dozen 'test-lamps' out of an
old 'C7' string of Christmas lights (`tis the season!).
Editor's note: where a voltage doubler (or 220 VAC input) is involved, use two
such bulbs in series.
ESR (Equivalent Series Resistance) is an important parameter of any capacitor.
It represents the effective resistance resulting from the combination of
wiring, internal connections, plates, and electrolyte (in an electrolytic
capacitor). The ESR affects the performance of tuned circuits (high ESR
reduces the Q factor) and may result in totally incorrect or unstable
operation of devices like switchmode power supplies and deflection circuits
in TVs and monitors. As would be expected, electrolytic capacitors tend to
have a high ESR compared to other types - even when new. However, due to
the electrochemical nature of an electrolytic capacitor, the ESR may indeed
change - and not for the better - with time.
When troubleshooting electronic equipment, electrolytic capacitors, in
particular, may degrade resulting in a significant and unacceptable increase
in ESR without a similar reduction in uF capacity when measured on a typical
DMM's capacitance scale or even a cheap LCR meter.
There commercial ESR meters and kits available ranging from $50 to $200
or more. Here are a couple of sites to check out:
* http://www.ozemail.com.au/~bobpar/esrmeter.htm
* http://www.awiz.com/cwinfo.htm
There devices can generally be used to measure really low resistances of
non-inductive devices or circuits as well (they use AC so inductance would
result in inaccurate readings). Since their lowest range is at least 10
times better than a typical DMM (1 ohm full scale - .01 ohm resolution),
they can even be used to located shorted components on on printed circuit
boards.
Note: always place the test probes on the capacitor terminals themselves if
possible. Any wiring between your meter and the capacitor may affect the
readings. While usually not a problem, very low resistance components in
parallel with the capacitor may result in a false negative indication - a
capacitor that tests good when in fact its ESR is excessive.
(From: Larry Sabo (ac274@FreeNet.Carleton.CA)).
I find my ESR meter invaluable for finding high ESR caps, and have never
seen a shorted cap that hadn't exploded. It's such a pleasure to zip
through the caps in a power supply that's duff and find the ones that have
had it, all without touching the soldering iron.
There have been days I wish I had the LC102 for it's leakage measuring
capability, but in my limited experience the 10% figure seems high. The
LC102 commends itself for the inductance ringer, too, but you sure pay a
premium. I'll build Sam's gizzmo first.
BTW, I built my ESR meter from a kit purchased from Dick Smith Electronics
in Australia, for $A 52.74 + $A 25.00 for delivery. It took about 8 hours
to assemble, but I'm a fuss-ass.
(From: Michael Caplan (cy173@FreeNet.Carleton.CA)). Before I bought my ESR meter I too wondered--what exactly did it measure? Nevertheless, having heard so much about the meter, I went ahead and bought one. It works, and that's the real bottom line. A recent question about what exactly in being measured (DF or Q) piqued my interest again. I think I have the answer -- 'think' being the operative word. Here's my interpretation. In summary, the ESR is indeed related to Dissipation Factor (DF), but it is not the same. A DF measuring device might not as readily identify a bad capacitor as does the ESR meter because the reading varies and is not direct, as described below. Capacitors may be thought of as having pure capacitance (C) and some pure resistance (R), the two being in series. An ideal capacitor would have only C, and no R. However, there are the leads and plates that have some resistance and constitute real R. Any R in series with C will reduce the capacitor's ability to pass current in response to a varying applied voltage, as in filtering or DC isolation applications, and it will