Hi Mike, At 11:30 PM 3/6/98 -0500, you wrote: >On Fri, 6 Mar 1998 20:10:40 -0500 Sean Breheny writes: >>BTW, how do these super caps achieve such high capacities? Are the >>plates >>so much closer than older electrolytics or do they use some >>tremendously >>good dialectric, or a combination of both? > >Mostly the third variable in the capaictance equation: surface area. >Lots of surface area. They are made from activated charcoal, one of the >most "porous" materials known. An elecrolytic process creates a very >thin dielectric (of what? I don't know) over this huge surface, and a >very large capacitor is born. > Sorry, but I still don't get it. I understand that the activated charcoal has a huge surface area, but how do they get two conductive layers of charcoal which are separated from each other? >Since carbon/charcoal isn't a very good electrical conductor, a >relatively large internal resistance results compared to something like >the aluminum used in standard electrolytic capacitors. In aluminum >capacitors, the surface is first etched to make a larger active surface >area, then a very thin dielectric (Al2O3?) is formed. I think that >advances in the etching process and controlling the thickness of the >dielectic to be uniformly just greater than the voltage rating are >responsible for the decreasing size of electrolytic capacitors. The >plates are seperated by a fiber soaked in conductive fluid. Since the >fluid is conductive, no electric field occurs across the seperator. It >just keeps metal-on-metal contact from happening and forms a liquid >"plate" right next to the thin dielectric. The capacitance effect >happens entirely across the thin Al2O3 layer. Carbon supercaps have a >similar liquid filling that permeates the carbon. > >At 50F 2.5V, there would be >>a >>whopping 125 coulombs of charge on each plate, an unheard of amount of >>charge in usually physics, enough to suck a statically charged comb >>right >>through a solid brick wall from several meters away, if it weren't for >>the >>oppositely charged plate, that is. > >125 couloumbs of charge on a comb definitely spells "bad hair day". > Wow, I don't even want to think about having 125 C on any object close to me!! Actually, what I meant was that even a normal comb, carrying something like a few micro-couloumbs of charge, would feel a force of something like 2800 pounds of force at a distance of 3 meters from a 125 Couloumb charged object! Of course, I realize that the oppositely charged plate in the capacitor cancels this out, but it is still a scary thing to think about! Sean +--------------------------------+ | Sean Breheny | | Amateur Radio Callsign: KA3YXM | | Electrical Engineering Student | +--------------------------------+ Fight injustice, please look at http://homepages.enterprise.net/toolan/joanandrews/ Personal page: http://www.people.cornell.edu/pages/shb7 mailto:shb7@cornell.edu Phone(USA): (607) 253-0315