Re: Maxwell 31159 cap on eBay

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Ed Phillips <evp-at-pacbell-dot-net> 

Tesla list wrote:
 >
 > Original poster: "Dr. Resonance" <resonance-at-jvlnet-dot-com>
 >
 > Your assumption is correct --- they are built differently.  The difference
 > is a Maxwell trade secret that they will not reveal.
 >
 > Dr. Resonance
 >  >
 >  > "If a HV pulse cap is not rated for high % voltage reversals, there is
 >  > significant corona formation at the edge of the foils.  This corona and
 >  > surface tracking begin to heat up the local dielectric area (di/dt) and
 >  > will
 >  > eventually produce enough heat to begin melting through the dielectric.
 >  >
 >  > Dielectric breakdown at the edge of the foil is the number one killer of
 >  > HV
 >  > pulse duty caps (according to Maxwell Sr. Engineer Bob Cooper).  Second
 >  > on
 >  > the list are small voids in the dielectric material, which also produce
 >  > corona in the voids due to the different dielectric constant in the void
 >  > material, and, again, leading to local intense heating effects that
 >  > damage
 >  > the dielectric.
 >  >
 >  > Pulse caps not rated for high voltage reversals literally "can't take
 >  > the
 >  > heat".
 >  >
 >  > Dr. Resonance"
 >  >
 >  > 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

	OK - trade secrets can be very important.  Presumably the capacitors
with smaller reverse voltage ratings are built that way because they can
be sold for a lower price and/or smaller in size.  Any of that correct?

Ed