"NoSPAM" wrote in message news:... "William
Sommerwerck"
wrote in message
. ..
In theory, if the cap has a voltage rating much higher than the actual
circuit voltage (say, a 100V cap in a 50V DC circuit), the capacitor is
more
likely to deform.

That's the theory. I've never seen any systematic study of it. Anyone
know
"fer shure"?


Yes, the 100 volt electrolytic will eventually become a 50 volt
electrolytic
(or whatever voltage is actually impressed on it). The aluminum oxide film
that is the actual dielectric of the capacitor will hydrolyze eventually
dissolving the oxide back into the electrolyte. This is why relatively new
electrolytic capacitors sometimes need to be reformed. In most cases, this
is seen as an elevated leakage current in the first few hours of operation.

In the old days when vacuum tube rectifiers were in vogue, power supply
electrolytics would be subjected to a higher voltage than their normal
operating voltage until all the tubes were "warmed up". This is why high
voltage electrolytics often had a surge voltage rating high enough to
prevent breakdown during those few seconds that the supply was not
providing
current.

I wrote a number of posts about 9 to 10 years ago on electrolytic
capacitors
and their chemistry. Most of these can be found archived at
http://yarchive.net/electr/electrolytic_caps.html.

Dr. Barry L. Ornitz WA4VZQ