Re: EMMC Value Drift
>From: Tesla List <tesla-at-pupman-dot-com>
>Subject: EMMC Value Drift
>Date: Mon, 05 Jul 1999 19:46:15 -0600
>Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
> 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
>27.7nF. 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
>real. After lasts week's run, it seemed to drop by about 0.3%. I ran
>very hard this 4th and it seems to have drifted another 0.4% with that
> I run this 16000 volt cap at about 22000 volts so it is stressed in
If the caps are self healing than you are definitely seeing the effect
of this self healing. As I said before in my previous post every time
there is a significant overvoltage the voltage punches trough the
dielectric and blasts the metal layer surrounding the punch trough. The
cap continues working, typically only loosing about half of its
original charge in the event, but the capacitance drops when this
happens because there is less metal and hence less dielectric is used.
If you have strings in parallel than the effect will be much more
pronounced. It takes quite a few punchtroughs to cause a capacitance
drop as large as the one you’re seeing! As the weak spots are taken out
the tendency is for it to start loosing less and less capacitance with
progressive runs. Or die…
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