[Amps] MOVs

[David Cutter]

Roger

Did those SCRs have an L/C filter out to the mains?  I fit a MOV directly 
across the SCR or Triac.

David
G3UNA


> On 11/25/2010 2:14 PM, David Cutter wrote:
>> It is my experience that MOVs should not be placed directly across the 
>> mains; they should have some limiting impedance, just like zeners do in a 
>> dc circuit.  I always fit them downstream of the mains filter so that as 
>> they conduct there is a limiting Z for the period of the glitch.  They 
>> are not meant to be sacrificial but they would be if placed directly 
>> across the mains with essentially extremely low source resistance.
> MOVs should be installed down stream of fuses or breakers.
> I should have said no flammable objects should be placed close to them.
> Carl is correct, MOVs don't normally catch fire, but a shorted one can get 
> very hot, or even explode.  I've seen a pair of #6 wires sticking straight 
> out from where a 50,000 joule MOV "used to be" and I've also been in the 
> next "cage" over from one when if blew. It takes a lot of power to explode 
> a 50,000 joule MOV and the results are spectacular.  The idea was to 
> protect a pair of 1200 Amp SCRs (bout the size of a hockey puck and water 
> cooled) from spikes, which they sometimes did. <:-)) The fuse they'd take 
> out was pretty spectacular for size too.
>
> All in all even if the SCRs went they were still worth the investment.
>
> 73
>
> Roger (K8RI)