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Thread: Decoupling App Note
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picon face BY : Russell McMahon email (remove spam text)



From: Morgan Olsson <KILLspammorgans.rtspamKILLspamTELIA.COM>


>Russell McMahon wrote:
>>Ecaps die quicker when run markedly BELOW their rated voltage.
>
>I have heard differing opinions about that, but most say electrilytic caps
live longer when operating below rated voltage, epecially if operating
temperature is high.
>Any links or explanations to why they would break at lower voltage?
>What about zero voltage?  I have some 30 year old never electrified caps
and I checked they are still OK...


A "wet" aluminium (actually vaguely damp) electrolytic capacitor separates
its two aluminium plates by a VERY thin layer of oxide purposefully built on
one electrode. The thickness depends on (and establishes) the rated voltage.
AFAIU running below rated voltage tends to de-form the layer.

Running a cap at zero voltage is the same as having it off - if you do this
at elevated temperature it will indeed die sooner tyhan if it is on as the
applied voltage maintains the layer in the presence of temperature. I am not
acquainted with the fine detailo of the chemistry but a look at
manufacturers' spec sheets re storage at high temperatures will confirm the
above.

>I heard an guru once say that lagre electrolytics should be powered up
slowly first time, and then be held at nominal voltage.  As i understood it,
it would be because it could heal eventual cracks in the aluminium oxide.


Especially true of "good old daze" caps. The oxide layer tends to dissapear
with time and is 'reformed" by the application of lower voltage (actually
full voltage with a resistor to limit current to a very small value) for a
period. This does not seem to be a problem with modern caps of substantial
size but again, I don't know what chemical magic they have worked to make it
so.


Russell

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