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From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: net.periphs
Subject: Re: MOV's and tubes
Message-ID: <869@kitty.UUCP>
Date: Wed, 12-Mar-86 11:36:50 EST
Article-I.D.: kitty.869
Posted: Wed Mar 12 11:36:50 1986
Date-Received: Sat, 15-Mar-86 21:36:45 EST
References: <11600005@uiucdcs>
Organization: Recognition Research Corp., Clarence, NY
Lines: 32
Summary: Testing Metal Oxide Varistors (MOV)

In article <11600005@uiucdcs>, carey@uiucdcs.CS.UIUC.EDU writes:
> 1)  I think this has been discussed on the net before, but I didn't 
>     follow it at the time--is there any way to test whether a MOV
>     is still in working condition?  I have a power strip that
>     has three MOV's the number on them is:
> ... 
>    Anyway, I think the strip might have had lightning go through it.  Besides
> just replacing them, is there any way to TEST whether they are still good.

	The only effective way to test a MOV is by using a semiconductor curve
tracer.  We have a Tektronix curve tracer with external high current adapter
which will simulate the overvoltage/current necessary to test the breakdown
capability of the MOV WITHOUT DESTROYING IT.  A voltage vs current curve of
a defective MOV stands out like a sore thumb.

> While we are at it, in what cases are MOV's effective--small power surges, 
> voltage variations caused by large motors, lightning strikes, induced voltage
> from lightning, static charges.

	MOV's are effective in clamping transients from "small power surges",
inductive transients from motor starting, and induced voltages from lightning
strikes.  MOV's are often NOT effective in preventing damage from _direct_
lighting strikes because such transient voltages are longitudinal in nature;
i.e., the hot, neutral, and protective ground wires feeding the equipment are
ALL raised above ground potential.

==>  Larry Lippman @ Recognition Research Corp., Clarence, New York        <==
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