Re: Dead MMC

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

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Winston,

"metalized polypropylene" suggests that the plates are super thin vacuum
deposited metal film.  This film cannot make good enough contact to the end
plates where they tend to fail when they get hit with like 100 amps.  You
really need "foil" which is a far thicker separate aluminum foil conductor
that can really take high currents.  If you tear one of the caps apart, you
may see burning around the end caps where they join the layers.  So one
needs high-current metal-foil polypropylene caps.  Unfortunately, there are
many kinds of caps of which only one works.  Also, beware that some
discount and surplus dealers have no idea as to the inner details of caps
and have been known to mess up in the descriptions especially on no-name
caps.  Also if they get a "bad batch" of caps, they often get sold to the
surplus houses.

Sorry it didn't work :-(  MMCs are wonderful, but the type of capacitor
used is very critical.  Maybe we should emphasis that a bit more...

http://hot-streamer-dot-com/TeslaCoils/MMCInfo/good-bad.txt

Cheers,

	Terry

At 10:17 PM 1/25/2002 -0800, you wrote:
>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
>
>
>
>Tesla list wrote:
>> 
>> Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-qwest-dot-net>" 
><Gary.Lau-at-compaq-dot-com>
>> 
>> Could you please elaborate on just how your failed cap was built?  This
>> would help others not make the same mistake, and help identify the
>> boundaries for safe MMC operation.  What kind of caps did you use, what
>> were the ratings, how many in series and how many strings, what kind of
>> power supply/NST, and how wide is your gap?
>> 
>> I DON'T recommend building PE caps, I've made two and both failed after a
>> very short life.  And a .1uF PE cap would be huge!
>> 
>> Gary Lau
>> MA, USA
>> 
>>  -----Original Message-----
>> From:   Tesla list [mailto:tesla-at-pupman-dot-com]
>> Sent:   Friday, January 25, 2002 10:16 AM
>> To:     tesla-at-pupman-dot-com
>> Subject:        Stupid Capacitors!
>> 
>> Original poster: "Marry Krutsch by way of Terry Fritz <twftesla-at-qwest-dot-net>"
>> <u236-at-earthlink-dot-net>
>> 
>> Hi everyone.
>> 
>>         Well, my MMC bank just failed catastrophicaly (burst into
flames) for
>> the 5th time, and my wooden primary form just carbon tracked.  I have
>> about 5 min. total run time on my coil, so this is going to get
>> expensive pretty soon!  All but one cap failed open circuit, so the lead
>> to plate junctions must not be good enough.  I don't have enough $$$ for
>> a Geek MMC, so I'll probably build a plate cap.  It's going to be big,
>> since I need 0.1 uF.  I can get PE sheet rather cheaply.  Which is best,
>> high or low density PE?  Any suggestions?
>> 
>> Now working feverishly,
>>         Winston
>