Re: EMMC Value Drift

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
> 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