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How to test capacitors in-circuit?

Discussion in 'Electronic Basics' started by phaeton, Sep 20, 2007.

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  1. phaeton

    phaeton Guest

    How do you accurately test a capacitor in the circuit? Aside from
    bulging packages, how do you know a capacitor has failed? Do they
    usually just short and start passing DC?

    Sure you can desolder it, test it out of the circuit, and resolder it
    back in. But by time you've done all that you could have just skipped
    testing the suspect and soldered in a known good one.


  2. Phil Allison

    Phil Allison Guest


    ** Generally, that is impossible.

    ** Only electros bulge.

    ** Film and tantalums caps sometimes do.

    ** Electros can be checked - in circuit - with an ESR meter.

    But this does not find leaky ones.

    Particular kinds of circuit misbehaviour indicate a faulty cap at a certain

    ........ Phil
  3. Are you trying to find something specific, or is this a general

    One old trick is when you suspect a capacitor, jumper it with
    another capacitor of the same value. If things improve, then
    it may be the capacitor. But of course, this implies that the
    original capacitor is open or has lost capacitance, rather than

  4. phaeton

    phaeton Guest

    It is a general question.

    (and not a homework question!)

  5. For electrolytics in most circuits, an ESR meter works pretty well. In some
    circuits, the capacitor will still have to be removed for testing.

  6. Guest

    You have a look at either side of it with a 'scope and estimate what
    you should be seeing e.g. if it is a large coupling capacitor in an
    audio path then both sides should be the same size.

    If it is some sort of first order filter then work out the capacitor's
    Xc and compare that to the "load" and if e.g. they are the same then
    half the signal should be dropped across a working cap.

    If it is part of a UHF oscillator (that won't) then do as the other
    poster says i.e. bridge it (hold in place with end of plastic trim

    A worse case is where you have a large output transistor with four
    caps soldered to strip-line pcb layout at UHF and the output power is
    10Watts instead of 20. This is straight off the production line and
    has never worked before. Then you have no option but to remove all
    four and measure them with a bridge.

  7. Rich Grise

    Rich Grise Guest

    If you're getting paid by the hour, it's probably cheaper to just
    swap out the cap.

    I once worked repairing video game PCB's, and when we'd get a really
    knotty problem that took too much time to diagnose with the scope,
    we'd "shotgun" it - just replace every chip on the board, because
    it was cheaper to take an hour to replace 20 or so chips at $0.30
    apiece than to spend four hours troubleshooting at $30.00/hr.

    And I've been in the business since 1968, and have never seen a
    capacitor tested in-circuit, let alone done it.

    Good Luck!
  8. Jamie

    Jamie Guest

    I have a nice B&K tester that works very good with
    in circuit stuff. It's an LCR meter and it's been
    around for a bit but it's what I use for in circuit

    That model isn't have I have but it's
  9. Jamie

    Jamie Guest

    I posted a URL of what I use which works very well, I can tell you
    a basic theory of how it works.
    The in circuit uses the resistive load in the circuit to offset the
    reading from the test cycle on the capacitor.
    It's not perfect but it's very close for doing debug work.
    what I have found is this. In many cases where a cap test
    resulting in much higher value than should be, may indicate a
    leaky cap. Of course, one that test much lower is most likely
    a weak cap.

    As far as doing repairs, I haven't yet pulled a cap from a board
    using this LCR meter that showed a below normal reading to be false.

    I have seen where at times, when it shows far to much Uf on a cap
    in circuit that it may be a false reading. But this hasn't happen to

    Don't go for a cheap cap meter. Get one that is designed for
    in circuit tests.
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