Re: PC board cap is a no-go

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

Original poster: "Kurt Schraner by way of Terry Fritz <twftesla-at-qwest-dot-net>" <k.schraner-at-datacomm.ch>

Hi Shad,

I had similar experience as you, regarding the corona with a PC board
cap, however my conclusion differs from yours, regarding the "no-go":
it seems very important, not to try the finished PC boards, without
having submerged them in oil. In my earlier TC trials I buildt a 18nF
cap, which, despite me not having observed the above, after submerging
it in transformer oil, did quite a good job, till I began to overvolt
it by the typical beginner's mistake of overvolting it badly (~0.8"
safety gap =8-( !) in resonant mode. See some pic's and the whole
story (sorry, partly in german) at my site:

http://home.datacomm.ch/k.schraner/pcb_cap.htm

I "think", if MMC's were not much easier, I would give this kind of
cap another trial, if the situation would arise. BTW: why is it not
possible to design an MMC with sufficient current capability for your
application?

Cheers,
        Kurt Schraner

Tesla list wrote:
> 
> Original poster: "Sundog by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<sundog-at-timeship-dot-net>
> 
> Hi All,
> 
>    I was looking at etching the copper from PC boards and using them as
> elements of a flat stacked cap for the GU10A tube coil.  I *finally* got
> the chance today to set up a test.  12kv NST,  fullwave rectifier, and a
> variac on the NST input.
> 
>   It failed pretty miserably.  There was a lot of corona losses at the
> edges of the plates, even at ~11kvDC.  Puncture resistance was okay, but
> the sustained corona quickly weakened the dielectric enough for it to punch
> through after about 2 minutes of sitting at ~13kv DC (that'd be pretty hard
> on the GU10A).
> 
>    Yes, I could stack 2 plates together in series to halve the voltage
> across each one, but that would immediately quadruple the space needed, and
> also the cost.  Where before I'd need about 30 plates, I'd now need
> 120.  At around $4 each, it's now real expen$ive.
> 
>   So I need an alternate plan here guys.  I'm up the creek without a
> suitable cap.  MMC's are out of the question.   Too much capacitance, not
> enough current handling ability.  transmitting caps are hard to find and
> expensive, and I don't know of any home-made cap that will take the raw
> current (much higher sustained current than a TC can deliver).   I get the
> feeling LDPE and aluminum flashing caps would get too hot and disentegrate
> internally, leading to a spectacular and probably fatal (to the tube)
> failure.   I'm open to suggestions.
> 
> Thanks all!
> 
> Shad Henderson
> G3-1203