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Re: Maxwell 31159 cap on eBay

Original poster: "Jim Lux" <jimlux-at-earthlink-dot-net> 

 > I understand the breakdown phenomena and also the problem with corona
 > in voids under AC operation, but don't understand how the construction
 > of the "high voltage reversal" capacitors differs from the "low voltage
 > reversal" ones.  Are they built differently or just screened?  When I
 > was at Hughes Aircraft we had a group building special capacitors for
 > radar modulator pulse-forming networks (where dV/dT can exceed 10^7
 > volts/sec and operation is at several thousand pps) and they used to
 > screen capacitors after construction by applying a high AC voltage and
 > listening for radio noise as a symptom of corona in unfilled voids.  In
 > this case they were pitched out because the construction techniques
 > including vacuum impregnation were the best they knew about.  Does
 > Maxwell do something like this but salvages the rejects by issuing them
 > under a different part number?
 > Ed

I'm going to guess that the big difference is just in the derating.  If you
look at the physical size of the high reprate or high reversal capacitors,
they are larger by just the amount that you'd expect from the derating
formulas in their conference papers.  That is, they would build the
capacitor as a 50kV cap for 25 kV use, getting a huge increase in life from
the 7.5 exponent.  Then, the decrease in life from the reversal (which I
recall is in the 1.5 power range) is less important.

The other thing to contemplate is what a "shot" is, in the context of lots
of reversal.  If you figure that the life is a "charge/discharge" cycle,
then a critically damped circuit (zero reversal) gives you one
charge/discharge cycle per shot.  A 20% VR is almost the same (because the
second half cycle is such lower voltage, and that 7.5 power factor comes
into play) as no cycle, life wise. At 90% VR, the succeeding half cycles are
at almost full voltage, and probably count as "shots".

This is probably where the exponent for Q in the life equation comes in: as
some sort of combination of the exponential decay raised to the voltage/life
power factor.