Re: A Couple of Questions (voltage reversal)

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Original poster: "Jim Lux" <jimlux-at-jpl.nasa.gov> 


> >Also, on the Maxwell cap specs page that Jim Lux so gracious-
> >ly made available to us (thanks Jim), there was some spec 
> >about 20% reversal current, or something like that. Can some
> >one explain this to me? I would appreciate any responses on 
> >this to help better educate this dum-dum. :^)
> 
> Apparently some caps don't like to go from positive to negative voltage.
> This is not a big problem with polypropylene caps but some types of
> dielectric tend to act a little like batteries and are not graceful about
> voltage reversals.  Perhaps someone who knows more about this could
explain
> it to both of us better ;-))

I believe (but am not sure) that the impact of voltage reversal is the
mechanical stress on the dielectric.  As Terry mentioned, some dielectrics
tend to "take a set" or remain polarized, with various relaxation times
(why you should leave a short across the cap after discharging it). 
Imagine that you charge a cap up with one polarity... Now discharge it and
then remove the load(or short).. the cap will rebound to some voltage. 
Either due to polarization of the dielectric, or actually some mechanical
shift in the positions of everything.  If you charge it with the opposite
polarity, you are actually working against more than just the no-charge
situation..

Any good explanations?

It does have a significant effect on life (although not as big as
voltage... which is a mechanical stress issue)