a leaking cap

[simonwai999]

hi folks
any easy simple ways to tell if an electrolytic cap is leaking or bad?
thanks

[Mooly]

If we are talking electroylitics leakage is usually visible, corroded look, green/furry looking etc.
How to tell if it's bad, measuring it's value is not the way to do it, you have to measure it "goodness" which involves measuring the ESR or equivalent series resistance for starters.

[simonwai999]

hi buddy

thanks for your reply
but how do i
measure its "goodness" which involves measuring the ESR or equivalent series resistance for starters?

regards

[Mooly]

Hi,
I wondered if you would ask that. Again we are talking electroylitics. You can do it with a 'scope and signal generator or you can ( I think -- never bought one ) get ESR meters. I have homemade circuit, not of my own design, that actually goes one better than ESR and measures phase angle or TAN Theta, of the voltage across and the current through the cap and displays the result on a DVM. It's quite a complex bit of kit really. One of Cyril Batemans "inventions" from about 10 years ago.
It's interesting comparing different caps, but to be honest most modern components are pretty good these days, and if you suspect a cap is playing up it's cheap enough to renew them all and have done with it.
Have you a 'scope ?

[MRupp]

Just test the leakage current I'd suggest to apply the rated voltage through a resistance of ca 10 - 50 Kohms. After a some minutes, depending on the capacitance/voltage/resistance, it should reach it's rated voltage, except for the very small leakage current. You can calculate the expected leakage current of a cap from formulas given in the datasheets, and if you do not have the right datasheets, those leakage currents are usually in the same ballpark for different brands.