dissipate heat which is wasteful and could lead to failure of the component. As with ESR, a lower DF (or higher Q, it's inverse) may be equated with better performance, all other things being equal. Now I get a bit more mathematical, but only using basic electronic theory and formulas so I hope most will be able to follow this. DF is defined as Rc/Xc, the ratio of the R in the capacitor (Rc) to the reactance of the capacitor (Xc). The higher the Rc, the higher the DF and the "poorer" the capacitor. So far so good. The reactance (Xc) is a function of frequency. Xc=1/(2*pi*f*C). So, as the frequency goes up, Xc goes down. Now look back at the formula for DF. DF is an inverse function of Xc. As Xc goes down, DF goes up, and vice-versa. So DF varies proportionately with frequency. Here's an example using the ubiquitous 22 uF, 16 V electrolytic that seems to be at fault too often in many switched mode power supplies. At 1000 Hz, this capacitor has an Xc of 7.2 ohms. If the series Rc is only 0.05 ohms (pretty good), then the DF is 0.0069. At 50,000 Hz, this same capacitor would have an Xc of only 0.14 ohms. At this frequency, the DF is 0.36, again good. Now, change the Rc from 0.05 to 25 ohms. At 1000 Hz, DF = 3.4. At 50,000 Hz, DF = 178. So we see that DF is a function of the test frequency. The higher the frequency, the higher the DF. DF is a measure of the capacitor "quality", but the figure is valid only at the frequency of the test. (A good capacitor, with an ideal Rc of zero, will have a DF of zero regardless of frequency.) DF can indeed be used to identify a bad capacitor, but the user must interpret the level of measured DF that would indicate a bad component. Any 'go/no go' tables of DF values would be valid only at the specified frequency. As an alternative, the user can calculate the Rc by first measuring both DF and C, and then, knowing the test frequency, determine if the Rc is excessive. (Rc=DP*Xc). The ESR meter measurement system, however, does not appear to be a function of Xc. It measures the voltage across the capacitor resulting from the application of a very short pulse of current. This short pulse is not enough to charge the capacitor so the voltage being measured across the capacitor's leads is primarily a function of Rx, which is not frequency sensitive. And, with the 'tables' of typical ESR (=Rc) that is provided with the ESR meters I have seen, there is no need to do any further calculations. The ESR meter is not going to be reliable with very small capacitors. In this case, they will become more fully charged by the applied current at the time the meter samples the voltage. Even if the Rc is an ideal zero ohms, the meter will now read the voltage built up on the capacitor and interpret it as a very high (possibly off-scale) ESR. Thus its advantage, and main purpose, is in testing electrolytics which tend to be larger value capacitors. (Note: The inability of the ESR meter to test low value capacitors is true only if the meter does not distinguish between in-phase and quadrature voltages, and it does not. If it did sense only the in-phase voltage that is produced across Rx (i.e. in-phase with the applied current), then it would not be sensitive at all to the delayed (minus 90 degrees) voltage built up on the capacitor's plates.) All testing I have done with small capacitors (less than 0.001 uF) seems to suggest that the (Bob Parker) ESR meter is not phase discriminating and Bob Parker has confirmed this. This is not a great disadvantage. The objective of the ESR meter is to identify capacitors that have gone bad. This is more the case with electrolytics where the dielectric compound tends to dry up. Smaller capacitors usually are not electrolytic and therefore tend to be relatively stable. Faults in the latter (e.g. ceramic, mica, polystyrene) are more likely to be open, shorted, or leaky, all of which will be detectable by capacitance or resistance measuring devices.)
