Re: Maxwell 31159 cap on eBay

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

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
 >
 >
 >