Re: MMC cap bank

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

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

Hi DC,

At 09:01 PM 7/3/2003 -0500, you wrote:
>My info comes direct from Beau Meskin, Pres. of PCI in Chicago, who has
>visited my lab and commented on the MMC project.  He said they would work
>for a while but would ultimately fail due to excessive corona at the caps
>edge plate surface.  He said the only way to prevent this from occuring is
>to immerse them in oil, usually silicon oil, which is the way all of their
>commercial caps are mfg.
......................

>MMC's and Terry's work with them is terrific.  I only meant to point out
>that failure would be a probable mode when operated as the poster posted on
>the Tesla List.  Running at DC ratings in a non-oil cap just won't work for
>extended periods of time.
>
>Dr. Resonance

When a poly cap gets more than about 320VAC across the plates (in air), 
ionization starts.  This well known "ion inception voltage" is what 
controls the AC rating.  In poly caps, there are actually two poly layers 
in series so the voltage is doubled to ~630VAC.  You will note that 
practically all poly caps have an AC voltage rating limit of ~630VAC  It is 
just a fixed limit.

Above the ion inception voltage, the dielectric gets a real pretty blue 
glow as stray ions bombard the dielectric.  In time, the dielectric gets a 
rainbow hue to it as the long poly molecules get broken down by the 
ions.  You can slightly see this when an old well used MMC cap is taken 
apart.  Of course, this effect does not occur under DC conditions.  At DC, 
pure dielectric punch through limits the voltage.

So why do MMC work so well?  "Time".  It takes a long time for the ions to 
cut through the poly.  As far as I can tell, we are looking at about 1000 
hours.  Far too short for most commercial applications needing 10's of 
years of life, but a lifetime to most Tesla coils.  There are all kinds of 
equations and guides, but one really has to test a given cap for ion 
failure time.  A lot depends on the thickness and purity of the dielectric.

Commercial caps cannot take a single over voltage event.  You have to rate 
them high in voltage since one hit and they are gone.  MMCs simply don't 
have that problem, so we rate their voltage right to the limit (and well 
beyond).  Josh's filter with the "poped" poly caps was pretty cool.  All 
the case plastic got blown off from the dielectric breaking down.  But they 
all self healed and still work fine!!  MMC caps can (and have) taken 100's 
of thousands of hits with little real harm.  After a while they look pretty 
bad and the dielectrics look like Swiss cheese...  but they keep on going.

Since Geek Group caps are so common, a simple test would be to string say 5 
of them together and run 7.07kV across them from an NST (be sure the cap 
value is not resonant with a given NST!).  Then just sit and wait... to see 
how long it takes for them to fail.  One has to be careful since it may 
burn one's house down if they flame out when your not there and there is a 
big electrical hazard and all that.  But that test would pretty well define 
the lifetime.  It is interesting to note that at first they will just 
self-heal.  But eventually, they will obviously go "bad".  As far as I 
know, no MMC has ever failed from ionization.  Just to hard to get 1000+ 
hours of run time on a coil ;-))  Perhaps your coils get run much more, so 
you may have to worry far more than most.  Of course, you buy caps most of 
us can't afford too ;-))

I am not sure if Mark Broker is looking for a science experiment ;-) but 
maybe the Geek group could run this test safely.  It would be interesting 
to really know...

Of course, MMC caps can be configured to any value to match one's NST, are 
easy and fairly cheap to get, well known, "almost" fool proof ;-)), self 
heal if they get "hit", easy to make....  For the average home coiler, they 
are just about the only way to go. I don't know how many half finished 
rolled poly oil cap projects got dropped when MMC caps hit the scene, but 
it was a lot...  Very early on in the MMC thing, rolled oil caps became 
extinct from Tesla coiling...  So MMCs may not be ready for 1000 year space 
duty, but considering what we used to do ;-)))  I don't think anybody has 
made a rolled cap for about two years now unless they didn't know about MMCs.

One big deal too is the MMC have those bleeder resistors.  That has saved a 
lot of our rear ends ;-)))

Of course, the commercial cap makers don't sell to "us" for obvious 
reasons.  I remember the great "stir" that caused as people worried where 
they would get good caps.  I wonder if the commercial guys noticed that 
they got "designed out" :o))  Cornell-Dubilier sure noticed when they got 
"designed in" :-))  DigiKey sure notice as thousands and thousands of those 
0.056uF Panasonic caps kept getting ordered...  I remember the great gasp 
we all took when they went on back order for four months (I ordered the 
last 2000 and had the market cornered :o)))...  I think the valued used on 
the "terry filter" is still on intermittent back order to this day.  I 
think caps are just about the only consistent thing coilers "will" spend 
real money on...

I guess I'll stop...  I still have not quite gotten out of the mode of 
"defending" MMC's...  We won that one a long time ago :o)))

BTW - "Terry" did not invent the MMC, I just worked a lot on them.  Many 
many others deserve most of the credit there.

Cheers,

         Terry