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Electrolytic Cap's Leakage & Voltage Division

Discussion in 'Electronic Basics' started by Blake, Feb 8, 2006.

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  1. Blake

    Blake Guest

    I recently read that the leakage current in an aluminum electrolytic
    capacitor will increase with higher applied voltge. This seems reasonable.
    In fact, I have a vague memory of having measured leakage current and
    finding that it rises sharply as the applied voltage nears the rated
    voltage.

    But I also remember reading that when connecting electrolytics in series,
    there is a danger of unequal voltage division which might overstress one of
    the caps.

    This seems to present a contradiction. If one capacitor takes more than its
    fair share of the voltage, wouldn't that cause an increase in leakage
    current and a subsequent reduction of the voltage?
     
  2. Phil Allison

    Phil Allison Guest

    "Blake"

    ** Course it does.


    ** The rule for connecting two high voltage electros is series is that they
    are both the same ( age, type, ratings, temp) plus the combined voltage is
    comfortably over (by say 25% ) the actual max DC voltage expected.

    The caps will then sort it out themselves.

    Ballast resistors are optional.



    .......... Phil
     
  3. Yes it does tend to do that.
    Yes, the danger can be very real.
    Correct. But the caps have to be fairly well matched and have a certain
    minimum voltage spec over the working voltage in order for them to
    "self balance". So you might get away with it in many cases, in other
    cases you could be asking for trouble in the long term, and in some
    cases ballast resistor will be essential. Good practice is to put a
    ballast resistor in parallel with each cap unless you have a specific
    reason not to. The required resistor value can be calculated by:

    R=(CapLeakRes * (MaxCapVolt-(Vrail/2)) / (Vrail-MaxCapVolt)) * 2

    Dave :)
     
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