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Using DMM on Possibly Charged Caps

Discussion in 'Electronic Repair' started by Rob, May 29, 2007.

  1. Rob

    Rob Guest

    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?
     
  2. Meat Plow

    Meat Plow Guest

    Short them out first then you won't have to worry about it.
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  3. Eeyore

    Eeyore Guest

    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
     
  4. Have you tried Google?
     
  5. Rob

    Rob Guest

    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.
     
  6. 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 .
     
  7. Guest

    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
     
  8. Jamie

    Jamie Guest

    by standard, they have 10 Meg ohms ..
     
  9. 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.
    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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  10. Jim Yanik

    Jim Yanik Guest

    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.
     
  11. 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.
     
  12. Ignore the several obnoxious replies. These sorts of questions are perfectly
    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
     
  13. 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
     
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