electrolytic caps testing

[Dave H]

I hooked one up to my DC supply to check for leaks. What happens next confuses me.

<+24v> to <+capacitor-> to <+ volt meter lead-> to <PS ground>

This measured 24v, regardless of polarity of the capacitor on all four caps. I've always tested coupling caps this way with good results, why is this time different?

The difference is a coupling cap is 0.1u but these are 4700u so they are going to take an eternity to charge.
Assuming the meter resistance is 10M the time constant for a coupling cap is 0.1u x 10M = 1 sec. The time constant for 10M and 4700u is 47000 seconds or 13 hours so you'll have to wait a while

[Mick Bailey]

For anyone interested in building a leakage tester, here's an interesting article;

DC leakage and reforming electrolytic caps | PA4TIM's opvangtehuis voor buizenbakken

[J M Fahey]

I build so I buy a lot and from "supplier of the day" which may vary wildly.
Found that most (as in 98% ) modern generic parts are very good, but in case of suspicion , sometimes only justified by reading on the capacitor carton "Another product from Lucky Dragon firecracker factory, established 947 C.E.", I test them.

What is my time proven (decades) self calibrating electrolytic test jig?

I hook them (forward polarity of course) to a way too high voltage supply, say 100/120V for typical SS supply capacitors through a 10k resistor or 600/700V for Tube type ones, through a 100k resistor, and let them charge for a few minutes.

They eventually "zener" and stop rising , usually some 30% or 40% above rated voltage (if good quality) , no danger because resistr limits current to a safe level, so no overheating or bursting.

That alone gives me a first idea about quality, I know that same voltage applied non limited will *explode* the capacitor.

Then I disconnect it from the supply and let it self discharge, for, say, 10 minutes.

Itīs usually 10% to 20% above rated voltge, again in good quality ones.

NOW I know I am safe applying *up to* 80/85% of the self discharge voltage I just measured.