Using DMM on Possibly Charged Caps

[Rob]

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?

 

[Meat Plow]

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?

Short them out first then you won't have to worry about it.
--
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COOSN-266-06-25794

 

[Eeyore]

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?

What on earth do you have in mind ? Why would you do that ?

Is it ever safe to use a DMM to measure a voltage in your mind ?

Graham

 

[Malissa Baldwin]

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?

Have you tried Google?

 

[Rob]

Have you tried Google?

I certainly did. The answer I was looking for was that DMMs have
effectively infinite impedance when measuring voltage and would not
discharge a capacitor when connected across it.

 

[Malissa Baldwin]

I certainly did. The answer I was looking for was that DMMs have
effectively infinite impedance when measuring voltage and would not
discharge a capacitor when connected across it.

O.K. so if you can find your answer using Google then why post your
question here?

If you do it again I will send a complant to [email protected].

 

[[email protected]]

I certainly did. The answer I was looking for was that DMMs have
effectively infinite impedance when measuring voltage and would not
discharge a capacitor when connected across it.

The answer you were "looking for", or the actual answer? And since
when is impedance to be taken for resistance?

Luckily, the Flukes should have internal fuses and should *just* be
able to accept a charge from a small electrolytic cap without
permanent damage. Capacitors will deliver a near-infinite amount of
*current* for a very brief period... the bigger the cap, the ever-
closer-to-infinite-current it will deliver. Can you say OUCH!!

I am not quite sure what your point is after all. Are you attempting
to test the quality of the cap? The voltage in the circuit? Both?
Neither? If you are attempting to test a cap, you need a cap tester.
Preferably one that will test the cap at full operating voltage. After
that, an ESR meter. Short of this, a VOM will only just barely test a
cap, and only after full discharge and only then on the Ohms setting
for internal resistance, and only then to an _extremely_ limited
degree with results that are only just better than none at all.

I keep a very nice Fluke with an internal capacitance checker... this
measures capacity only with leakage (on electrolytics) tending to show
up as excess capacitance. In the field, it is better than nothing, and
it is very useful testing new caps when precision values are required.
But when I am testing electrolytics, a meter that will test at full
voltage is the only way to fly.

Peter Wieck
Wyncote, PA

 

[Jamie]

I certainly did. The answer I was looking for was that DMMs have
effectively infinite impedance when measuring voltage and would not
discharge a capacitor when connected across it.

by standard, they have 10 Meg ohms ..

 

[Sam Goldwasser]

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.

Ignore the several obnoxious replies. These sorts of questions are perfectly
appropriate for this newsgroup.

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Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.

 

[Jim Yanik]

by standard, they have 10 Meg ohms ..

Not necessarily;the cheapo Harbor Freight DMM I have is only 1 MegR,and
I've seen a 22 MegR DMM.

but even 1 MegR would not discharge a cap very fast.

 

[Ancient_Hacker]

Is
it safe to use the DC voltage measurement function to see if the cap
has been safely discharged?

If you put the DMM on the 200 volt range.

Or if the DMM has an "Auto range" feature. With Auto-range, the DMM
has to be capable of standing full rated voltage even on the most
sensitive range. You see with the leads disconnected, the auto range
feature is going to change ranges down until it's on the most
sensitive range, typically 200mv full scale. Then when you put the
leads on a high voltage, for a few milliseconds the meter is getting
up to 2000 volts into the most sensitive range. The design of the
front-end has to be able to handle this. Typically they use a large
series resistor and clipping diodes to limit the peak voltage going
into the A/D chip. No problem at all.

But in general it might be simpler to just have a discharging resistor
handy. A 5-watt wirewound resistor of around 1000 ohms will do the
job safely and quietly. Solder some stranded wires with clips on the
ends.

 

[Esther & Fester Bestertester]

Ignore the several obnoxious replies. These sorts of questions are perfectly

appropriate for this newsgroup.

Ah, Sam to the rescue (c:. Glad we can count on you to actually answer his
inquiry without feeling the need to shame the OP for asking a basic
question...

"Good on ya' ", Sam.
FBt

 

[Matt J. McCullar]

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