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Re: electronic components



On 12/17/96 "Daniel L. MacIsaac wrote:

The supercaps have a significant internal resistance.

Does it mean the discharging is slow even when the cap is short-circuited?
How does the internal R appear in an equivalent circuit, in series or in
parallel? What is the physics of large R?
Ludwik Kowalski

Yes. According to R. Crane's HOW THINGS WORK in TPT 34(3) [March 96],
his 1F supercap half discharged from 5V in a half second. I did not take
measurements with my own supercaps, but they did not completely discharge
quickly even when shorted. They seemed very "bouncy" in terms of taking
and releasing charge quickly.

Crane's article is one of the better in-print descriptions of supercap
physics I've found, and he cites several other refs. The immense surface
area of activated charcoal electrodes (gives you the monster A) and
as a result:

The coating on the electrodes has to be thin: the porous carbon
has a resistance, and the tortuous path the electrolyte has to follow
introduces further resistance. (p 163)

The double-charge layer where charges are separated by only one or so
molecular diameters (so long as voltage is too low to decompose the
electrolyte, hence the low voltages on these caps) gives a teensy d. Immense
A and teensy d make the supercap possible; several have to be combined in
series to get any decent overall voltage so there are actually several
supercaps in series in the can of a 5V 1F supercap and each of these has
a value exceeding the rated capacitance.

Get two and hacksaw one apart. At least, this was my immediate reaction :^)

Dan M

Dan MacIsaac, Assistant Professor of Physics and Astronomy, Northern AZ Univ
danmac@nau.edu http://www.phy.nau.edu/~danmac/homepage.html