[time-nuts] Replacing electrolytics - any disadvantages of high temp ones?

[Chuck Harris]

Dr. David Kirkby wrote:

>> I think the "usual" thumbnail calculation still work for caps if you can
>> reduce the temperature by 20 degrees they will last at least 4 times as
>> long. That is an activation energy (Arrhenius eqn) of about 1ev. I also
>> believe though I cant quote that they are best run at about 75% of their
>> specified working voltage. I have always wonderd about this but it would
>> seem to be a mistake to have too low a voltage on electrolytics ...maybe
>> something to do with the strength of the instulating layer formed.
>
> Yes, I have heard this before about not using them at too low a voltage.
> I don't know whether its an old-wives tale, or if there is any truth to it.

It is discussed in ITT's Reference Data for Radio Engineers.  It certainly
was true at one time.... not necessarily anymore, though.  In the old days,
the oxide layer that formed the dielectric was mostly formed in place by
applying a controlled current to the capacitor at a voltage above the intended
operating voltage.  Now, the aluminum electrode foil is anodized before the
capacitor is assembled, and the electrolytes are specifically formulated to
not damage the oxide layer.... and yet, I find that some electrolytic
capacitors that have been run at lower than normal voltage improve markedly
when "reformed" by applying  rated voltage through a 10K resistor for a
couple of hours.

-Chuck Harris