Capacitor Polarity: Determining Polarity Visually & Beyond

[steve9983]

How can you determine the polarity an electrolytic capacitor (if it is indeterminable from a visual inspection)

 

[berkeman]

Use your DVM. Measure the resistance both ways -- you'll get basically an open circuit (or charges up to an open circuit) measurement when you match your +/- DVM leads to the +/- of the cap. You'll get a different reading when you measure it backwards, because you'll get a small leakage current and a couple of volts across the cap. I just tried it in the lab and got a negative resistance with two different DVMs -- I'm not sure where the negative sign is coming from, but whatever.

Look on the datasheet for electrolytic caps, and they'll list a reverse leakage current at some reverse bias. Always use a high impedance source (like a DVM, or a power supply with a 10K Ohm resistor) when doing this kind of measurement. Hook an electrolytic cap up backwards to a low impedance power supply, and you'd better be wearing safety glasses! Boom.

 

[dlgoff]

... I just tried it in the lab and got a negative resistance with two different DVMs -- I'm not sure where the negative sign is coming from, but whatever...

Did you discharge the cap after the first measument?

 

[berkeman]

Did you discharge the cap after the first measument?

Yeah, I tried that. Then tried a 2nd capacitor in the reverse polarity first. I'll have to figure this out sometime (no time now). Thanks, -Mike-

 

[Averagesupernova]

Yeah, I tried that. Then tried a 2nd capacitor in the reverse polarity first. I'll have to figure this out sometime (no time now). Thanks, -Mike-

What you are seeing I believe is caused by dielectric absorbtion. Try a circuit with a lower source impedance than what your DVM has in ohms range.

 

[frogdogbb]

It is possible that it is a bi-polar cap so orientation is not important. Unless it is really old polerised electros almost always have a stripe running vertical on the case or horizontal for an axial, indicating the negative side. The positive side always has a longer lead.

 

[berkeman]

It is possible that it is a bi-polar cap so orientation is not important. Unless it is really old polerised electros almost always have a stripe running vertical on the case or horizontal for an axial, indicating the negative side. The positive side always has a longer lead.

Not always. I've seen some mislabeled electrolytics. Yikes. Always wear your safety glasses when you power up a new circuit...

 

[rachmaninoff]

What happens when an electrolytic cap fails? Does it explode? Does the aluminum combust or oxidize? Are there any pictures of what happens?

 

[berkeman]

What happens when an electrolytic cap fails? Does it explode? Does the aluminum combust or oxidize? Are there any pictures of what happens?

Yeah, big pop/boom. I'm not sure of the mechanism, but I've been there when it happens. That's one of the reasons that electrolytic caps have scored metal tops -- to help aim the blowout.

Just for fun -- We have a poster on the wall in my lab, called the "Doctor Destructo" poster. We have blown up parts and pictures of blown up instruments pasted to the poster. We only have a couple blown up electrolytic caps taped to the poster, though. More caps would pull the poster off the wall...

 

[Matrax]

Polarized Electrolitic caps use a Electrolitic solute as the dielectric for the two internal metal plates. When powered up with correct polarity it forms a non-conductive oxide on the positive plate. When you reverse the connections, the dielectric oxide film breaks down and allows very high currents to flow thus heating the capasitor and forming a gas by boiling off the water in the electrolytic. The gas expands and blows the capasitor's casing.

Hint: Never use a polarized cap with pure AC without using a DC polarizing voltage across the capasitor.