Re: Detecting the Polarity of a Large Electrolytic Cap


Jan Panteltje wrote:
On a sunny day (5 Dec 2006 10:27:52 -0800) it happened "EdV"
<ed_vogel@xxxxxxxxxxx> wrote in
<1165343272.390457.56280@xxxxxxxxxxxxxxxxxxxxxxxxxxx>:

a 39000 uF 25V Nippon Chemicon. It is hard to see the tiny stamped "+"
and the the terminals are symetrical to the case so incorrect
installation is pretty easy to do. Currently about once every 15
assemblies.

The vent hole has worked succesfully to prevent anyone from being
injured but boy it would be nasty if it didn't work just one time.

This assembly (terminals, cap and inductor) is an open circuit until it
is installed in the system.

Any thoughts on a non destructive test?

Thanks,
Ed V.

Yes, charge via say 1 MOhm, to 25V.
If it does not reach 25V in a few minutes, or if a constant current keeps
flowing (uA meter) it is the wrong way around likely.
No guarantees, just try some tha tway to look for how it behaves.

1e6 ohms * 3.9e-2 farads = 3.9e4 seconds: over ten hours. The rated
leakage current for such a large capacitor may well cause significant
drop even after days. Better use a lower value resistance. Even John
Fields' suggested 510 ohms yields a time constant in excess of 20
seconds, so his "momentary dip" in voltage is actually going to go on
for rather a long time; it would be over half a minute to reach 20
volts. I'd expect the technique to work, and you don't need to wait
till the voltage reaches that high a value to know if it's backwards,
but do set expectations correctly about the duration of the test. You
probably do need to test to near the rated forward voltage; I just ran
a similar test on a 25V 10,000uF cap, using 25V and 1100 ohms, and
noted that it allows 10V reverse. Your mileage could vary--a lot.

Perhaps an even better way to test would be to use a current-limited
supply that will deliver, say, a constant 100mA until the voltage
reaches the preset voltage limit, say 25V. With 3.9e-2F and 100mA, the
charging rate will be about 2.5V/second. Ten seconds should get you
to close to 25V. If you reach 25V and the current drops to a low
level, you're OK, but beware the effects of dielectric absorption that
may result in noticable current (significant fraction of a mA) for
quite a long time (minutes).

Cheers,
Tom

.

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