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
oups.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
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?