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Re: Maxwell 31159 cap on eBay

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

"Ed and all,

Pulse capacitors are constructed with a number of equally sized sections
connected in series or series-parallel. During the design phase, a
sufficient number of sections is used such that each section will "see"
worst case voltage in the range of 3500-5000 volts/section. This reduces
the peak voltage stress seen (both at the edges of the metallization and
within any voids) below the point where corona normally forms - called
"corona inception voltage". Capacitors that are designed to withstand
voltage reversals are constructed with more individual sections than
simpler DC or lower percent voltage reversal capacitors. A capacitor
for 80% voltage reversal (Q~7 - highly oscillatory) is actually
with a dielectric system that can withstand 180% of the rated
DC voltage. For example, compare the x-ray of the 15 KVAC Plastic
Capacitors BNZ series pulse cap (20 sections in series) with the
80 kV LN series cap with only 8 sections in series in Mark
recent article, "X-Ray Pictures of High Voltage Capacitors".

Pulse capacitors designed to endure severe voltage reversals sometimes
a layer of oil-saturated kraft paper between the foil and the
(PP) film. The oil-soaked kraft paper acts as a higher dielectric
(k ~ 4-6) buffer zone between the foil and the film, helping to even out
the e-field stresses seen by the film. Because of the lower dielectric
constant of the PP film layer (~2), the film actually sees most of the
voltage stress in the cap. The oil-kraft paper layer is more resistant
corona damage (if it should occur during accidental overvoltage
making for a more robust dielectric system during highly oscillatory
discharges. The combination of a thicker dielectric system and more
capacitor rolls/cap also mean that caps rated for high Q discharge duty
will be significantly larger than their DC-rated or low % reversal

Best regards,

-- Bert --"

	Thanks for the very clear answer.  The last sentence really had the
dope I was wondering about.