Identifying polarity of axial electrolytic capacitor leads

Discussion in 'General Electronics Chat' started by DaveH, Mar 21, 2009.

  1. DaveH

    Thread Starter Active Member

    Jan 1, 2009
    Hi folks,

    I have a a 100uF 50v capacitor, with a typical electrolytic look to it, photo attached (the can is about 1 inch long).

    I'm starting to wonder if this really is a polarized electrolytic or polarized capacitor of any kind.

    The leads are exactly the same length.
    The are no markings on the end or side of the can to indicated which is +ve.
    There is no indentation around the can on just one side, but they are on both sides!

    In other words all 3 of conventions used to distinguish which lead is +ve are not there.

    Here's a photo (which I've cropped to exclude the embarrassing melting of my breadboard when I connected a UA741 circuit wrongly and it caught fire, I was trying to make an oscillator but created a fantastic incendiary device, the design was correct but I connected the balanced supply rails the wrong way round on the board)
  2. SgtWookie


    Jul 17, 2007
    Can't read the markings, but does it have "N.P." or "NP" on it?
    If so, those are non-polarized electrolytic caps.

    Don't feel bad about roasting the 741. I managed to roast a quad 741 (LM358) a few months back the same way you did. Quite a pyrotechnic display with a blue flame jetting out one side. Toasted a spot on my breadboard, too. :rolleyes:

    Try using a newer opamp, like an LF353 or a TL082. They're about the same price as a 741, just as available, but have much higher bandwidth.
  3. thatoneguy


    Feb 19, 2009
    The two grooves on each side look like many of the non-polarized electrolytics I've seen, and the right two characters repeated across the top row appear to be "NP".

    If you cannot find that exact value, you can use two 220uF 50V polarized caps, and connect their - sides together, then you have a 110uF non-polarized.

    I also have a breadboard with shrunken holes and dark spots, it happens to everybody.
  4. DaveH

    Thread Starter Active Member

    Jan 1, 2009
    I was looking for "NP" as you've both suggested (that was a very bad photo taken with my cellphone). Actually the top row of marking say "C E - B.P" and that is just repeated across the cover material.

    It has to be non polarized as you say, so that means you can connect it anyway and the electrolyte won't break down. It's also interesting the suggestion about connecting polarized caps together to make the whole thing non polarized. Perhaps that's how non polarized electrolytics are made internally.

    As for my breadboard, I feel much better now that you've said that - it's obviously the mark of a real professional engineer.