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how to tell if a capacitor is faulty?

Discussion in 'General Electronics Chat' started by confounded, Jun 3, 2009.

  1. confounded

    confounded New Member

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    Hi,

    I suspect i have a faulty capacitor in my circuit.

    I connect the capacitors to my multimeter and i get the right capacitance.
    But I think one of them is shorting to ground.

    Am i right in thinking measuring capacitance does not prove a capacitor is ok?
    If so what other tests can i carry out to see if they are ok?
    I dont have a specific capacitor tester just a capacitance function on my multimeter.
     
  2. edeca

    edeca Active Member

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  3. pcbheaven.com

    pcbheaven.com New Member

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    1. Measure with multimeter the capacitor in Ohm scale. The multimeter must shows at first small resistance and as time passes the resistance must increase.... This means the capacitor is still working.

    2. If electrolytic capacitor, charge it with 3 volts and put it across an LED. It should turn on for some fractions of secconds and then start fading out.
     
  4. dave

    Dave New Member

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  5. confounded

    confounded New Member

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    thanks for replying guys

    I've skimmed through those pages and i think i can use my multimeter set on resistance measure to see if the caps are shorted. :)
     
  6. edeca

    edeca Active Member

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    Good job. Sorry for simply pasting links, but there are lots of good tutorials and repeating them here would be silly :)
     
  7. whiz115

    whiz115 Member

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    use an analog multimeter...
    a good capacitor should gradually show high enough resistance...
     
  8. crutschow

    crutschow Well-Known Member Most Helpful Member

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    If it's an electrolytic cap it may have excessive leakage at its operating voltage.

    Connect a multimeter set to measure current in series with the cap and connect to a DC power supply. Slowly increase the voltage to the rated cap voltage. After the cap is charged, the current should be no more than a few µA.
     
  9. stevez

    stevez Active Member

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    I read some articles on reforming old capacitors and within those articles were tables of leakage current for various electrolytic capacitor values. You might use those tables with what crutschow has recommended as possibly go/no-go points.

    A relatively recent EE graduate took some large electrolytic capacitors that I had laying around and performed a number of tests on them. The professionals who helped him said the results were no surprise (all of the caps were 20+ yrs old) - the economy capacitors behaved as capacitors but at about 10% of their original value. What I don't know is if leakage current was predictably high or is totally unrelated to the degradation. You might have to measure both the value and the leakage among other parameters. Not sure if ESR is something that can change or degrade - or it just is what it is. I suppose it all depends on what is important to each application.
     
    Last edited: Jun 4, 2009
  10. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    ESR is the most important thing that changes, probably 95+% of all electrolytic failures are for high ESR. The vast majority of faults in modern electronics are high-ESR electrolytics - an ESR meter is of far more value for servicing than an LCR meter (but an LCR meter is still a nice toy :D ).
     
  11. Willbe

    Willbe New Member

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    Charge to 9v with a 9v batt. Discharge through a 1 megohm resistor. When the voltage reaches 37% of initial value, the time in seconds is the value in uF.
     
  12. stevez

    stevez Active Member

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    Nigel (or anyone who can answer) - may be a silly question but I'll ask anyway. As part of common failure modes for an electrolytic capacitor, would you expect to find a capacitor that shows a high ESR to also show an inappropriate value of capacitance or extraordinary leakage?

    Another way of saying it - can a capacitor retain it's capacitance yet have ESR go out of specification?

    I would think that all of those are related - but maybe they aren't.
     
  13. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Yes it can, and most often does - although the capacitor is probably lower capacitance than it originally was, but still usually within tolerance. Leakage isn't likely to be a problem at all.

    The two ways to test are an ESR meter, circuit unpowered, or an oscilloscope, circuit powered.

    The Peak Electronics ESR meter I now use at work tests both ESR and capacitance, so it's doubly useful - and low cost as well.

    Peak Electronic Design Limited - Atlas ESR - Equivalent Series Resistance Meter and Capacitor Analyser - Model ESR60
     
  14. Sceadwian

    Sceadwian Banned

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    The reason ESR going up is bad is pretty easy to understand if you just think of every electrolytic capacitor in your circuit over time turning into a low pass filter.
     
    Last edited: Jun 4, 2009
  15. Hero999

    Hero999 Banned

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  16. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    I've got one of those as well, but it's not much use for servicing purposes, and really very few capacitors and resistors are unknown, but for inductors it's handy (not needed it yet though).
     

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