Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!husc6!cmcl2!brl-adm!umd5!cgs From: cgs@umd5.umd.edu (Chris Sylvain) Newsgroups: sci.electronics,comp.misc,misc.consumers,rec.audio Subject: Re: Power supplies & 3B1's Message-ID: <2097@umd5.umd.edu> Date: Thu, 3-Dec-87 14:56:32 EST Article-I.D.: umd5.2097 Posted: Thu Dec 3 14:56:32 1987 Date-Received: Mon, 7-Dec-87 04:56:23 EST References: <942@woton.UUCP> <1805@ukecc.engr.uky.edu> <2257@kitty.UUCP> <2803@ncrcae.Columbia.NCR.COM> <2626@catuc.UUCP> Reply-To: cgs@umd5 (Chris Sylvain) Organization: University of Maryland, College Park Lines: 32 Keywords: secondary surge lightning arrestor Xref: mnetor sci.electronics:1867 comp.misc:1715 misc.consumers:3587 rec.audio:4643 In article <2626@catuc.UUCP> doug@catuc.UUCP (doug) writes: >I did some research into this area and was told that MOV's have a prpblem >in that they degrade each time they absorb a surge and that the amount of >energy they can absorb each time becomes less and less. Eventually they >become ineffective. If you have a GE data book handy, and look up the GE-MOV's, then you'll find a graph that indicates the amount of energy the device can dissipate (as heat) decreases with the number of pulses (surges) the device has already dissipated. Why is that? MOV's are constructed of particles of zinc oxide, the metal in Metal-Oxide Varistors. The particles of zinc oxide fuse together when they are heated. So, in time, yes the MOV becomes ineffective as a surge dissipater, as the zinc oxide finally conducts all the time, which dissipates a GREAT DEAL of heat. My experience with them is when they were used in cable TV satellite receivers I repaired. The MOV's are installed AFTER the power line fuse so that when they do fail, they open the fuse. They do fail spectacularly, too.. Zinc splattered (really no worse than a two "blob" solder-splash) over the personnel shield (a piece of plastic to keep fingers from 120VAC), the PC board charred under the MOV, and a blackened hole in the MOV. Replace the MOVs (three used), and the fuse -- the receiver functions literally like new. The power supply board isn't as pretty as when it was new, but with a little epoxy and a few pieces of buss wire, it's as good as it needed to be. -- --==---==---==-- .. and the mome raths outgrabe. .. ARPA: cgs@umd5.UMD.EDU BITNET: cgs%umd5@umd2 UUCP: ..!uunet!umd5.umd.edu!cgs