Re: Another MMC question...

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

Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>

Hi,

MMC caps need to have high RMS current capacity.  Very high dV/dT. Strong 
end cap construction, super low loss, self healing...  Only the very best 
polypropylene caps will do all this.  There are many polypropylene caps 
that do not have all these features to make them cheaper.  Good MMC caps 
are the most expensive and best of the bread of polypropylene caps.

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

http://www.cde-dot-com/film/9422000.htm

The favorite CD 942C series 2000V  0.15uF caps have an RMS rating current 
of 13.5 Arms, a peak current of 432 amps, and a dV/dT of 2879 V/uS.

Those high numbers are critical for MMC caps.  I always get worried when 
someone uses some different cap since we have no idea if they are going to 
work or not without spending a lot of time checking data sheets.  Only 
about 5% of the polypropylene caps available will work.

See if you can find some of the known good caps from the list above or make 
arrangements to get them sent over to you if you can't get them locally.

Cheers,

         Terry

At 12:30 PM 11/4/2003, you wrote:
>Hi there everybody,
>
>I have yet another question about my MMC. After laboriously making a very 
>eye-pleasing and neatly soldered MMC with my 100 caps (see my last 
>question for details) I do believe I know whether or not the concept 
>works. You see, it doesn't. More than twelve caps went up in smoke 
>(ruining my Lexan!), which made me realise that these were indeed not 
>suitable for TC use. So it's back to the drawing board. I still have one 
>relatively inexpensive option. I can use 23 Farnell 440 VAC 680 nF caps in 
>series to get 10120 VAC total string voltage and 29.5 total capacitance. 
>These are polypropylene capacitors, and they should be suited for TC use, 
>based on info from the data sheet. However, there is one thing that could 
>be a problem: the caps are rated at 250 V/us max! I read somewhere that I 
>shouldn't use these for an MMC, as only capacitors rated at 1000 V/us will 
>suffice. Will using these caps result in absolute failure or more Lexan 
>destruction, or can will it still be better than what I am using now 
>(which is a very large ceramic capacitor MMC)? Please help...
>Thanks in advance & happy coiling,
>
>Koen