I'm still using my 20+-year-old trusty Fluke 77 DMM for this very thing, and I will testify that it handles the job perfectly well. When checking these caps for any voltage that they may still hold, put the meter on DC volts. This is what the meter is supposed to do anyway, right? Measure DC voltage. Even a couple of hundred volts DC won't hurt the meter at all. In fact, this can save you from a nasty shock... checking big caps for any charge. Better your meter probes find out before your fingers do. If a big electrolytic capacitor still has a big charge on it, though, I really doubt your DMM will discharge it for you automatically. (My Fluke 77 won't, anyway.) Better to keep a resistor of, say, 470 ohms at at least 2 watts handy for this purpose, along with clip leads. Connect the meter probes to the resistor and you can see the voltage decrease as you hold it in place across the cap terminals. I've found out accidentally that the Fluke 77 has an interesting feature that I'm not sure the designers intended: If the meter is set on resistance but there's a voltage in the device you're measuring, the digital display will show NEGATVE resistance. This is, of course, impossible. This has saved my bacon more than once because I repair big electronic motor drives and sometimes we'll get one trucked in, sitting in its sealed crate for up to a few days, and when I open it up and do some preliminary testing with my meter (checking fuses and such), I'll find a big electrolytic filter capacitor in the drive that still has a BIG charge on it... as in over a couple of hundred volts!!!! I discovered this once with my meter set on resistance instead of voltage, and the negative resistance shown in the display led me to check the power bus and -- yaha! Saved myself from getting a nasty shock. The darned capacitor had sat there for well over a week, and still held a mess of electrons. (Yes, there was a bleed resistor connected across the terminals, but it was open.) I've never had the Fluke 77's internal fuse open up as a result of this, either. I call it an "undocumented feature." Doesn't knock it out of calibration or anything, either. I cannot speak for what this might do to any other model or manufacturer of DMM, though. Matt J. McCullar Arlington, TX "Rob" wrote in message news:1180448482.253675.153690@p47g2000hsd.googlegroups.com... I have several DMMs, including a Fluke 77. Let's say I'm testing a > high voltage, possibly charged cap such as a motor run capacitor. Is > it safe to use the DC voltage measurement function to see if the cap > has been safely discharged? What about on cheaper DMMs, is this Yes, as long as you are on a range that exceeds the voltage rating of the cap since there may be no way of knowing how high it is charged. Or, if an autoranging meter, then the maximum voltage of the DMM is more than the maximum possible voltage of the cap. The input resistance of most DMMs is 10M ohms. They will not be effective at discharging any but the smallest uF caps in finite time though. But to check if a cap is charged, sure. In fact, it's a good habit to get into. > typically possible? Does it make sense to measure the resistance > across the leads of a cheap DMM with another DMM when it is in DC > Voltage mode to see if this would be the case? Try it, though the resistance will be stated in the specifications. One thing I should add is that the heat from unsoldering an electrolytic cap can cause its ESR to drop quite substantially, so one needs to let them cool down again before doing a measurement. Another issue which pops up occasionally is that an electroytic cap can have an acceptable-looking ESR figure but be low in capacitance and disturb circuit operation that way. I suspect that good ESR/low capacitance caps might be quite common, but most circuits tolerate them so no-one notices. Has anyone here ever investigated this? Sam's correct that inductive components in the circuit can have an effect on measurements, though probably more so if the cap being tested has a high ESR. I think we all agree that ESR testing in-circuit isn't infallible, though most times it works OK. BTW, I'm just another technician who's certainly no expert on capacitors. :) Most of the time I use the scope. It's great for finding the little problems that you can't see but will cause grief in the near future. I will then test the suspected caps for both capacitance and ESR with a Sencore LC103. I will also test similar caps in the equipment. My reasoning is that if one is bad, the others may also be on their way out. This doesn't mean I'll test every one. If I test 2 or 3 of the same value (or close) and age and they're OK then I will assume that it was only that one that failed for one reason or another. But if I find that they are drifting in value, are thermal, or are higher than normal in ESR then I will usually replace all of that value and voltage and check others of similar values. I do this until I feel I have a reliable repair. There has only been a few times where I have replaced all of the electrolytics and when they were tested I found most of them bad. ohn Kimball wrote: >What about if there are, let's say, 3 caps in parallel, and one has high >ESR. Would any tester find the fault? I don't see how it could. You'd have >to test out of circuit, correct? Hi John, That's the kind of trap with using ESR meters I was referring to. In one of these threads, Bob Urz mentioned this happening to him with a VCR power supply. Conversely, unless things have changed since I got out of doing repairs on a routine basis, it's been my experience that it's pretty unusual to find paralleled electroytic caps in most kinds When I worked at Tektronix,repairing and calibrating TEK professional video test equipment,I often encounted boards with 40+ 100uf/35v miniature electrolytics,some of which were marginal or failing,causing noise,jitter,and other problems. Using the DS ESR meter aided me greatly in replacing all the bad caps.Trying to use a scope to find bad caps was useless. Extreme case...a few desolders (one pin, two caps, max) and you isolate the culprit...go with the flow...ESR testing is THE front-line method for finding these problems... Tom