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There are many different ways to test capacitors. Using a capacitance meter, using a DMM and an analog meter.

In general, is it safe to assume that a capacitor is considered functional if it's capacitance measurement is +- 20% of it's declared value without doing the ohm/voltage test? Are these test overkill after the capacitance value has been verified?

Just so we are on the same page, the ohm test I am referring to us to check to see if the resistance value is not constant and that it increases until OL is displayed.

The voltage test is just to charge the capacitor and check that it is properly charging/discharging. Could these test be mutually exclusive?

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  • \$\begingroup\$ what if the capacitor was a 1% COG. Would you still consider a reading +-20% to be a good indication? \$\endgroup\$
    – user16222
    Commented Feb 19, 2017 at 14:25

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Short answer: No. A capacitance measurement will only give you part of the picture. You also need to measure the ESR, especially for electrolytic capacitors. You could have an electrolytic capacitor that measures exactly what its rated capacitance suggests, but the cap will not work at all in the circuit because its ESR is too high.

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Functional? Maybe, it depends on the specified tolerances of the part. But there are many other things to factor in (ESR, ESL, ripple current, etc.) If you just want it as a quick functional test before soldering the part to a PCB, it may be good enough for you.

HOWEVER... Reliable? Who knows!

If maximum ratings are exceeded during part testing, it may well be the case that the part is mostly functional but not reliable any more. Even benign operating conditions that are within the design range may precipitate a failure much sooner than expected from a random failure calculation.

Be very careful when testing parts on your own.

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So, the question, as I understand it is, can a capacitor suffer a failure in a parameter such as ESR or leakage and yet still have a correct main value as measured by a hand-held tester (for example)? The people who service things like central air etc who see a lot of failed motor-run and start capacitors tend to only go by the main value. The hand-helds they have can only measure that. However, these are mostly polypropylene film capacitors. Other equipment has aluminum electrolytics that often have high ESR failures. I think that measuring the capacitor uF may not catch these because there are hand-held ESR meters for these capacitors. The answer then may YES, but it depends on the capacitor type.

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