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Re: Re. Cheap 20nF 20kV pulse caps - If it would work??
Tesla List wrote:
>
> Original Poster: Gary Lau 31-Oct-1998 1731 <lau-at-hdecad.ENET.dec-dot-com>
>
> >Original Poster: Terry Fritz <terryf-at-verinet-dot-com>
> >
> >Hi All,
> >
> >I was reading through my catalog looking at caps and a thought occurred to
> >me. I would like to know if this would work.
> >
> >I can get 630WVDC 0.10uF polypropylene film capacitors for $0.43 each.
> >They are designed for current pulse, high frequency, low inductance
> >applications although not quite what we have in mind ;-) They are
> >Panasonic ECQP6104JU types. They are about 1 x 1 x 0.5 inch in size.
> >
> >What I had in mind is this. If I string 8 parallel rows of 40 caps in
> >series (320 caps) I would get one large cap rated at 25kVDC (18kVAC) at a
> >value of 20nF. It would cost $138 plus a container, oil, and a bunch of 1M
> >ohm balance resistors (I could put 40 transorbs in also to protect the caps
> >for not too much more money). Perhaps it would be ok left in free air??
> >It wouldn't have the 3X rating we like but these are commercial caps with
> >the usual 5 seconds at 250% over voltage rating. Each string may see
> >around 30 amps peak during a firing pulse. It would be interesting in that
> >each cap or string could be easily replaced if something blew.
>
> Hi Terry:
>
> Sounds like "Many dry caps in series" to me ;-)
>
> I can't comment on how well it would work, but I did have a question that
> may be applicable to my proposed 20 section cap. If each of the series
> caps are reasonably close in value, why is it necessary to use balance
> resistors?
>
> Good luck and regards,
> Gary Lau
> Waltham, MA USA
Gary and Terry,
For Tesla Coil application, it isn't. Equalizing resistors are necessary
if the series of capacitors are subjected to extended operation at HVDC.
Each cap may have significant variation in leakage resistance, and under
extended DC excitation, the voltage distribution can be quite uneven,
particularly if the capacitors came from different manufacturing runs.
Adding equalizing resistors prevents this from happening - the large
value equalizing resistors are still much lower than the leakage
resistance of the caps, swamping out the unequal leakage resistances.
Because of the very long effective Rleakage*C time contants, this effect
only shows after extended DC operation. For Tesla Coil operation, each
cap have about the same equivalent series resistance (ESR) so that tank
current heating effects are shared relatively equally. Using identical
caps from the same vendor should pretty much guarantee this as long as
the peak currents are not extreme.
Offhand, I don't see any real problem in going with the proposed design
approach for Gary's homebrew or Terry's commercial series cap, since the
voltage stresses should be below the point of internal corona formation.
Terry's approach has the added benefit of sharing the current stress,
and that a cascade failure would be constrained only to a single chain.
And Terry's design should not require an oil bath for cooling, since 320
individual caps would provide excellent power dissipation to free air.
Good luck to both of you!
-- Bert --