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Re: Maxwell 31159 cap on eBay
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.
> "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
> eventually produce enough heat to begin melting through the dielectric.
> Dielectric breakdown at the edge of the foil is the number one killer of
> pulse duty caps (according to Maxwell Sr. Engineer Bob Cooper). Second
> 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
> the dielectric.
> Pulse caps not rated for high voltage reversals literally "can't take
> 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?