Re: MOVs as an SG

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Chuck,

At 09:57 AM 7/21/2001 +0100, you wrote:

>snip
>
>Is this MOV  behavior the same as that of two Zener diodes connected back to
>back? 

Yes.  Very similar.

>I am also curious about the failure mode of the MOV. Do they fuse and
>become shorted, or do they blow apart and become open circuited when
>subjected to excessive currents due to voltage spikes? 

At high power dissipation, they over heat and melt to a shorted blob.  If
you really hit them, they may explode but more likely they will burn to a
cinder.

>It appears that  they
>would fail every time in a pig type transformer secondary circuit when over
>voltage is experienced.

Not really, think of the current.  If a pig pushes about 5 amps on the
secondary (remember it's giant inductance will not allow super high instant
currents) then the current on the primary is 300+ amps.  Something else
will blow first.  The Pig's ballast would limit the energy...  Of course,
with a pig, MOVs may not be needed anyway but they would have to be the BIG
MOVs since the pig can heat them up so much faster.  Another thing is that
the MOVs only conduct current when the voltage is high.  The instant the
voltage drops, they turn off.  So they really only clip the high voltage
peaks further lessening their power load.  I have only used MOVs on NSTs so
there may be unforseen problems in using them with pig systems.

Cheers,

	Terry

>
>Chuck
>