RE: Dead MMC

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

Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Gary.Lau-at-compaq-dot-com>

Hi Winston:

Thanks for sharing this.  I think we learn more by failures than by
successes, I'm sorry it had to be yours.

In addition to the unknown nature of the film metalization noted by Terry,
caps also have a maximum dv/dt rating that basically determines the maximum
AC current that the cap can withstand.  If you look at a manufacturer's
data sheet, this dv/dt rating goes down drastically as the uF-rating of the
cap increases.  The 1.0 uF rating of the caps you used is at least 10 times
what is typically used in MMC's.  (I'm a little leery of the Geek's .15 uF
caps for this reason, but I guess you can't argue with success.)  I suspect
that your caps just couldn't support the high current demanded by this
application.

Regards, Gary Lau
MA, USA

Original poster: "Marry Krutsch by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<u236-at-earthlink-dot-net>

Hi everybody! (again)

	At Gary Lau's request, I will elaborate on the failures of my dying
MMC.

	My MMC is a 0.1 uF unit, using 90 630VDC 1uF caps that are supposedly
metalized polypropolene.  I use 30 in series, then put three 30 unit
banks in parallel for a grand total of 0.1 uf at 18KVDC.  I did most of
what you arent supposed to do in building an MMC.  My caps are square,
so I taped them together with fiberglass tape, then taped them to
phenolic strips.  There is no space between caps.  It's possible that
they flashed over, but with less than 500 volts on each cap, I doubt
it.  I used 10 megaohm equalizing resistors on each cap, which all still
test good, even on the failed caps.  All failures occured with my gap at
around 3/32", or about 10kV.  I use 2 GE copier xfmrs in series.  If
anyone has a C&H Sales catalog, they are in there.  Don't buy them! 
Four out of 5 failed open circuit, so the lead to plate junctiones can't
take the abuse.  These are not "big brand" caps.  They were made in
Portugal by some company I've never heard of.

	I should say that my MMC isn't really dead YET, but with 5 replacement
caps left out of my original ten, and 5 hard minutes run time on my
coil, I'm expecting about 3 minutes more out of my cap.  It then makes
the magical transformation from a $100 capacitor to a $100 paperweight
=:-0.

	These might work for a higher voltage, lower current system, but I sure
wouldn't buy any more myself.

Hope everone else's coiling is going better than mine,
	Winston