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Re: EMMC Value Drift

Hi All,

	Many thanks for the great ideas about the drift problem.  This has a lot
of interesting implications.  The fact that Finn has seen the same thing is
very interesting!!

	If the caps are arcing internally, it shows that the self healing feature
really does work even under nasty Tesla coil conditions.

	This tends to suggest that the caps are seeing voltages high enough to
puncture the dielectric!  DC testing indicated that they should hold up
fine.  It was very obvious when they arced internally during DC testing
which was pretty sensitive.  The caps were fine till about 3000 volts so my
string of ten should never break down.  Perhaps there is some mechanism
that causes these high voltages inside the caps that would be very
important to understand.  When I blew the caps under DC conditions they
often cracked through the case or had other obvious damage that I don't see
with my real cap.  However, they continued to function.

	I wonder if the high dv/dt is helping the dielectric breakdown sooner?
Perhaps a little stray inductance is driving the internal voltage up...  I
would think that there would be a weaker cap in the string that would be
degrading more than then others...

	Tonight I will carefully inspect the caps for damage.  Then I will test
their values and pick a few to dissect (I have spares to replace them
with).  These caps unroll pretty easily and the arcs should be obvious if
the DC testing is any indicator.  When the dielectric blew under DC, it
left a very obvious blown area in the dielectric...

Cheers,

	Terry


At 06:40 PM 7/6/99 +0200, you wrote:
>Mine are drifting down, too.
>After the initial rise in capasitance after the first run, where the
>caps setteled in at 34,3 nF. Next time I measured it, I had changed the
>battery in the meter, and it had gone down to 33,9, so I thought this
>might just be the battery.
>After the last run, however it had dropped another 0.2, so that it now
>measures 33.7.  Hey, this could get expensive!
>The heating that I noticed last time (but forgot to _measure_ with the
>thermometer) was mainly in the center strings. I had fitted a perspex
>cover to keep the tapping strap from tempting to arc to the capacitor
>when it passed over it. This stopped the convection, and was the cause
>for the lack of convection cooling of the center strings.
>There are _so_ many things to wacth out for, to a (relative) beginner
>like me, that I forget to have an eye on all of them at a time.
>
>About the voltage that I am using: The pig is wound for 20 kV with 400 V
>in, but the variac will supply 480 V, and since I am a sucker for a long
>spark, the caps may see 34-35 kV. Now with a bit of rise on top----- :-)
>Gottta make those fiberoptic probes soon....
>
>Cheers, off for hollidays 3 weeks, Finn
>
>Tesla List skrev:
>> 
>> Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
>> 
>> Hi All,
>> 
>>         I seem to be noticing a very slight capacitance value drift in my 
>EMMC
>> cap.  It has 10 caps per string and has 5 strings of 1600 Volt 0.056uF
>> Panasonic high voltage pulse caps.  The total value has gone from 27.9nF to
>> 27.7nF.  Each individual string seems to have drifted the same amount
>> (-0.7%).  This is close to the limits of my LCR meter but the drift seems
>> real.  After lasts week's run, it seemed to drop by about 0.3%.  I ran it
>> very hard this 4th and it seems to have drifted another 0.4% with that use.
>>  I run this 16000 volt cap at about 22000 volts so it is stressed in
>> voltage.  The strings seem to be fine current wise.  I wonder if the dv/dt
>> thing is degrading the internal connections??  Perhaps it will level out
>> with more use but it seems to be continuing in a linear way with the amount
>> of use.  If it continues, eventually it will fail and I can dissect the
>> remaining caps to see what the cause is.
>> 
>> Cheers,
>> 
>>         Terry
>
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