Re: MMC voltage ratings

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

Original poster: "Jason Petrou by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jasonp-at-btinternet-dot-com>

Jonathon,

I find that a resonant tank cap works very well for a static gap, as long as
the spacing isnt too wide =8-) If you dont go for resonant cap (21nF) then
go for slightly smaller than resonant. That way, the caps will reach their
peak voltage and the gap will fire much more reliably than if the caps were
LTR. I tried this, and STR or resonant works much better than my LTR cap.
However I cant stress enough the need for MOVS and a safety gap when using a
resonant cap.

> I have 22 caps, .15uf/2000VDC. The Geek site says 2 strings of 10 will be
> ok, this would be .03uf/20kv. Does this have an adequate safety margin?
No, not really. What I would do is to set up 2 strings of 15 caps, so you
have a holdoff of 30KVDC... so you've got the right idea. As you increase
the frequency at which your circuit resonates it decreases the ability of
the dielectric in the caps to stop arcing over. Basically when a cap is
rated for 2KVDC, this means at DC frequency (i.e. pure DC) they will
withstand 2KV. However when the frequency starts to get really high like the
200KHz that usually runs small coils, the ability of the dielectric in the
caps to hold off the voltage is vastly decreased. For this reason you can
use a 10KV cap on a 10KV transformer to smooth the output of say an HV
rectifier, but when it comes to high frequency applications such as TCs you
really need to push up the voltage rating of the caps.

> I could go closer to resonant and get 8 more
> caps, and have 2 strings of 15 for .02uf/30kv. Is .02 close enough to
.0212
> (the value listed as resonant) to be resonant?
Anyway, got sidetracked :) build each MMC string to 30KV, which should give
you 10nF strings. Put 2 of these in series, and you will get a slightly STR
cap, which will reliably fire on a static system. I would reccommend
building a rotary, and then moving to a 2x LTR cap, at say 240 breaks.

IMHO ;)

Best regards,
Jason