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Thought on MMCs...
Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
In October of 1998, Chip blew up his Halloween coil while it was throwing
sparks of the roof of his house (Chip is into that :-)). His commercial
caps blew up.... Twice over...
That is when I got onto this "there has to be a better cap" craze...
Chip's nice commercial cap just was not enough for his powerful coil.
I can't believe that was only ~2.5 years ago =:o Since then, MMC cap
technology has virtually dominated and obsoleted all other Tesla coil caps.
Since many today did not live through those "few" years, I thought I would
summarize the situation...
Many people have tried stringing together smaller caps to make bigger Tesla
caps over the years. Those caps worked on smaller coils but the caps of
the "old days" just were not good enough. However, many capacitor
manufacturers were working on low-loss fault-tolerant caps in the early
90's and they actually figured it out!! Polypropylene with aluminum foil
electrodes plus a little engineering to provide high-volume machine
manufacturing was the real key. Very low loss caps at substantial voltages
could now be made. However, those little caps where only 2000 volts at
around 0.050uF and they were expensive. You could string them together for
a Tesla coil cap but if you needed real power, the big high voltage 35kV
0.50uF caps were still the way to go. But three things changed everything.
1. Everyone wanted them. All the capacitor manufacturers on Earth raced
to retool to make high RMS current, high voltage, small poly caps. WIMA
was always at the front but the Japanese could make very large quantities
with flawless quality. Late in 99 there was a world shortage of these caps
(Terry stocked up :-)). So giant manufacturing efforts were unleashed to
produce them. It was only a matter of time before the availability and
price problems went away... Today the Geeks sell 2000V 0.150uF 13 amp RMS
caps for $2.25... That beats even salt water caps in many cases...
2. Self healing. Poly caps have a very thin metal layer plate (millionths
of an inch) in them. This layer will vaporize and act like a fuse if the
caps arc over. The metal film will vaporize and clear the shorted area of
metal and the cap is virtually as good as new again. A whole lot of effort
went into perfecting this but it ended up working better than anyone had
imagined. The old caps were dead (exploded and leaking oil) if the voltage
arced the plates over only "once". But, these new caps could arc over
perhaps a million times and still perform!! The plates can literally be
blown into Swiss cheese or "machine gunned" with arc overs and still be
perfectly good. For Tesla coilers, this was a tremendous advantage. Tesla
coil caps have a habit of being abused :-))) A cap that could survive an
over voltaged one, two, three thousand times simply eliminated the concern
about building caps to 2X or 3X the voltage. In fact many MMCs are now
built at or well exceeding the rated voltage since they just self heal...
I guess "I" started that one :-)) But the new "cheap" MMC caps make the
3. Design. At the same time, powerful tools were emerging to better
understand what voltages, currents, and operating conditions Tesla coil
caps needed to be "designed" to. All the wild guess work was being
replaced by solid theory and real numbers. We were now able to "know" the
peak voltage, RMS current, power dissipation, heat generation... of Tesla
coil caps. We had real numbers that caps had to meet to work. MMCs and
those little caps were perfect. One could buy 100 of them and test them to
destruction to find exactly what they could take. That was not possible
with the $700+ plus, hand made, wildly varying,... commercial caps. A
little MMC cap could be analyzed in high detail and matched to a specific
Tesla coiling application.
As far as I know, Reinhard Walter Buchner was the first person to put
together a modern version working MMC in the simmer of 98. I jumped in
(too late :-)) in November... A lot of people contributed to the
"invention of the MMC... I always placed most of the thanks to the
engineers and scientists at the capacitor companies that really made those
poly caps work not only for coilers but for many other application as well.
Those people really worked the long hours figuring out how to make a MMC
capacitor. All we did was hook them together :-)) It is fun to know that
those same cap manufacturing folks are "spying" on us these days since we
exceeded the "art" a bit beyond what they knew :-)) Tesla coils are the
nastiest application for capacitors out there...
I remember more than one night sitting at the keyboard with may hands
shaking as I realized that the day's experiments were showing that this MMC
thing was really BIG :-)) I remember reading of people trying to find
sheet poly for their rolled oil caps thinking "Oh! That guy is wasting his
time... But still to early..." I know a few of you were left with half
finished poly caps that were started with great hope but fell "obsolete"
before your eyes. Sorry bout that :-)))
Chip's MMC he made with 200+ MMC caps didn't blow up the next Halloween.
Homemade oil/poly caps are... gone... as well as every other Tesla cap with
the exception of the Salt water caps that are so easy for the new
coilers... Some cool versions like the party cup cap are still very active...
I guess I still can't belive it has only been 2.5 years... Seems like
20... So how much does a 200nF 20kV LTR pig cap cost these days?
Ten deep, 13 across, 20kV (takes 40kV booboos ;-)), 130 caps, 170 amps RMS,
indestructible... - $300.00