Re: Garry's MMC cap failure explained.

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

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

Stan,

We are thinking this was either a bad batch or something else is wrong.  I
did notice that one seemed to heat on just one side??  Garry had the leads
bent pretty close to the case.  I wonder if the end connections were
damaged by sever lead bending?  I'll do more tests...

Cheers,

	Terry

At 01:21 AM 12/2/2000 -0500, you wrote:
>Terry, I am VERY puzzled by your tests.
>
>I bought 75 or so 940C 30S47K 's from Newark.  These are listed as
>polypro, and a quick check on Cornell Dubilier's website confirms this. 
>Type 940C is listed as "Polypropylene pulse and high frequency".  It is
>"double metallized", with dV/dt up to 2500, peak A up to 1000, and RMS
>current up to 20 A.
>
>So now I am totally lost.  Unless I'm missing something, these are some
>pretty good MMC caps on paper.  My specific ones are .047 mfd -at- 3000
>VDC.
>
>-Stan
>
>Tesla list wrote:
>> 
>> 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.
>> 
>> First I checked the resistances of all the caps and found a few of the
>> bleeder resistors were damaged.  This appears to be due to them being too
>> close to other objects or from voltage transient damage.  So my first new
>> "rule" is:
>> 
>> 1.  Keep bleeder resistors at least 1/8 inch away from any other objects.
>> This will prevent corona and eliminate the possibility of arc over.
>> 
>> The resistors all seemed like they died due to other stuff going wrong
>> rather than they being the cause.
>> 
>> I then tested all the caps to 3000 volts DC and found a few that had been
>> damaged but it was not obvious.  Most of the caps tested fine and the few
>> internal arcs cleared just like they should.  However, I did note that the
>> strings of caps were very close together.  So the second rule:
>> 
>> 2.  Leave at least 1/8 inch distance between caps and 1 full inch between
>> strings.
>> 
>> The cap to cap voltage is not real high but if something goes wrong, an arc
>> over to the next string can cause the damage to propagate into other
>> strings.  However, that really did not seem like a likely cause either.
>> 
>> 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
>> .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
>
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