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Capacitor polarity -- how to tell with confusing markings?


Don Bruder

Jan 1, 1970
larry moe 'n curly said:
In this picture of four electrolytic capacitors:

Point the first: Those look a lot more like tantalum caps than any
electrolytic I've ever encountered. Point the second: According to how
I learned to count, there are *FIVE* caps in that picture.
Which lead is the positive one for each cap?

Look for the "longer" lead when the dots are confusing. If there isn't
one obviously longer, look for the one that's "straightest" coming out
of the body of the cap - Notice how on the three right-most ones, the
marked lead is significantly more "looped" than the un-marked lead?

Which makes me think that the best first-guess for the second from the
left would be the rightmost lead. The leftmost cap I can't really tell -
It's cut off too short to try to "read" the leads. However, it *APPEARS*
that the leftmost lead is going to be the one that's "supposed to be"

John Fields

Jan 1, 1970
"larry moe 'n curly"

** Tantalums actually ......

Yes, but electrolytics nonetheless.

What 'electrolytic' denotes is that the dielectric is formed by an
electrolyte, just as it is in aluminum electrolytics. That's the
reason why electrolytics are polar; the electrolyte chemically
forms the non-conducting dielectric film on the 'plates' when
voltage is applied between them in one direction and removes it when
applied in the other direction, shorting out the plates.


Jan 1, 1970
larry said:
In this picture of four electrolytic capacitors:

Which lead is the positive one for each cap?

The black and white dots on the two capacitors on the left are actually

exactly in the middle of the leads.

Good morning, Dr. Howard, Dr. Fine, and Dr. Howard. The other posts
have given good advice. However, if all else fails, you can do a
leakage test like this:
| .------.
| | |
| | 1K 2W| 10K
| +| .-. ___
| 12V --- | |<-o--|___|--o--.
| - | | | | |
| | '-' | | o+
| | | | _ |
| | | | / \ | DUT
| | | '--( V )--'
| | | \_/ o-
| '------o----------------'
(created by AACircuit v1.28.6 beta 04/19/05

This test setup works on the concept that, if the Device Under Test has
the correct polarity in the fixture, leakage current will be very small
up to the rated voltage of the cap. However, if you plug in the cap
backwards, leakage current starts increasing rapidly once you get past
a couple of volts. The 10K resistor limits current to 1.2mA max, which
should keep the cap from damage, and also provides a means of measuring
current with the voltmeter.

Use a high impedance DVM set to the 20V range. Starting at 0V, connect
the DUT and slowly bring up the voltage at the wiper of the pot. Look
for the voltage across the 10K resistor to increase if the cap is
hooked up with the polarity reversed.

Be careful not to raise the applied voltage any higher than necessary
to see significant reverse leakage current across the 10K resistor (1V
or so). While the 10K resistor limits reverse current to no more than
1mA or so, tantalums generally don't like reverse voltage, so don't
crank up the pot any higher than necessary. Turn the pot voltage down
before removing the DUT.

If you get leakage current going both ways, the cap is bad, and should
be thrown away (thos caps look pretty ancient).

Don't try this without the 10K resistor, or you'll end your test by
explosively determining that the cap is _now_ bad.

For Duty And Humanity!

Phil Allison

Jan 1, 1970
"John Fields"
"Phil Allison"
Yes, but electrolytics nonetheless.

** But "tantalum " is the distinguishing NAME used in the trade.

The ones in the pic are "solid tantalum" ( aka "bead tantalum" ) types which
have a dry electrolyte ( manganese dioxide) unlike "wet" tantalums or
aluminium electrolytic capacitors.

.......... Phil