Re: Dead MMC

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Winston,
           
On 26 Jan 2002, at 9:47, Tesla list wrote:

> 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.

So do I.

  I used 10 megaohm equalizing resistors on each cap, which all still
> test good, even on the failed caps.

The equalizing resistors have no effect whatsoever when the caps are 
being used with high frequency AC. The only thing they do is 
discharge the caps after use and offer some protection if the MMC is 
charged to some high DC voltage and held there. In this case, they 
counteract any imbalance in leakage currrents between capacitors.

  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

Three strings is nowhere near enough for serious operation. As 
always, you get what you pay for. A business-like cap costs business-
like prices.

Regards,
malcolm