Testing a capacitor for polarization

[sodderboy]

I have a big Elna can on a 70's era PCB, 2.2µF 200VDC, where it physically looks like an electrolytic with polarization (stripe at the bottom of the can), but the person totally down with the circuit says and the schematic shows a non-electrolytic cap.
Is there any way, short of reverse biasing it to see if it heats-up, to test if a mystery cap is polarized?
Mike

 

[tv]

I think that the "answering machine" is a 1N4007 in series with a 40W bulb, going straight from the mains into the CapUnderTest.

Perhaps you could manage to scare your neighbours shitless?
Post back with photos!

 

[JohnRoberts]

With non-polar film caps there will sometimes be a line on the body to indicate which lead is connected to the outer wrap. In high impedance circuits it is worthwhile, connecting the outer wrap to the lower impedance circuit node. 

If you believe it is non-polar, you can confirm that it doesn't breakdown at less than rated voltage in either polarity. 

JR

 

[abbey road d enfer]

Any markings on the cap?
Usually, polarized caps are clearly marked with a + and/or - sign.
A 2.2uF/200V electrolytic cap of the era should be no larger than 20mm diameter/ 40 mm long. You say big; how big is big?

 

[sodderboy]

It is just smaller than a chapstick in both dimentions, the traditional aluminum can with a plastic, not rubber, cap, and a thin blue line at the bottom of the can opposite the opening.
Mike

 

[Samuel Groner]

Is there any way, short of reverse biasing it to see if it heats-up, to test if a mystery cap is polarized?

Well you don't need to wait till it heats up--just measure leakage with both polarities. A couple of volts should be enough. Otherwise I'd say that a distortion test should be able to determine polarization--a polar cap will very likely show substantial amounts of even-order harmonics (along with some odd-order) while non-polar parts will have mostly odd-order harmonics. Put the 2.2 uF cap in a high- or low-pass filter configuration with say a 3k3 resistor and measure at 100 Hz/+20 dBu.

Samuel

 

[emrr]

Size alone suggests it's a film type, given the value, and lack of definite +/- marking.  I'd assume the line marks outer foil. 

 

[tv]

No "ka-boom" yet?

 

[sodderboy]

Nope.  I save boom for things that are designed to go boom.  The howitzer does not get cleaned and oiled until mid May.
Samuel has me on the correct path.  Thanks!
Mike

 

[CJ]

just use an ohm meter.

if you get the same ohms both ways, it is non polarized.

if you get a low reading one way, and a high reading the other way, it is polarized.

vary the range sw if you have a non auto range meter.


now do you know the "outsdie foil" test?

use a cap meter, lay the cap on a metal plate with the black lead hooked up.

then use the red lead on the cap leads. the most C means that the foil is right on the chassis.