Newsgroups: sci.electronics
Path: utzoo!henry
From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re: High-value Button capacitors: Where to get?
Message-ID: <1991Jan9.232842.14165@zoo.toronto.edu>
Organization: U of Toronto Zoology
References: <1991Jan9.200343.12783@watmath.waterloo.edu>
Date: Wed, 9 Jan 1991 23:28:42 GMT

In article <1991Jan9.200343.12783@watmath.waterloo.edu> mwtilden@watmath.waterloo.edu (Mark W. Tilden) writes:
>capacitors which deliver 47000uF in a small thimble-sized
>can (max 5.5v).  They also had 1.0F caps about the size of an
>oreo cookie...
>As an aside, does anyone have information on how these things work?

If they're the ones I'm thinking of, they are made by taking a conductive
powder, packing it loosely so there are lots of pores, and then treating
it to make the surface a dielectric.  The powder is one electrode, and a
conductive material filling the pores is the other.  The enormous surface
area inside the pores is what gives the massive capacitance.

A consequence of this setup, however, is that internal series resistance
is quite high, which limits the applications to ones in which frequency
response is not an issue.  They are primarily battery replacements for
keeping nonvolatile memories alive.  Don't use them as filter capacitors,
they aren't built for it.  You might even overheat them.
-- 
If the Space Shuttle was the answer,   | Henry Spencer at U of Toronto Zoology
what was the question?                 |  henry@zoo.toronto.edu   utzoo!henry