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Further EMMC testing

Hi All,

	Today I simply removed caps from the string one by one and ran it for 15
seconds at 20kV in the tank circuit.  A cap blew with four devices in the
string.  I cut it open and it appears the internal heat started to soften
and melt the poly.  There is a large hard area in there.

	However, this is very good news!  Voltage wise, each cap had 5000 peak
volts across it!  The RMS current was 10 amps.  That is 10^2 x 0.011 = 1.1
watt per cap!  That is about 10 times their dissipation "rating".  So, they
are blowing up just about when they should :-)

	I over voltaged the ones in the torture test and new ones too.  All the
breakdowns of new ones were around 4000 volts.  The torture test did not
seem to degrade the remaining three caps from that test which is a
surprise.  In fact, they all seemed to be over 5000 volts so I suspect that
they arced but self healed!  Interestingly, once they arc, they seem to
self heal well.  I got one to go to 10000 volts before it shorted so bad it
would not heal!  I don't think the self healing feature is something to
rely on though :-))  However, the self healing feature my be defeated by
interconnecting parallel strings of caps!!  I would no longer recommend
doing that based on this.  If a cap shorts internally, it may be able to
successfully absorb the pulse.  However, if there are say five other caps
in parallel with it, it will probably not recover!  Of course, I always
recommend using drain resistors across each cap to equalize voltage and
drain the charge when power is removed.

	One really great thing about these MMCs is that one can do this testing
very easily and cheaply.  Their value is easy to adjust and if you blow a
cap, you just reach into the bag and grab another...  One thing I will say
about any poly cap.  If it gets warm, start to worry!!!  Good news is, it
takes a LOT to get them warm!  Apparently they are pretty immune to over
voltage as I suspected.  About 2.5 times their rating.  The RMS current and
their internal resistance is the key.

	Of course, this is still very experimental!  All seems to be going great
but I have not powered a real coil with these yet.  However, that is now
the next step.  That should be a far far easier test than what they have
seen so far.  The voltage stress will still be there but they should be
fine.  In my LTR coil they should see an RMS current of only 2.5 amps each.
 I am thinking about fan cooling after all...

Cheers,

	Terry