Good design philosophy tends to suggest that when selecting a capacitor for a given
application, that the working voltage of the capacitor be as much as twice the applied
voltage. This philosophy is often not practiced in consumer-type equipment, but is
considered in high reliability and military/aerospace designs.
I fail to see how the concept of "reforming capacitors" has any relevance in the instance
under discussion.
Roland F. Harriston, P.D.
****
Stefan Trethan wrote:
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That's just not plausible. The dielectric layer will always be sufficient
for the applied voltage, it is a self regulating process. It might not be
sufficient for the rated voltage if operated for very extended periods very
much below it, same as if it was stored without any voltage, but even that
condition can be rectified by reforming. There is just no mechanism that
causes a capacitor to fail sooner if operated at lower voltage. Just the
opposite, if you use a 400V capacitor in place of say a 25V one it will last
much much longer simply because the can is physically much larger which
always leads to a higher lifetime. I regularly replace capacitors in old
test gear with the largest can size that can be accommodated, which means a
higher voltage rating at the same capacity.
There might be things tech learn through experience, but there might
actually be things the capacitor manufacturers know for sure and techs may
just have misinterpreted.
Anyway this whole reforming issue isn't of all that much importance unless
you are dealing with decades old equipment. Even then it is usually not
needed. Only very old gear (with linear supplies) may be powered up with a
variac, and even there are dangers such as overloading components with the
low input voltage, regulator valves for example.
The jury is divided over the idea, and the only undisputed way to reform
capacitors in such situations is by feeding the internal supply rails with
external current limited supplies. Not worth worrying about in all but the
most exceptional circumstances.
ST
On Wed, Jan 26, 2011 at 11:08 PM, AlienRelics <alienrelics@... <mailto:alienrelics%40yahoo.com>> wrote:
Running an aluminum electrolytic capacitor at well under its rated
voltage
will cause the oxide layer to not reform correctly and the leakage
current
may rise. Then the heat can cause pressure that causes leaks around the
seals, drying out the electrolyte.
