Aluminum Electrolytic Capacitor Construction.

Discussion in 'General Electronics Chat' started by nathan1972, Feb 6, 2015.

  1. nathan1972

    Thread Starter New Member

    Feb 22, 2011
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    Hello.

    I have a bit of experience with capacitors, but this question stumped me. In a basic aluminum electrolytic capacitor, it's construction consists of two sheets of foil (one with an oxide layer) separated by a tissue soaked with an electrolyte.

    The question I was asked is, what keeps the electrolyte from leaking around to both sheets of aluminum and shorting between the non-oxidized side of the anode and the cathode.

    It's probably a simple answer, but one that I don't seem to be able to come up with.

    Thanks in advance for the help.
     
  2. paulktreg

    Distinguished Member

    Jun 2, 2008
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    Perhaps soaked is the wrong word in that it's a liquid or gel electrolyte impregnated paper, it's there but not in a quantity that will "leak"?

    Perhaps the large leakage currents and ESR are a consequence of some minor leaking?
     
  3. ian field

    Distinguished Member

    Oct 27, 2012
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    The oxide is formed electrolytically by applying a current limited voltage across the plates - of the plate that has the oxide layer, any part of it that's exposed to the electrolyte gets "formed".

    The electrolyte is caustic, and the oxide layer gradually breaks down over time, aluminium electrolytics that have been stored for some time, may need to be re-formed, as the oxide will be much thinner - if not perforated in some places. This will cause at least high leakage if not practically a short. The forming voltage must be supplied by a current limited supply, the current must be low enough to avoid heating the capacitor - you gradually raise the applied voltage up to the rated voltage, usually over a 24h period.

    IME; if you set the current limit low enough, you can forget about constantly adjusting the voltage upwards - just give it free reign and it'll find its own level.
     
  4. paulktreg

    Distinguished Member

    Jun 2, 2008
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    ? What's that all about!
     
    Last edited: Feb 6, 2015
  5. nathan1972

    Thread Starter New Member

    Feb 22, 2011
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    Thanks for the help Paul. I was thinking along the same lines as you, I just didn't trust my opinion.
     
  6. ian field

    Distinguished Member

    Oct 27, 2012
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    ESR is series resistance - not parallel.
     
  7. alfacliff

    Well-Known Member

    Dec 13, 2013
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    thats the reason that electrolytics hae to be used at their "working" voltage. if a cap is rated too high a voltage than its used at, it deforms and fades away. trhey used to be marked in "wvdc" as well as max voltage rating.
     
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