How do you test capacitors?

[Joe Hart]

How can I test electrolytic caps to see if they still work? Do I need to take them out of the circuit? What am I looking for when testing? Just continuity or the actual value? Thanks.
-Joe Hart

[dolhop]

Usually they work or they don't.  Essentially, you need to charge it up, and see that it holds the charge.    You'll get best results if you do this out of circuit - but in circuit look to see that the voltage you're expecting it hold is actually there.  If it is sagging, then you could have a leaky cap.

[phillip]

In the past I've found bad capacitors by measuring their resistance...they should have a very high resistance.  On my meter their resistance climbs until the meter reads OL.

A bad capacitor on my Tyco clone once caused it to sound gated, and after measring the resistances of the caps, I found that it was the 220uF bypass cap on the emitter of Q3 that had gone bad...the resistance between the two leads was very small.

I don't know if measuring the resistance of capacitors will always determine if it's good or bad, it's just a method that I've used in the past to find bad ones

Phillip

[dolhop]

It is a good start if the cap is out of the circuit.  However, it won't tell you too much about a somewhat leaky cap.  But yes, it is a good means to test caps.

[Fret Wire]

Time to discharge is also a way to figure uf on an unknown cap.
Resistance also helps. A cap meter is nice to quickly get the value, will also pickup shorts on polarized caps. Helps to sort out caps that you have laying around.
http://www.electronickits.com/gold/capacitancemeter.htm

[petemoore]

I use a similar method when diagnosing engine spark.
 Instead of paying  60 to have myself tols which of the ignition parts were faulty, I would pay 60 for all the ignition parts and start swapping the ones I thought were bad first. It was usually the points.
 I usually socket some caps on a build anyway, and have a slew of circuits without boxes around, not having a 'cap checker', I do it the old fashioned way..by swapping. A good cap is good I think, haven't noticed much differences from one to the next except maybe uf value, when they're bad you can probably tell they're bad, cause the problem will cease when you find it...another + for sockets...with these switching to a different cap is too easy.
 Since it'll have to be half pulled from the circuit for testing [I would think], why not just replace it?

[niftydog]

I use a couple of methods, the cheapest being a "low ohms meter" kit.

It basically gives you an indication of the ESR of the cap, which you can compare against a known good cap (or a table of values provided with the kit).

Gottem here in Oz, don't know if there's other kits available elsewhere. You can purchase this kit from O.S.

Or, if you've got some spare cash sitting in the sock drawer, an RCL meter is the bee's knees!