Re: MMC resister problem

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

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

> >Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <evp-at-pacbell-dot-net>
> >       If the capacitances aren't identical, when the series is shorted (or
> >discharged through a resistor), the smaller capacitors will have
> >reversed polarity if the voltages across each capacitor have been
> >equalized by a resistor shunted across each one.  Just ohm's law.
> >Consider a large capacitor and a small capacitor in series with the
> >series string charged to some voltage.  If the voltages on each are the
> >same (again by means of a resistor divider) then the larger one will
> >have more charge than the small one.  On discharge the current will be
> >the same for each capacitor, and the smaller capacitor will end up with
> >reverse voltage.
> 
> I think the assumption that the voltages are being equalized by virtue of
> the bleeder resistors is invalid.  A 0.15uF cap with a 10 Meg resistor
> across it will have a time constant of 1.5 seconds.  About 180
> charge/discharge cycles will have occurred in this timeframe.  For the case
> where there are no bleeder resistors, the residual charge problem is most
> evident.  When using reasonable-valued bleeder resistors (or none), the cap
> voltages will divide based just on the capacitance values.

	Are we talking about the voltage left during normal operation, or
residual voltage/charge left when the TC is turned off?  If the former,
I agree completely; thought the discussion concerned the latter.  At
least to me "discharge resistors" imply safety devices.

> >       In general, capacitors also have some hysteresis due to a process
> >called dielectric absorption.  This manifests itself as a rise in
> >terminal voltage after the capacitor is shorted and then open
> >circuited.  For good dielectrics (quartz, polystyrene, teflon) the
> >effect is very small.  For ordinary oil-filled paper capacitors the
> >recovery can be several percent of the original voltage.  It is fairly
> >common practice to wrap a shorting wire around the terminals of HV
> >filter capacitors, to prevent serious shock.  The effect is often
> >modeled by considering the capacitor to be composed of one perfect
> >capacitor in parallel with a number of different series RC circuits of
> >smaller capacitance and different time constants.  I have measured very
> >good capacitors intended for use in precision analog differentiators,
> >and even for them current flow (very small at the end) can be measured
> >for hours.
> >
> >Ed
> 
> Agreed.  I didn't think the definition of hysteresis included
> time-dependant effects, hence my confusion with the term.  Maybe my
> definition is too narrow.  I'm still not sure if this dielectric
> absorption-effect is pertinent to the issue at hand.

	It probably isn't.  "All depends on the numbers."
 
> Regards, Gary Lau
> MA, USA