[Amps] MOVs

[Roger]

On 11/25/2010 6:06 PM, David Cutter wrote:
> Roger
>
> Did those SCRs have an L/C filter out to the mains?  I fit a MOV 
> directly across the SCR or Triac.
That was over 23 years ago, so you have already heard my complete 
memories of the hook up. <:-((

73

Roger (K8RI)
>
> 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) 
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