Need help identifying polarity of capacitor

Discussion in 'General Electronics Chat' started by jut, Sep 10, 2007.

  1. jut

    Thread Starter Senior Member

    Aug 25, 2007
    224
    2
    I bought this capacitor for an experiment, but I'm not 100% sure of it's polarity. One lead is long, while the other is rather short. The "NEG" marking on the cap doesn't tell me that much because it's an axial cap.

    I posted a picture below:


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  2. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    Your pic ap007 shows the long lead is connected to the can. That is the negative lead.
     
  3. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
    The small indented ring at the end with the rubber cap is usually positive, the case is negative. In most cases if it were non-polarized there would be a ring at each end. If it's non-polarized no problem, if it were designed by a psychotic and the polarity is reversed from standard it will fail. Since the lead is cut it's either pulled from a circuit or it was cut with the intention of installing in a circuit board.

    IMHO it is incompetently marked, modern parts would be more explicit. That appears to be a surplus military part and it's common for some parts to be badly marked.
     
  4. jut

    Thread Starter Senior Member

    Aug 25, 2007
    224
    2
    Excellent, thank you both for the valuable information... and correct information, as I was able to use this cap in my test circuit and it didn't blow up in my face -- I had a big plastic cup over the cap and safety glasses on, just in case.
     
  5. Eduard Munteanu

    Active Member

    Sep 1, 2007
    86
    0
    Isn't it safe to test such a capacitor with a multi-meter? It should show good conduction if the capacitor is reverse-biased. Would those low currents degrade it at all?
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    If it's leaky enough to show low ohms in reverse, it's a goner.

    Be aware that the seal may be imperfect. If so, the electrolyte has dried, and the cap will fail. Make sure you know the voltage rating and stay well below it if experimenting.
     
  7. Eduard Munteanu

    Active Member

    Sep 1, 2007
    86
    0
    Well, it should show some conduction because electrolysis occurs in reverse-biased polarized capacitors. On the other hand, if it's forward-biased, it's resistance should be too high to measure. My question/doubt was whether or not any significant and/or irreversible damage occurs to the capacitor (electrolysis is somewhat reversible AFAIK).
     
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