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From: wmb@sun.uucp (Mitch Bradley)
Newsgroups: net.analog
Subject: Re: Big Capacitors
Message-ID: <1829@sun.uucp>
Date: Sat, 1-Dec-84 20:17:17 EST
Article-I.D.: sun.1829
Posted: Sat Dec  1 20:17:17 1984
Date-Received: Tue, 4-Dec-84 05:37:28 EST
References: <4682@fortune.UUCP> <6146@brl-tgr.ARPA>
Organization: Sun Microsystems, Inc.
Lines: 39

> >	The physics of the cap is as follows. Early capacitors
> >stored charges between plates of material. Large caps took large
> >plates and rolled them into tight cylinders. This worked O.K. but
> >the surface area was still small, and the larger the surface
> >area, the higher the capacitance. The supercaps cover smaller plates
> >with activated carbon particles. Each particle multiplies the surface
> >area of a portion of the plate by a large amount. A plate covered with
> >particles has a huge surface area, and hence a very high capacitance.
> >
> >	I have NECs liturature on them so more info is available...
> >
> >						-Jim Wall
> >					....!amd!fortune!wall
> This sounds like they should be non-polar then.  ...

Probably they are polar.  The original article didn't mention that
electrolytic capacitors (and I believe the basic technology of these
"Supercaps" is electrolytic) use a chemical film as the dielectric.  The
film is formed on one of the plates by a process similar to electroplating.
If you reverse the polarity, the electrolytic cell is reversed in polarity,
the opposite plate doesn't have a dielectric formed on it, and the capacitor
conducts DC.  Generally it overheats and blows up, usually dramatically.
(I am reminded of the time when a PDP11-45 blew a cap, which knocked off
one of the panels.  Several keypunch operators who happened to reside in
the same room were terrified, and huddled in a corner.  But I digress...)

Electrolytic capacitors that have been sitting around out-of-circuit for a
long time (like the ones you get in surplus places) should be re-formed.
This may be done safely by applying a low DC voltage (of the correct
polarity) to them from a
current limited supply, and leaving them that way for several days.

It is also possible to change the polarity of an electrolytic capacitor
by very carefully applying a reverse voltage to them, which will re-form
the dielectric on the other plate.  I do not recommended doing this, as
the conditions must be carefully controlled to prevent explosion;
also, there is no point in doing it anyway.

Mitch Bradley