Slow and fast venting caps

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  • Tom41
    Badcaps Veteran
    • Oct 2005
    • 336
    • England
    #1

    Slow and fast venting caps

    What exactly is it that causes a cap to vent slow or fast? Most vented caps I've seen on motherboards have a domed top, and sometimes a small layer or mound of electrolyte on the top. That would seem to suggest that the cap failed gradually, slowly exuding the electrolyte after doming up.

    I also tried connecting a cap up to a DC supply, set to twice its rated voltage at reverse polarity. I expected it to dome up slowly and start oozing electrolyte - but imagine my shock when the cap literally exploded with a noise like a gunshot, and a cloud of smoke! That must have been a 'fast' vent.
    You know there's something wrong when you open your PC and it has vented Rubycons...
    Tags: None
    • Sparky
      High voltage
      • Jan 2007
      • 234
      • USA
      #2
      Re: Slow and fast venting caps

      Can we say boom?

      When a cap on a mobo vents it is still usually at its rated voltage at the proper polarity. Reversing polarity is a good way to vent one fast, and then doubling the voltage, well, you saw what happened

        Comment

        • PeteS in CA
          Badcaps Legend
          • Aug 2005
          • 3578
          • USA, Unsure of Planet
          #3
          Re: Slow and fast venting caps

          Venting is due to hydrogen gas released when the aluminum oxide layer is "formed". This could happen if the existing layer is too thin for the applied voltage - either an over-voltage condition or the existing layer has been attacked by the electrolyte. In the case of mobo caps, this forming is slow, as is the build-up of gas (and internal pressure). If you apply a significant over-voltage or a reverse voltage, the forming and gas production is rapid ... until it makes a loud noise.
          PeteS in CA

          Power Supplies should be boring: No loud noises, no bright flashes, and no bad smells.
          ****************************
          To kill personal responsibility, initiative or success, punish it by taxing it. To encourage irresponsibility, improvidence, dependence and failure, reward it by subsidizing it.
          ****************************

            Comment

            • bgavin
              Badcaps Legend
              • Jan 2007
              • 1355
              #4
              Re: Slow and fast venting caps

              Similar to a Noise Emitting Diode (NED) when driven over-voltage.

                Comment

                • gastorgrab
                  Badcaps Veteran
                  • May 2007
                  • 320
                  #5
                  Re: Slow and fast venting caps

                  Yeah, i got an uncle who caught some shrapnel back in 'Nam when a grenade 'vented' nearby.


                  .

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