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Re: Garry's MMC cap failure explained.
- To: tesla-at-xxxxxxxxxx
- Subject: Re: Garry's MMC cap failure explained.
- From: "Tesla list" <tesla-at-xxxxxxxxxx>
- Date: Sat, 02 Dec 2000 13:54:41 -0700
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Original poster: Terry Fritz <twftesla-at-uswest-dot-net>
I just keep looking and looking at that data sheet... 3.8 amps RMS...
These caps I have here would be lucky to sustain 0.5 Arms... I wonder if
the peak amps of only 57 is the key. Perhaps the high pulses simply ripped
them apart and destroyed their current handling ability. However, even the
good ones I have could never do 4 amps, not even close... Perhaps, they
get super lossy at high frequency compared to other caps (I test at
350kHz). It appears the Cornell Dubillier 940C caps are very bad for MMCs.
The 942C series seems to do fine but their 170A peak still seems low. I
do note that the end cap construction in Panasonic, WIMA, and other high
power caps is enormous compared to the ones here. Of course, it is
possible this batch of caps had something go very wrong during manufacturing...
I put a 56nF Panasonic into the test and they seem
At 06:14 PM 12/1/2000 -0800, you wrote:
> Hi Terry, all,
> These caps are supposed to be of metallized polypropylene
> construction, according to CDE. The 940C-series data is at
> and detailed data on Garry's .033uF/2000v cap is at
> Robin Copini has used (1600v) 940C-series caps sucessfully
> 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: Terry Fritz <twftesla-at-uswest-dot-net>
>> I received Garry's MMC cap that kept wildly blowing up
>> on him tonight by mail and I immediately set to work on it.
>> I then got out the RF power supply and ran a little current
>> through each. 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
>> 2000VDC CDET.
>> There are a bunch of pictures at:
>> Unfortunately, there is nothing that can be done to fix the
>> cap. But I'll get it sent back to Garry on Monday.
>> 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.