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RE: MMC resister problem



Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Gary.Lau-at-hp-dot-com>


A series-string of caps will all experience the same current flow and
(assuming they are all roughly the same capacitance value) will all gain
the same incremental voltage as they charge.  If one cap starting at zero
volts charges up to 2000V, another cap starting out at 500V will charge up
to 2500V.  The voltage on one cap can't affect the charging of others.

In a perfectly linear world (i.e. circuit simulators), all caps starting
out at zero volts WOULD guarantee that they charge and discharge uniformly
and all end up at zero volts.  But it appears that some non-linear effects
are complicating things.

Gary Lau
MA, USA

Original poster: "david baehr by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<dfb25-at-hotmail-dot-com>

If some caps have that 500v and that string is recharging again, wouldn't
that extra charge just get passed on to the others too??? If all caps were
at zero volts, is this a garrantee that they will all take on an even
charge??? Heck, I don't know : )

>From: "Tesla list" 
>To: tesla-at-pupman-dot-com 
>Subject: RE: MMC resister problem 
>Date: Tue, 04 Jun 2002 08:01:20 -0600 
> 
>Original poster: "Lau, Gary by way of Terry Fritz " 
> 
>I can vouch from experience that a charge sufficient to make a healthy 
>"snap" when shorted exists on individual MMC caps after the power is turned 
>off, even though the MMC end-terminals are shorted by virtue of the NST 
>secondary. 
> 
>It's not at all clear how these asymmetrical residual charges came to be. 
>If individual caps have slightly different capacitance values, then their 
>respective voltages will scale inversely with their value, but there's no 
>(apparent) way that they can develop a voltage reversal with respect to 
>others in the string. Same thing with unequal leakage resistance. 
>Something non-linear must be going on. Could there be some rectification 
>occurring as the corona inception voltage (AC voltage rating) of the caps 
>is exceeded? 
> 
>But the fact that such unequal residual voltages does exist has some 
>troubling implications for the MMC's voltage rating. Say we have a string 
>of ten 2000V caps and we want to charge the whole thing up to 20kV. Fine, 
>that's 2000V per cap. But if one cap has an initial charge of 500V, then 
>we've exceeded it's rating by that much. 
> 
>So, it may be that the value of individual bleeder resistors goes beyond 
>personal safety and benefits the well-being of the caps. 
> 
>Gary Lau 
>MA, USA