From: SFPhysics@aol.com
Date: Thu Mar 10 2005 - 00:44:39 PST
From: SFPhysics@aol.com Message-ID: <196.3a84939a.2f6162f7@aol.com> Date: Thu, 10 Mar 2005 03:44:39 EST Subject: 1 Farad Capacitors
In a message dated 3/9/2005 11:16:32 P.M. Pacific Standard Time,
pinhole@exploratorium.edu writes:
hey all
does anyone know a good source for 1 farad 5 volt capacitors? we've been
looking through the Jameco catalog and online and its a bit overwhelming and
hard to find what we want. We ordered some from Jameco and they actually turn
out to be battery back-up capacitors which release their charge very slowly
(see below). We have one NEC brand capacitor which works great. any ideas?
for the physics types: how do you make a capacitor that releases its charge
slowly? the caps we got from jameco wont even discharge when we short
circuit them with a wire! so what's the deal? i mean, don't we tell our students
that capacitors can release their charge all at once like in a camera flash?
happy charging
-eric
Greetings Eric:
Let me answer the second question first. Capacitor storage capacity
measured in Farads is a function of the size of the two opposing capacitor plates,
the space between the plates, and the dielectric factor of the sandwiched
insulator. To get 1 full Farad is an astounding feat of engineering. This is
akin to putting two football field sized plates a micron space of air apart.
The way engineers did this was to use activated charcoal specially prepared to
have a huge surface area filled with polyacrylonitrile for the electrolytic
insulator. A figure of 900 square meters per gram of substance has been
achieved. On charging the polyacrylonitrile polymerizes and stores electrons as
a chemical change as opposed to a regular capacitor which stores energy in
the electrostatic field. This is why the discharge is not immediate and
explosive though a short circuit discharge is very hard on the internal structure
of the "super capacitor." Because of the use of carbon with it's own high
resistance and the storage of charge in a chemical matrix, the discharge rate
is slowed down drastically. This is a good thing and prevents the explosive
discharge seen in large oil type capacitors or aluminum electrolytic
capacitors (Xenon camera flash).
NEC is the leading brand and has the best patents to date but other
researchers are hot on their trail. These capacitors are used in circuits where a
power supply output must be stabilized beyond any reasonable doubt. Some have
built in resistance to lessen the charging shock to the power supply which
would see the super capacitor as a dead short and automatically shut down. The
NEC units were some of the first on the market and relied on external
resistance to stop the surge load.
The last ones I got when I was still teaching full time was from Electronics
Goldmine but a quick search shows they may be out of them as they are not in
the current listing online.
_The Electronic Goldmine_
(http://www.goldmine-elec-products.com/products.asp?dept=1028)
All Electronics had some super caps but did not list the specs:
_All Electronics Corp._
(http://www.allelectronics.com/cgi-bin/category.cgi?category=140280&type=store)
Look under "capacitors - memory back up" which is where in computer circuits
they are needed to provide an uninterruptible source of power.
Kindest regards to everyone on the list,
Al Sefl
Who finds advancing age makes it more difficult to charge up anything...
Except the stairs to Pasta Pomodoro...
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