On Fri, 6 Mar 1998 20:10:40 -0500 Sean Breheny writes: >At 08:58 PM 3/6/98 +0100, you wrote: > >> >>Not that high! Today I=B4ve seen ranges from 220R (NEC 5,5V 0.047 F) >down= > to >>only 0.08R (Elna 2,5V 50F) =20 >> >>Yes, 50F!! Almost like an accumualtor. Exept the price... >>/Morgan >> >>/ Morgan Olsson, MORGANS REGLERTEKNIK, SE-277 35 KIVIK, Sweden \ >>\ mrt@iname.com, ph: +46 (0)414 70741; fax +46 (0)414 70331 / >> > >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. 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". _____________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com Or call Juno at (800) 654-JUNO [654-5866]