While, the techniques described below can in principle be applied to any capacitor, they will be most useful for electrolytic types. Of course, make sure to observe the polarity and voltage rating of the capacitor during testing! (From: Ron Black (ron.black@pstbbs.com)). An inexpensive way (for the cost of a resistor) to measure the ESR of a capacitor is to apply a squarewave signal through a resistor in series with the capacitor under test. Monitor the waveform on the capacitor using an oscilloscope. When using a sensible squarewave frequency (a few KHz - not one where the inductance of the circuit becomes an issue) there will be a triangle waveform with a step at the squarewave transition times. The amplitude of the step will proportional to the ESR of the capacitor. Calibrate things by adding a known small value ESR simulating resistor in series with the capacitor. This doesn't have to cost anything if you have a squarewave generator, or can build one cheaply. (From: Gary C. Henrickson (gary@aloha.net)). Motivated by the discussions on the virtues of ESR testing, I ordered a genuine ESR meter. While waiting for it's arrival, a large pile of dogs were accumulating in my shop. To crank out these repairs quickly in the meantime, I constructed an 'ESR meter' by cabling a (50 ohm) function generator output to the scope input and, via a T-connector, on to a set of test leads. With the test leads shorted, mere millivolts displayed on the scope. Across a good capacitor, mere millivolts. Across a sick capacitor, mucho volts. The defective caps stuck out like a sore thumb. Wow, this is too easy. Instant in-circuit (power off) fool-proof testing of electrolytics. I wish I had thought of this 50 years ago. I used 100 KHz and 5 V p-p. With scope set at 0.2v/div you can also check diodes surrounded by low ohm transformer or inductor windings. (Editor's note: to avoid the possibility of damage to semiconductors due to excessive voltage, use a lower amplitude signal - say .5 V p-p - for in-circuit testing. This will also prevent the most semiconductor junctions from conducting and confusing your readings. (From: Bert Christensen (70461.2507@compuserve.com)). I have been reading the various messages about ESR checkers and while I don't doubt their value in electronic servicing, I think that the use of these devices adds an extra and IMHO unneeded step. My method of diagnosing possible electrolytic fault is to use just a scope. Remembering that electrolytics pass AC or signals through them, a scope should show *the same* waveshape on both sides of the cap. If the cap is a bypass cap to ground, then the waveshape should just be a flat line on both sides; if it is a coupling cap, the waveshape should be the same on both sides. There are some exceptions, one being a cap that is used for waveshaping in a vertical circuit but such applications are few. Most electrolytics are either coupling or bypass. Using 'my' scope method has several advantages. The main one is that it tests caps dynamically in the circuit they are used in and using the actual signals applied to them in real life. The method is fast because you just have to go from one to another (if you are using the scatter-gun approach) using just the scope prod. But, best of all, it seamlessly integrates a total dynamic approach to servicing using the set's own signals or lack thereof. If you are tracing a video circuit, you can find an open cap, an open transistor, or a defective IC using the same piece of equipment. I have been running a service business for over 40 years. Most of my business today is doing tough-dog service for other service companies. But, I must admit that sometimes I fix sets just by changing the caps that are swollen. ;-} (From: Clifton T. Sharp Jr. (clifto@megsinet.net)). I still do just enough work that I'll one day break down and buy an ESR meter (I always give in and indulge myself with the toys of my "trade"). For now, though, the quickie method I use is the oscilloscope. It goes something like this: 1. Scope positive lead. Any significant AC? If not, go to next cap. 2. Is the AC more than about 5% of the DC? If not, note this location and go to next cap. 3. Scope negative lead. AC here roughly the same as on positive lead? If so, go to next cap. (If this lead is *obviously* grounded, skip this step.) 4. Set off; note value; jumper in roughly same value at safe voltage rating. (Note: make sure both caps are discharged! --- sam) Set on; scope positive lead. Significant difference? If not, note this location and go to next cap. 5. Replace cap. Test set. If not okay, go to next cap. If that doesn't catch it, a quick review of the "noted locations" often does. This fixes 98% of cap problems. Not exhaustive or perfect, nor is it intended to be. Close cover before striking. Probably causes cancer in laboratory rats. Your mileage may vary. So there!
(From: Gary Woods (gwoods@albany.net)). Thanks to a friend with a scanner, ESR meter schematics, theory of operation, and sales literature (From a company that, alas, no longer exists) are on my personal page: http://www.albany.net/~gwoods/. Boat-anchor relevance - although the device is sand-state, it's just the ticket for checking out those old 'lytics!
(From: John Whitmore (whit@hipress.phys.washington.edu)). First, you need an AC ripple current source. Then, you tune to the frequency of interest (120 Hz for rectifier power supply filter capacitors is usual) and apply both the AC current and a DC voltage bias. Measure the phase shift between the current and the voltage (for a perfect capacitor, this is 90 degrees) and measure the induced voltage (for a perfect capacitor, this is I*2*pi*f*C). Take the tangent of the difference of the phase shift and 90 degrees. (This is 'tan(delta)' and appears on the spec sheet for the capacitor...) Then remove the AC, and crank the DC bias up to the voltage surge rating; measure leakage current. Ramp the DC bias down to the working voltage rating; measure leakage current. Raise temperature and repeat the capacitance, phase shift, and working-voltage measurements at the max temperature the capacitor is rated for. Yes, it DOES sound rather elaborate, but that's the test that the manufacturers use.
