Newsgroups: sci.electronics.components
From: Henry Spencer <henry@zoo.toronto.edu>
Subject: Re: WTD:  Construction of >1F capacitors
Date: Tue, 5 Aug 1997 16:30:55 GMT

In article <33e278ef.3454076@news.doit.wisc.edu>,
Melissa & Jim <mgkepner@facstaff.wisc.edu> wrote:
>Does anyone know the construction principle of the rather new
>capacitors that are now available in sizes over 1 Farad.  There must
>be some new new materials/construction techniques involved.  Does
>anyone know what it is?

One electrode -- can't remember which one offhand -- is basically carbon
powder, packed just densely enough to put the particles into good
electrical contact with each other, but not densely enough to close up the
pores between them.  The result is a compact electrode with a huge surface
area, which is just what you want for large capacitance.  The other
electrode is in contact with the conductive fluid that fills the powder
pores.  As in any electrolytic capacitor, the dielectric is formed
electrochemically as a very thin layer on the surface of one electrode (in
this case, obviously, the powder one).

That's the general idea.  There are undoubtedly lots of highly-proprietary
variations on it by now; these capacitors have been around for a while.

The thing you have to bear in mind, when using these puppies, is that they
have a fairly high series resistance, because of the long and convoluted
current paths through the powder electrode.  So they can store a lot of
charge, but you can't get it in or out terribly quickly.
--
Committees do harm merely by existing.             |       Henry Spencer
                           -- Freeman Dyson        |   henry@zoo.toronto.edu