How to test a capacitor with an ESR meter ?

Discussion in 'General Electronics Chat' started by ranatungawk, Nov 17, 2014.

  1. ranatungawk

    Thread Starter AAC Fanatic!

    Oct 30, 2008
    How to test a capacitor with an ESR meter ? Esr meter just shows a number (certain Ohms value ) but how to decide whether the Cap is good based upon that number? is there a range for good capacitors ???
  2. ericgibbs


    Jan 29, 2010
    Read thru this PDF.
    For more info' Google: capacitor esr values
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  3. alfacliff

    Well-Known Member

    Dec 13, 2013
    the mfgrs specs should give you the esr value for the cap. generally if it is very high, the cap is bad. esr is one of the causes of heating in caps.
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  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
    ESR is, equivalent series resistance. Essentially, how much internal resistance in the capacitor. An ideal capacitor has zero resistance but every real capacitor has some resistance as the cap is charged or discharged. This is just one attribute of a capacitor.

    If you are testing a capacitor, then you also need a capacitance meter to test that attribute.

    Maximum working voltage, polarization, working temperature and working lifetime are other specs of a cap that you cannot really test but mostly trust the manufacturer.
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  5. ian field

    AAC Fanatic!

    Oct 27, 2012
    Some ESR meters have a table of suggested values printed on the front panel - or in the user manual.

    A lot depends on what application the capacitor is used in - say for instance a 220uF supply rail decoupling cap in an audio preamplifier, a few Ohms ESR might not be that serious - but on the supply rails of an SMPSU, you want the ESR to be well under 1 Ohm.

    For years I got by without an ESR meter, if the ESR is too high in an SMPSU capacitor - it will run hot.

    Before you go sticking your finger on the caps in a PSU primary side - the common point isn't ground!!!
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  6. wayneh


    Sep 9, 2010
    I was wondering about this exact question a few years back when I first built myself an ESR meter. More recently I replaced all the electrolytics on a power supply board, and the rule-of-thumb I learned in that project was that any electrolytic showing more than 5Ω is suspect. I bought new caps from 1µF to 150µF, and not one of the new caps in that range showed more than 5Ω of ESR. Some of the caps I replaced showed 40Ω. They had no visible damage and still had rated capacity, but were clearly failing.

    Measuring against specifications requires expensive test equipment, but field measurement with a cheap meter can give useful diagnostic information.
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  7. ian field

    AAC Fanatic!

    Oct 27, 2012
    AFAIK; temperature stability is also an indicator of whether an ESR value is normal or faulty.

    My first homebrew ESR meter eliminated Xc by putting the cap in a HF bridge - the heating effect of that HF current made the meter pointer creep towards zero somewhat conspicuously.

    It was also evident that capacitors heated by unsoldering needed leaving to cool to get the true reading.

    Temperature effect is less on a good cap, another clue is on some equipment; the symptoms fade as it warms up.