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Path: utzoo!watmath!kpmartin
From: kpmartin@watmath.UUCP (Kevin Martin)
Newsgroups: net.analog
Subject: Re: large capacitors -- low resistance??
Message-ID: <10101@watmath.UUCP>
Date: Sun, 2-Dec-84 08:46:05 EST
Article-I.D.: watmath.10101
Posted: Sun Dec  2 08:46:05 1984
Date-Received: Tue, 4-Dec-84 05:20:05 EST
References: <17@tekig5.UUCP>
Reply-To: kpmartin@watmath.UUCP (Kevin Martin)
Organization: U of Waterloo, Ontario
Lines: 59

>> So it was with great expectation that we charged this thing up.  Got a piece
>> of 16 gauge wire.  Taped it to a stick.  Sloowly approached the terminals
>> with it ... and ... nothing happened.  Apparently these things have a fairly
>> low internal resistance so they discharge shortly after the current is
>> removed.  Sigh.
>
>     Where did you learn your physics?  These capacitors have a large internal
>resistance.  That is one of the trade-offs that had to be made in order to make
>a capacitor with 3.3 F of capacity.  They certainly would not be too useful
>if they discharged shortly after current was removed.  The main use of these
>capacitors is for such things as battery backup.  They will produce a small
>current for a long time, but don't try to filter the high frequency ripple
>out of a power supply with them.  It won't work.  If you want to melt wires,
>get a large computer power supply capacitor.  That is what they were made
>for (and you thought it was to filter power supplys).


I think we are getting into confusion over leakage resistence vs. series
resistance. The former determines how fast a capacitor discharges when
its terminals are left open. The latter determines how fast it discharges
when you short it out.



                                    leakage
                             ______/\  /\  /\_____
                             |       \/  \/      |
                             |                   |
                             |        +--        |
                             |        | #        |
                   series    |        | #        |
       __________/\  /\  /\___________| #____________________
                   \/  \/             | #
                                      | #
                                      +--

Electolytic capacitors, in general, have a lower leakage resistance than
other types (e.g. mica or mylar), since the insulator is just a chemical
film between a piece of metal (usually) and some semi-liquid gunk. If you
apply reverse voltage, the film gets electrolysed out of existance, which
is why the electrolytics are polarized.

By varying the composition of the electrolyte (the gunk), the thickness of
the film can be adjusted. A thicker layer gives a higher voltage rating
and higher leakage resistance, but a lower capacitance.

To increase the capacitance without reducing the voltage rating, you
have to increase the (metal) plate's surface area. You can make it bumpy
by pressing a pattern into it; someone else mentioned coating the plate
with carbon particles; you can make the plates themselves thinner (and
thin out the layer of electrolyte too). The first two solutions are
the ones where big technological improvements are being made. The
latter solution increases the series resistance, since the conductors
via which the charge leaves the capacitor are thinner.

If you want to blow up a wire, you need a low *series* resistance, so the
capacitor will discharge rapidly. You should have no problems with the
capacitor discharging through its leakage resistance.
                  Kevin Martin, UofW Software Development Group