(From: Jeroen H. Stessen (Jeroen.Stessen@ehv.ce.philips.com)). Electrolytic capacitors like to be kept cool! If there's anything that these capacitors can't stand, it's heat. It causes them to dry out. Electrolytic capacitors exist in (at least) two different temperature ratings: 85 C and 105 C. The latter are obviously more temperature resistant. Unfortunately they also tend to have a higher ESR than their 85 C counterparts. So in an application where the heat is due to I^2 * ESR dissipation, the 105 C type may actually be a *worse* choice! If the heat is due to a nearby hot heatsink then 105 C is indeed a better choice.
(From: Ralph W. M. (ralphwm@aol.com)). Electrolytics have a shelf life. Electrolytics can go bad (i.e., dry out) on the shelf even though they were never used/turned on even once. Technically, an "stale" electrolytic (more than one year after it was manufactured) would have excessive DC leakage, and should be properly re-formed before using it. In practice, I have never found this to be a problem 99% of the time (only exception is critical timing/direct coupled circuits; very rare these days). The worst I have even noticed, when installing a stale electrolytic, was that the circuit was slightly unstable for 15 minutes, but cleared up and was fine thereafter and NEVER "bounced". (all bets are off if something so old it has "whiskers" is tried though). How old is too old? I would offer that up to 5 years on the shelf, in practice, should not be a problem. But 10 years stale MIGHT upset things a bit. Technically, if you read electrolytic specification sheets, you will find that the best (i.e., lowest) DC leakage is not until it has been ACTUALLY used for at least 10% of the total projected lifetime, (i.e., a 1,000 hour @105C electrolytic would not achieve the lowest DC leakage until it was used for 100 hours @ 105C (or used for 600 hours @ 65C; but that conversion is another story). In practice, IMO, the vast amount of circuitry designs/type of circuits being currently designed, have built into it enough tolerance for above average DC leakage, that (these days), excessive/drifting DC leakage is rarely a problem. As far as "exercising" seldom used equipment; couldn't hurt. "I seem to recollect reading (or is it an old wives' tale?) that electrolytics last longer if you apply a voltage across them every so >often. This to me implies that seldom used devices should be turned on every now and again to make them last longer, not left sitting on the shelf. True or false?"
They are there to channel the debris in a known direction should the capacitor turn into a bomb. Really :-). However, exploding capacitors aren't all THAT common in properly designed equipment.... (Well, except for that EPROM programmer that had a tantalum electrolytic installed backwards at the factory. Six months later - K-Blam!) (From: Gary Woods (gwoods@wrgb.com)). If you look in a DigiKey catalog, they detail the 'Vent Test' in which an electrolytic cap is overloaded in a specified way and the can fails expelling the material *only* through that scored portion. Sounds like material for another urban legend; like the supplier who carefully tested each incoming fuse for blowing in a specified time at a specified overload. Of course, the people trying to *use* those fuses didn't appreciate how nicely they passed these tests! You can do a vent test by hooking up an electrolytic to your 'suicide cord' and plugging it into 110 VAC. Entertaining. (I did NOT recommend you do this, and am NOT liable!)
You may find non-polarized electrolytic capacitors in some equipment - usually
TVs or monitors though some turn up in VCRs and other devices as well. Large
ones may be found in motor starting applications as well. These usually do
need to be replaced with non-polarized capacitors. Since polarized types
are generally much cheaper, the manufacturer would have used them if it were
possible.
For small capacitors - say, 1 uF or less - a non-electrolytic type will very
likely be satisfactory if its size - these are usually much larger - is not a
problem.
There are several approaches to using normal polarized electrolytic capacitors
to construct a non-polarized type.
None of these is really great and obtaining a proper replacement would
be best. In the discussion below, it is assumed that a 1000 uF, 25 V
non-polarized capacitor is needed.
