Re: Garry's MMC cap failure explained.

["Tesla list" <tesla-at-xxxxxxxxxx>]

Original poster: "Steve" <seric48-at-yahoo-dot-com> 


Hi all,

  After talking off-list with Robin Copini about his MMC, he
told me it actually uses CDE 942C-series caps (the PP caps with
the film/foil construction), NOT the CDE 940C (metallized PP,
the ones Garry used).

  His original plan was to use 940Cs. This led to his having,
as he put it, "940C on the brain".

 Robin and I send our apologies for this honest mistake.

   Steve Copeland


PS-- Thanks, Ted, for helping us straighten this out. :^)



--- from Tesla List ---
>Original poster: Terry Fritz <twftesla-at-uswest-dot-net>
>
>Hi All,
>
>	I received Garry's MMC cap that kept wildly blowing up on him
>tonight by mail and I immediately set to work on it.
>
>-snip-
>
>I found two caps were open and removed them.  I then took one and then
>another and ran 1 amp through them.  After about 30 seconds they puffed 
>up and melted down.  They are NOT polypropylene!!  Comparing to my other 
>caps they must have about 20X the dissipation!  Tearing them apart, the 
>internal construction is basically good but they are built rather weakly.
>
>So, The caps had a real high dissipation factor and they burn up under
>1 amp of RF current in about 30 seconds.  A Panasonic cap with twice the
>current seem to run stone cold after a few minutes.  So these are high
>voltage "DC" caps (fairly good ones) with very poor AC characteristics.
>The caps are white wrapped tubular types that are 1 1/4 inches long and 
>a little under one inch in diameter.  The markings on them are:
>
>940C  20533K
>.033MFD+-10%
>2000VDC  CDET.
>
>There are a bunch of pictures at:
>
> http://hot-streamer-dot-com/TeslaCoils/Misc/blownMMC/
>
>Unfortunately, there is nothing that can be done to fix the cap.  But 
>I'll get it sent back to Garry on Monday.
>
>Cheers,
>
>       Terry
>
-------------------------------------------------------------------------
>
>Original poster: "Steve" <seric48-at-yahoo-dot-com> 
>
> Hi Terry, all,
>
>  These caps are supposed to be of metallized polypropylene
> construction, according to CDE. The 940C-series data is at
>
>     http://www.cornell-dubilier-dot-com/h940.htm
>
> and detailed data on Garry's .033uF/2000v cap is at
>
>     http://www.cornell-dubilier-dot-com/9402000.htm
>
>
>  Robin Copini has used (1600v) 940C-series caps successfully
> in an MMC. Maybe someone at CDE actually did 'spill pop in
> the cap winder', or loaded it with polyester film by mistake?
>
>  Steve Copeland
>
>
>--- from Tesla List ---
> Original poster: "Robin Copini" <rcopini-at-merlin-dot-net.au>
>
> Hi Gary,
>
>  The cap code you give '940C' implies a CDE part, i.e. 
> Cornell-Dubilier. Well..... I have to say I disagree with
> your assessment of these caps. I have a mmc built with
> CDE 940C caps (7x(33x0.33uF)) rated at 1600Vdc. The caps 
> are fairly plain looking with a cheap white stick-on label
> but... I run a pole pig rated at 33Kv/220Vin and I overdrive
> it with 270Vin, at over 80amps, the coil throws arcs over
> 4 metres and I cannot get these caps even warm.
>
> Best Regards
>
>  Robin Copini, Australia.
>