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leaky, open, shorted, low ESR....testing caps quesiton

Discussion in 'Electronic Basics' started by tempus fugit, Oct 27, 2004.

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  1. tempus fugit

    tempus fugit Guest

    Hey all;

    I've got a multimeter and a scope, but no capacitance meter. If a cap is
    open, it should show infinite resistance on a meter, without charging up. If
    it is shorted, it should show very low resistance.

    Am I right in these statements? I can test the ESR with the scope, and I
    read in an earlier post that the only way to test for leaky caps is to
    inspect them. What if they are dried up? Is there any way of checking that
    with a meter?

  2. Tom Biasi

    Tom Biasi Guest

    You are somewhat correct however, if the cap is of small capacitance the
    charge time may be so short that you see an open almost immediately.

    A short may not be a short at meter voltage but may be at operating voltage.

    Leaky refers to current leak not something leaking out of the cap.

    If they are dried up the capacitance will not be as indicated.


  3. Bob Masta

    Bob Masta Guest

    Do you have a sine-wave oscillator? If so, you can do all
    sorts of tricks. With a simple R-C circuit where the C is
    unknown you can look at Lissajous figures to measure
    capacitance. Accuracy is limited by your ability to measure
    heights and widths of an ellipse on the scope screen,
    but it's fine for ballpark. And it should be great for rapid
    testing of many same-value parts, since you can see at
    a glance if something is wrong.

    You can also make a Wheatstone bridge to measure
    the capacitance very accurately. You can do this
    with a battery-operated meter and no scope. There
    are several possible configurations. Conceptually
    simplest is to put the unknown cap in series with a
    known good cap of the indicated value. Put this
    series pair across the oscillator output, and also put
    a pot in parallel with the series pair. (10K to 100K or so
    should be fine.) Now put your meter between the
    wiper of the pot and the junction of the two caps.
    When there is no voltage difference, the ratio of the
    pot is the same as the capacitor reactance ratio.

    You can do the above even without an oscillator
    using a low-voltage line transformer to provide
    an isolated 60 Hz (or 50 Hz, depending on where you are).
    This will work best for big caps.

    Best regards,

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  4. tempus fugit

    tempus fugit Guest

    nice tipss Bob.

    I have a sinewave generator, but I'm not real experienced with my scope yet
    (but working on it!), so I'm not sure how to display the Lissajous figures -
    I've gread about them, but haven't messed around with them yet. How would I
    set up the RC circuit you mentioned?
    I forgot to ask in my original post - what should a meter (DMM?) read if the
    cap is dried up?

  5. Bob Masta

    Bob Masta Guest

    To use Lissajous figures your scope must have X-Y capability.
    You essentially have 2 separate channels driving the beam;
    the Y channel does the ordinary vertical deflection that you are
    used to, and the X channel drives the beam back and forth
    (instead of the sweep circuit doing that).

    So if you put the same exact signal in X and Y, you see a
    diagonal line because as X increases, Y increases proportionally.
    If there is a phase difference, the line starts to open out
    into an ellipse. At 90 degrees difference you have a perfect
    circle (assuming you set the X and Y gains appropriately).
    You can determine the phase angle by measuring the
    vertical extent and horizontal extent of the ellipse, and using
    simple trig functions. (Google on Lissajous for details.)

    The R-C circuit is just to give you a phase difference. You
    use the input to the R-C as the Y and the voltage across
    the C as the X (or vice-versa). The shape of the ellipse
    will be determined by the R, C and frequency.
    Hmmm, never actually tested this but I expect it to be close to
    an open circuit. Haven't played with those old paste electrolytics
    since the "good old days"!

    Best regards,

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
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