Testing aluminum electrolytic caps

[Conrad Hoffman]

This is a work in progress, but may have some useful info-

Testing Aluminum Electrolytic Capacitors

As usual, comments corrections and additions invited. Laughter and rotten fruit optional.

[Speedskater]

The red font hurts my eyes and is hard to read.

[Conrad Hoffman]

Well gosh Kevin, it was only two warning lines at the top, but not wanting to cause pain, I've dimmed them down a bit.

[Elvee]

Excellent article, and as usual, Conrad and I are in perfect agreement.
I recently made some tests on NOS vintage caps, and I arrived at similar conclusions:
Oldies but goodies!!!
The thread also includes the schematic of a cheap, vectorial esr-meter.

[phoenix710]

Thanks for a really informative and useful article! Thanks for taking the time to share a valuable testing technique.

I've permanently bookmarked it to my list of "must not lose" sites.
Dave

[a.wayne]

Hello Conrad,

Great effort and good read ....

[AndrewT]

Hurry up and finish this paper.
I want to read it all in depth.
There is a lot to learn and I want to start soon.

Thanks Con.

[Damon Hill]

Ditto. Bookmarked for future reference.

[ecir38]

excellant Thanks

[AmpliFire]

Conrad,

Great stuff and very accessable explanations.

If I may give some positive remarks:

- could you perhaps lighten up the grey background a bit? Makes the black text easier to read.

- filling the text out between left and right sides would make it even look better

- for DC leakage test, it would be usefull to explain how to measure it.... An easy way is to connect a 10 kOhm resistor in series with the capacitor, and reading the voltage drop across the resistor to determine the leakage current. Drawn from lots of experience, I must say that 10 kOhm is hardly enough for large (>5000uF) capacitors if the resolution of your DVM is less then 10 mV (which is the avarage DVM at hand), and that small capacitors (<500 uF) have such a low leakage current that even a 100 kOhm will give a few mA to read out (reminding that the leakage current is in uA and proportional to capacitor size). Another issue with this resistor method (which is feasible for anybody), is that the 'leakage current at 2 min' is not feasible due to the resistor slowing the charging process. Hence, two solutions are possible:

1) just wait until the current (i.e. measured voltage drop across resistor) is stable for 10 or more minutes. This week I did just that for a large number of large power supply caps (5000 ~ 22000 uF) and this method showed usefull results but only after 2 to 3 hours, giving stable leakage current only after 12 hours, For small caps, it should be fine within 10 to 15 minutes

2) connect a forward biased diode (suitable rated to handle the inrush current for large PSU caps! ) in parallel with the resistor. The diode will ensure the quick charge of the capacitor and will switch off when the capacitor's positive lead is less then 0.6 volt from the PSU voltage, in which case the resistor takes it over. Again, 2 minute sample time is not a valuable approach with this method, give it some time to settle out.

- what about the actual applied voltage during leakage test? The datasheets give little guidance, but I would say at rated voltage. Thinking of it, I will test if the leakage current drops if the applied voltage is lower.

- perhaps I stressing too much here about leakage currnet, but I discovered once again this week that a cap which had an almost normal ESR and dissipation profile showed complete failure under leakage test (around 100x more than other caps), which then explained why its resonance frequency came earlier than for the others. For another (new) cap it demonstrated to be a counterfeit. Following this experience, I would almost always start with leakage current test for aging caps, albeit that for PCB mounted caps it is not an easy thing to do.

- in fact, the above leakage test method is also reforming the cap... hence an integral subject.

- and to complete the story: after reforming the caps (between 6 and 25 months old), the ESR lowered between 5 and 20%.

I think I should send you a PM since I'm doing a lot of cap testing myself

Good job !

Oscar