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Re: Dead MMC



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

Hi Robert,

Of course, a 15kV RMS transformer has a peak and firing voltage of SQRT(2)
(1.414) times that as you state for a voltage of about 21,000 volts
peak/firing.

However, MMC capacitors can be run well over their "AC" rating.  A typical
polypropylene MMC cap may have an AC rating of only 630VAC but a DC rating
of 2000 volts.  We have found that you CAN use the DC rating for MMC caps
in Tesla coil primaries (in fact you can considerably exceed it as in
EMMCs).  I wound not worry at all with using ten 630VAC 2000VDC caps with a
15kVRMS NST.  The AC rating is limited due to the ion inception voltage
causing corona within the capacitor.  However, in our use, that is not a
concern.

I used 630VAC/DC caps in my first ('almost' THE first) MMC:

http://hot-streamer-dot-com/TeslaCoils/MMCInfo/orig/manyc011.jpg 

and it still runs fine.  In fact it will be used tonight :-) so Winston's
capacitor was fine except for the type of foil in the caps.

Even though MMC caps will arc over and self heal at about 2.5 times their
DC rating (Geek caps 5700 volts), they are the ONLY caps that do that.
Mica or commercial caps should be used conservatively since they only arc
once before becoming a door stop.  MMCs are far far tougher concerning over
voltage.  Both of my coils use 1600VDC caps that are run at 2000+ volts
peak with no problem (that's before I turn the variac to 140 :o)).  Of
course, "I" push them to the edge, but they seem to love over voltage.
Current is the killer.  If MMCs run so much RMS current that they start to
heat significantly, the polypropylene starts to melt which is definitly
"bad".  Thus, we are far more concerned about RMS currents than voltage.

Cheers,

	Terry


At 07:52 AM 1/27/2002 -0700, you wrote:
>I dont know what voltage your GE transformers are, but I would be concerned
>with your low capacitor voltage. 18kv is not much. a 15kv NST puts out 22kv
>peak voltage, That would smoke your caps.Your cap voltage should never be
>less than 1.5 X your transformer voltage, and that is not much safety
>margin. Rember the effective voltage of AC is its DC heating equivelent not
>the actual peak voltage of the power applied. 1.414 X the effective AC is Pk
>voltage so 1.5 X is only 8% higher, and that is nothing. Use 2X AC for Caps
>   Robert  H
>
>> From: "Tesla list" <tesla-at-pupman-dot-com>
>> Date: Sat, 26 Jan 2002 14:09:47 -0700
>> To: tesla-at-pupman-dot-com
>> Subject: Re: Dead MMC
>> Resent-From: tesla-at-pupman-dot-com
>> Resent-Date: Sat, 26 Jan 2002 14:13:33 -0700
>> 
>> 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
>>> 
>