[Amps] MOVs

Jim Thomson jim.thom at telus.net
Mon Jan 27 22:27:02 EST 2014

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.

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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