Re: Garry's MMC cap failure explained.

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

Original poster: "Stan" <sdarling-at-columbus.rr-dot-com> 

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

############################################################################
#########
This e-mail message has been scanned for Viruses and Content and cleared.
############################################################################
#########