Bob is right, and as a teacher told me, about electrolytics, that you ALSO
MUST consider DIODES in a supply, as in an earlier age, a item would have
bad diodes, you would shotgun them, only to see a problem develope in the
ELECTROLYTICS! (SHORT)! Turns out that that caps will derate their voltage
handleing capability
to the SUPPLY VOLTAGE, over time (see oxide thickness, discussion by BOB, it
diminishes over time, with lower voltage applied) ! , and that diodes build
up resistance, lowering their output voltage , over time. And, BTW, a
Electrolytic can be reformed to accept higher voltage, but the voltage must
be increased GRADUALLY, over time, say extra 20 volts/hour, until you get to
FULL RATED VOLTAGE. but to apply FULL STEAM all at once may lead to it
explodeing, or shorting!! Only places that over CAPACITANCE would come into
play would be in frequency passing circuits where a lower value would allow
a lower frequency responce to occur, or that a current inrush (I2xR ) would
exceed the capacity of supply xfmr to supply that current!
as info, Jim NN7K

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"Robert Grizzard" wrote in message
...
Zoran Brlecic wrote:


Bob wrote:

In general, electrolytic capacitors should be operated not terribly
much
below their ratings.

I am curious: where did you hear this and what is the rationale? I've
never heard anything similar before, although I suppose it is possible.

I tripped across the concept many years ago. Seems that electrolytic
caps are not created equal, said devices being dependent upon the plate
area and the reciprocal of the oxide layer thickness on the aluminum
electrode to supply the proper capacitance and on the oxide layer
thickness for the DC Working Volts rating. This oxide layer depends
upon the polarizing voltage for its maintenance, and the electrolytic
electrode is formulated to maintain the proper oxide thickness at the
capacitor's normal voltage. Insufficient voltage results in a thinner
than expected oxide layer, resulting in increased capacitance (not
normally a Bad Thing) and decreased breakdown voltage (which is a Bad
Thing). I don't know if it is possible for a derated cap to lose enough
oxide to drop its breakdown voltage below its normal working voltage, but
it would result in a dramatic failure if it ever did happen.

YMMV, IIRC, IANAL, and any other disclaimers one wishes to insert.
Perhaps there's a component engineer reading this group who could
speak to what I remember, or believe I remember, on this matter.

de kg7yy