Re: MMC cap bank

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

Original poster: "Crow Leader by way of Terry Fritz <teslalist-at-qwest-dot-net>" <tesla-at-lists.symmetric-dot-net>

Tesla list writes:
>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

Why is series, ad at a higher voltage? The failure of one will affect the 
voltage the others receive. You're going to need more than 5 caps to at one 
testing to declare a sound spec for the rest. I just had a new light bulb 
burnout after a few hours. I guess that means lightbulbs last 3 hours.
KEN
>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
>
>