[Amps] MOVs

Roger (K8RI) k8ri at rogerhalstead.com
Tue Jan 28 02:22:47 EST 2014

On 1/27/2014 10:27 PM, Jim Thomson wrote:
> Date: Sun, 26 Jan 2014 17:34:28 -0800
> From: Jim Brown <jim at audiosystemsgroup.com>
> To: amps at contesting.com
> Subject: Re: [Amps] MOVs
> On 1/26/2014 2:57 PM, Michael Tope wrote:
>> This is probably a dumb question, but why not put the MOV between hot
>> and neutral? If the spike is differential mode, that would kill it
>> without spiking current into the safety ground.
> Between hot and neutral on a 120V branch circuit is fine.  Also OK
> between Line and Line on 240V.

Be careful though, many an old house has had the neutral grounded in the 
outlet instead of back at the panel.


Roger (K8RI)

> 73, Jim K9YC
> ###  I mentioned in a previous post about wiring movs  from line to neutral.
> Whether you install  movs  from line to line or line to neutral....  make damn
> sure they are FUSED.  MOVs  consist of a lot of grains in series..and every
> time a transient, surge, spike  comes down the line, a few grains here and there
> will fail shorted.  Eventually the Mov V rating will keep dropping.   Once its low
> enough, it will conduct with just the normal 120 vac.  Or 240 if line to line.
> Boom, there goes the mov, blown to pieces.   I wouldn’t rely on the breaker
> in your main panel, or any fuses in the amp, end device etc,  to stop the destruction.
> You require the correct type of fuse and size to fuse a mov.
> ##  Newer style movs have a 3rd wire..with the center wire slightly offset to one side...so they
> cant be installed backwards in a pc board.  These types are internally fused..and are wired with
> a led etc. If you see the led not lit... you know the internal mov fuse has blown open...and you
> have no protection.   Several hundred house fires were caused in one year alone...all from movs  burning up.
> ##  MOVs  are not the only way to provide some protection  from transients and surges and spikes etc.
> Saturable reactor style transformers, like SOLA  constant voltage transformers  will also offer a degree of protection.
> Yet another  method is to use PI filters.  They use large torroids with heavy gauge wire plus UL rated caps plus movs.
> Windings used in both the neutral and hot sides of a 120 vac circuit. They can be used with several pi sections in series.
> The commercial 4 outlet box I use offers  50 db of suppression  in the 1st  2 outlets....and  70 db  in the 2nd pair of outlets.
> One PI filter feeds the 1st set of 120 vac outlets. A 2nd PI filter is between the 2  sets of outlets.  The 2nd pair of 120 vac
> outlets gets the benefit of both PI filters.
> Jim  VE7RF
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