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Capacitor Polarity

  1. Nov 8, 2005 #1
    How can you determine the polarity an electrolytic capacitor (if it is indeterminable from a visual inspection)
     
  2. jcsd
  3. Nov 8, 2005 #2

    berkeman

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    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.
     
  4. Nov 8, 2005 #3

    dlgoff

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    Did you discharge the cap after the first measument?
     
  5. Nov 8, 2005 #4

    berkeman

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    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-
     
  6. Nov 8, 2005 #5
    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.
     
  7. Nov 12, 2005 #6
    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.
     
  8. Nov 12, 2005 #7

    berkeman

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    Not always. I've seen some mislabeled electrolytics. Yikes. Always wear your safety glasses when you power up a new circuit...:eek:
     
  9. Nov 12, 2005 #8
    What happens when an electrolytic cap fails? Does it explode? Does the aluminum combust or oxidize? Are there any pictures of what happens?
     
  10. Nov 12, 2005 #9

    berkeman

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    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....
     
    Last edited: Nov 12, 2005
  11. Nov 27, 2005 #10
    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.
     
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