[Amps] MOV's

[Steve Katz]

> Steve, Richard:
> 
> I was looking at MOV's used in surge suppressors on telco and 
> power drops.  We wre zapping them with a measured charge, per
> IEEE, and observing the resulting waveform.  
> 
> One of the uncomfortable things is, they sometimes fail open..
> so you don't know they've failed.  
> 
	::Yep, I've seen that often and it is indeed a problem.  One of the
advantages of bipolarity silicon transient voltage suppressors is that they
can be built to fail as a short circuit, predictably and reliably.  It's
still a failure, but it's a more obvious one and thus more desirable than a
"failed open" device.

	-WB2WIK/6  

> Jim
> 
> -----Original Message-----
> From: Steve Katz [mailto:stevek@jmr.com]
> Sent: Friday, March 15, 2002 1:27 PM
> To: 'Richard'; jljarvis; AMPS
> Subject: RE: [Amps] MOV's
> 
> 
> Note abt MOVs:
> 
> One thing that unfortunately does not appear in the mfr's data sheets, or
> in
> any parametric/dynamic curves, is the definition of how the MOV crystals
> are
> arranged and how many joules it takes to fuse each grain.
> 
> Traditional MOVs (all that I've seen, anyway) are doped grains of metal
> oxides, primarily zinc oxide, that are placed in series, or in
> series-parallel combinations, to achieve the BV and J rating desired.  A
> large surge can fuse one or two grains and not the rest.  That results in
> a
> lower BV.  This isn't an open nor a short, it's just a modified MOV which
> has a lower BV than it originally had.  Unless you're in the habit of
> removing them to measure this characteristic after each surge, it's
> impossible to know what's happened within the MOV.
> 
> As the BV is reduced and gets close to the actual operating voltage,
> leakage
> current within the MOV causes heating (of course), and as the MOV grains
> get
> hotter, their BV drifts more until eventually -- bingo!  With luck, what
> you
> normally have is simply a blown fuse and an MOV you need to replace.
> 
> WB2WIK/6
> 
> "Each success only buys an admission ticket to a more difficult problem."
> --
> Henry Kissinger
> 
> > -----Original Message-----
> > From:	Richard [SMTP:2@mail.vcnet.com]
> > Sent:	Friday, March 15, 2002 9:42 AM
> > To:	jljarvis;  AMPS
> > Subject:	Re: [Amps] MOV's
> > 
> > 
> > 
> > >
> > >Careful, here.....MOV's shift characteristics with dosage, and
> > >may only be good for a few shots, depending on the charge it
> > >has to absorb, per shot.
> > >
> > EUR  MOVs come in a wide range of repetitive peak-current abilities.
> The 
> > little guys have thin wire leads and the biguns have strap leads.   
> > Picking the right one for the job at hand is not difficult.  When in 
> > doubt, $pring for the next size up. 
> > 
> > ><snip>
> > >>a MOV across the contacts cures the  problem better than a vacuum-
> > >>>relay since the reverse EMF still needs to be suppressed to prevent
> > >>>damage elsewhere.
> > >>
> > >>The problem is that because it's DC, even with no inductive load,
> > >
> > >EUR  If wires are used to connect a (pretend) purely resistive load,
> the
> > >load always looks iinductive.  And when the circuit opens, a reverse
> EMF
> > >at the opening contacts is guaranteed.  Good engineering practice is to
> > >suppress reverse EMFs with a MOV or bi-lateral Si transient suppressor
> > >diode.
> > ><snip>
> > >
> > >I did some work with MOV transient suppressors in a previous lifetime.
> > >If you zap them in opposite directions more or less equally, they can
> > >last a while.  Zap them in one direction, like reverse EMF, and they
> > >will become increasingly less useful.
> > >
> > EUR  When in doubt, read the fine print in the mfg specs.  
> > 
> > >Jim N2EA
> > >
> > >PS:  TWA's (retired) senior 747 check pilot at the time of the 'center 
> > >fuel tank' explosion, still believes it was friendly fire that downed
> the
> > 
> > aircraft.
> > >
> > EUR  As do many govamint conspiracy experts who have seen black
> > helicopters 
> > and Russky tanks in the corn fields of Kansas. 
> > 
> > cheers, Jim
> > 
> > -  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K, 
> > www.vcnet.com/measures.  
> > end
> > 
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