Here are three simple approaches:
* Connect two electrolytic capacitors of twice the uF rating and at least
equal voltage rating back-back in series:
- + + -
o----------)|-----------|(-----------o
2,000 uF 2,000 uF
25 V 25 V
It doesn't matter which sign (+ or -) is together as long as they match.
The increased leakage in the reverse direction will tend to charge up the
center node so that the caps will be biased with the proper polarity.
However, some reverse voltage will still be unavoidable at times. For
signal circuits, this is probably acceptable but use with caution in
power supply and high power applications.
* Connect two electrolytic capacitors of twice the uF rating and at least
equal voltage rating back-back in series. To minimize any significant
reverse voltage on the capacitors, add a pair of diodes:
+---|>|----+----|<|----+
| - + | + - |
o-----+----)|----+-----|(----+------o
2,000 uF 2,000 uF
25 V 25 V
Note that initially, the source will see a capacitance equal to the full
capacitance (not half). However, the diodes will cause the center node
to charge to a positive voltage (in this example) at which point the diodes
will not conduct in the steady state.
However, there will be some non-linearity into the circuit under transient
conditions (and due to leakage which will tend to discharge the capacitors)
so use with care. The diodes must be capable of passing the peak current
without damage.
* Connect two capacitors of twice the uF rating in series and bias the center
point from a positive or negative DC source greater than the maximum signal
expected for the circuit:
+12 V
o
|
/
\ 1K
/
- + | + -
o----------)|-----+-----|(-----------o
2,000 uF 2,000 uF
35 V 35 V
The resistor value should be high compared to the impedance of the driving
circuit but low compared to the leakage of the capacitors. Of course, the
voltage ratings of the capacitors need to be greater than the bias plus the
peak value of the signal in the opposite direction.
(From: Nicholas Bodley (nbodley@tiac.net)). Within the past 2 weeks or so (current date: 11-August-1997), probably prompted by an article in EE Times, I set Excite to dig for 'supercapacitors' and 'ultracapacitors'. I did find that when you use the 'More Like This option' enough, it gives you the same hits. Anyhow: What I found was fascinating to an old-timer. Capacitor technology is now at the point where it can do load-leveling to extend the life of electric vehicle (EV) batteries. The high power needed for EV acceleration can be provided by an ultracapacitor. The ultracap. can also absorb energy for regenerative braking, to limit the otherwise very high charging current for the battery. Noted in passing was a Mazda experimental EV that uses ultracaps. this way; it is called, believe it or not, the Bongo Friendee. No kidding. (I have a collection of 7 or 8 other such names...) Mentioned were capacitors of 1,800 farads at 2.3V. Yup, we're now in the kilofarad era, folks! The capacitor bank comprised a total of 80, in groups of two in parallel, 40 groups in series. Total voltage was 92. Other specifications noted in passing: Ultracaps. are now in the 0.1 to 8 kWh (kilowatt-hour) range. Some are made of carbon aerogels (that must not be news...) Maxwell has an 8-cell assembly rated at 24V, bipolar, 4.5 Wh/kg. The same company also has a monopolar cell (monopolar?) rated at 2,300 F, 3V; 5 Wh/kg. This one can provide over 100 A ! Some ultracapacitors apparently (pretty sure) do not use electric double layer technology. They use oodles of alternating layers of conductor and dielectric, stacked 'to the thickness of a credit card'. Some keen mind(s) have found out how to make a dielectric layer that is 'intrinsically free of defects'. These caps, fairly sure, use metal conductors; they have quite-low inductance. Multilayer thin-film caps can be made up to 25 cm^2, to 1,200 V (!), and store 10 joules / cm^2 with applied voltage just below breakdown. Also noted, but considering the topic, maybe a repeat: Carbon aerogel caps can go to 40 F /cm^3; work excellently as cold as -30 C, and can manage power over 7kW/kg. Self-discharge is in weeks. I found this info. utterly fascinating. When I get a decent job, I'm getting myself a 100F Elna. BTW, did you hear that a DMM uses a supercap. for power? I think the figures are that a 3 minute charge will run it for 3 hours.