Garry's MMC cap failure explained.

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

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