On Fri, 14 Oct 2005 07:08:21 GMT Rick Frazier
wrote:

I was in high tech engineering (computers and peripherals) for 20 years,
and we never ran electrolytics at anywhere near their rated voltage.
Permissible margins for low voltage DC circuits were 50% or more
margin. Typical rule of thumb was 100% margin (a 10 volt cap running on
nominal 5 volt line, etc.). Even in an industry where every penny of
component cost was significant, submitting a design with electrolytics
running with as little 25% margin typically got a less than stellar
response during design reviews, to say the least.

Your margin would be determined by your tolerance for failure. If
you're making 10,000 units per year, each with 10 caps in them and
want to keep your cap related failures to less than, say, 10 per year,
then you need to be a lot more conservative than if you have a single
device with 10 caps in it that you want to keep your odds for failure
under 1% per year.

For most of the items I fix around here, I like to approach the
problem in a way that I can think of the solution as permanent, but
when you have a sample of just one, a 1% annual failure rate is about
as close to perfect as you'll ever get,

I'd say that for 450V caps, you're being much too conservative. We
have banks of 450V caps at work, each bank with about 200 caps in it.
They are used for energy storage and charged slowly and discharged
rapidly about every 5 minutes all day long. We have about 10 such
banks. We have occasional failures, maybe 1 cap every 3-4 years. We
don't think that's too bad.

I'm currently working on some upgrade banks which will have about 90
16,000 uF, 450V caps per bank. These will be run at 450V, and we'll
have 16 such banks in the second phase of the project. United
Chemi-Con doesn't seem to have any problem with this. We expect
occasional failures, but we also realize that anything else is just
being unrealistic.

C-D lists computer grade caps rated at 500 and 550 V, each with surge
ratings higher than that. United C-C does, too, but they admit that
their etched alum foil for those voltages is not as advanced as at
450V, so the energy density is not as high. This does not sound like
it would be a problem for you.

Another thing we noted with interest was that the C-D catalog said
that grading resistors were not necessary when installing caps in
series for higher voltages, especially if all the caps were from the
same batch.

If I were you, I would just buy the caps you need/want at 450V, and
then buy a couple of spares from the same batch. You would want to
reform these if you ever needed them, but they would give you the
necessary assurance that you'd be set for life.

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Jim Adney
Madison, WI 53711 USA
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