Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84 SMI; site sun.uucp Path: utzoo!watmath!clyde!bonnie!akgua!whuxlm!whuxl!houxm!ihnp4!zehntel!dual!amdcad!decwrl!sun!wmb From: wmb@sun.uucp (Mitch Bradley) Newsgroups: net.analog Subject: Re: Big Capacitors Message-ID: <1829@sun.uucp> Date: Sat, 1-Dec-84 20:17:17 EST Article-I.D.: sun.1829 Posted: Sat Dec 1 20:17:17 1984 Date-Received: Tue, 4-Dec-84 05:37:28 EST References: <4682@fortune.UUCP> <6146@brl-tgr.ARPA> Organization: Sun Microsystems, Inc. Lines: 39 > > The physics of the cap is as follows. Early capacitors > >stored charges between plates of material. Large caps took large > >plates and rolled them into tight cylinders. This worked O.K. but > >the surface area was still small, and the larger the surface > >area, the higher the capacitance. The supercaps cover smaller plates > >with activated carbon particles. Each particle multiplies the surface > >area of a portion of the plate by a large amount. A plate covered with > >particles has a huge surface area, and hence a very high capacitance. > > > > I have NECs liturature on them so more info is available... > > > > -Jim Wall > > ....!amd!fortune!wall > This sounds like they should be non-polar then. ... Probably they are polar. The original article didn't mention that electrolytic capacitors (and I believe the basic technology of these "Supercaps" is electrolytic) use a chemical film as the dielectric. The film is formed on one of the plates by a process similar to electroplating. If you reverse the polarity, the electrolytic cell is reversed in polarity, the opposite plate doesn't have a dielectric formed on it, and the capacitor conducts DC. Generally it overheats and blows up, usually dramatically. (I am reminded of the time when a PDP11-45 blew a cap, which knocked off one of the panels. Several keypunch operators who happened to reside in the same room were terrified, and huddled in a corner. But I digress...) Electrolytic capacitors that have been sitting around out-of-circuit for a long time (like the ones you get in surplus places) should be re-formed. This may be done safely by applying a low DC voltage (of the correct polarity) to them from a current limited supply, and leaving them that way for several days. It is also possible to change the polarity of an electrolytic capacitor by very carefully applying a reverse voltage to them, which will re-form the dielectric on the other plate. I do not recommended doing this, as the conditions must be carefully controlled to prevent explosion; also, there is no point in doing it anyway. Mitch Bradley