[TowerTalk] MOV's

[Keith Dutson]

When a MOV is hit with a voltage (not spike) much higher than the rating, it
usually pops like a firecracker.  At this point it is open.  This happened
in one of my Astron power supplies (SS-25M) set for 120V when it was plugged
into 240V during the VP8WWW DX-pedition.  Oops!  Then we changed the switch
to 240V, plugged in again and the second MOV popped.  End of PS, or so we
thought.  Turns out we could have used it with both MOVs popped, but no one
was brave enough to try this.  Back in the US new MOVs were purchased from
Mouser and installed.  PS now works properly with protection.

73, Keith NM5G

-----Original Message-----
From: towertalk-bounces at contesting.com
[mailto:towertalk-bounces at contesting.com] On Behalf Of K8RI on Tower talk
Sent: Wednesday, August 03, 2005 3:29 PM
To: jimjarvis at ieee.org; towertalk at contesting.com
Subject: Re: [TowerTalk] MOV's




>
> If an MOV has accumulated enough charge such that it is close to 
> failure, and it fails open during an event, you are unprotected.

That is not how a Metal Oxide Varistor (MOV) works.  You *never* want one to
open unless the rating is too low.  They work by *shorting* transients.
[snip]