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Capacitor testing

Discussion in 'Electronic Basics' started by John, Apr 1, 2004.

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  1. John

    John Guest

    Hello.
    I have a question on capacitor testing (plenty on the web about caps but not
    basic testing).
    You have no access to a capacity meter, you only have a DVM, plenty of
    bulbs and some batteries.
    I know that using a moving coil meter you just select resistance, as the
    capacitor charges the meter movement moves.
    How can I test a cap with a DVM (no cap range) to indicate that it is
    functional.
    Thanks.
     
  2. CFoley1064

    CFoley1064 Guest

    Subject: Capacitor testing
    Hi, John. DVMs measure resistance the same way the older Simpsons did -- put a
    current across the resistor, then measure the voltage across it.

    You can do a quick cap check for larger value capacitors the same way -- set it
    to measure the highest resistance range, or use the range hold button to hold
    it in a meg-ohm measurement mode, and just attach the leads to the cap (red to
    cap +, black to cap -). You should see the reading on the DVM gradually go up
    from zero to full scale. Especially for the cheapie DVMs, you might want to
    make sure the cap is discharged before you test.

    If your cap isn't tantalum, you also might want to try reversing the leads, and
    seeing the reading go from negative overload up through zero and then to
    positive overload.

    If you don't have a range hold button but do have autorange, scrounge a
    high-value resistor at the lowest end of your highest resistance range (say, if
    your highest range is 20 Meg, get a 2.2 Meg resistor), and charge the cap up
    with the resistor in series. That will keep the autorange from shifting down
    (and also increasing the current across the DUT). This will also prevent the
    meter from starting at zero, but you will still see the reading gradually rise.
    You can't do the lead reversing thing here, though.

    Note that this is only a very basic test to see if the cap is dissipating
    current slower than the DVM is sourcing it -- I'd expect a little more from a
    cap test. Also, for smaller value caps, it won't tell you if the cap is open
    instead of shorted, which frequently happens. The voltage across the cap rises
    too fast to distinguish between the two.

    Good luck
    Chris
     
  3. John

    John Guest

    Thanks
     
  4. Bob Masta

    Bob Masta Guest

    Build a simple test circuit. If you build an
    oscillator with the frequency controlled by the
    cap, then you can measure frequency to
    determine capacitance. How to do that
    with a DMM? You need a frequency-to-voltage
    converter. A simple F-V is just a one-shot
    multivibrator (made from a logic gate) followed
    by a simple RC filter to smooth the result.

    Or, use the cap in a capacitive voltage
    divider driven by an AC source (such as
    the 60 Hz mains isolated by a low-voltage
    transformer such as the common AC-out
    wall-warts). The impedance of the cap
    can thus be compared with a known cap.
    If you have the standard cheapie DMM,
    it has no sensitive AC ranges so you
    will need to have a little AC-to-DC
    converter ahead of a sensitive DC range.
    You can make this with a couple of op-amps
    and diodes.

    If you do that, you can extend it even
    futher to a Wheatstone bridge, where
    you compare the output of the capacitive
    divider with an adjustable voltage from
    a potentiometer. When the voltage goes
    to zero you read the pot and compute the
    effective impedance of the cap.

    But heck, if you are going to have to build
    things like this, you may as well build a
    real capacitance meter!


    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
     
  5. it really depends on what you are testing for. If you are testing that the
    cap has not failed open, then you can test its resistance. If its less than
    about 1M ohm for an electrolyitc, and 10M for a non-electrolytic, it could
    be bad.

    A big old electrolytic may actually be OK with a lesser resistance; if its
    been sitting a while, the oxide layer may have thinned, thus allowing more
    current through. Putting its rated voltage across it (not right away, but
    gradually, over a day or so) will reform the oxide layer, and cause the
    resistance to go back up (unless it really has failed, in which case it
    won't go back up.)

    If you are testing values, then a capacitance meter would be a wise
    investment. You can get cheapo DMMs at sears that have fairly accurate
    capacitance settings, as well as frequency and temperature. I bought one a
    year ago for less than $30 US. It works well on values larger than about
    1nF.

    Regards,
    Bob Monsen
     
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