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.
David
G3UNA
> On 11/25/2010 12:53 PM, Ken Brown wrote:
> The original post for this thread has not shown up here yet.
>>> A similar thing happened with garden variety 130 v rated MOV's....
>>> too many house fires. Modern MOV's for 120/240 v use, will have a 3rd
>>> lead sticking out [offset to one side]
>>> and an internal fuse built into em.
>>>
>>>
> A 130V MOV on the 120V AC line would get hot in a hurry 1,414 * 120 =
> 169.7V so the thing would be conducting a high percentage of the time.
> You'd want at least a 180V MOV.
>> I don't understand how this can be a useful device. I thought that MOVs
>> were supposed to draw lots of current, instead of letting it got
>> elsewhere causing damage to more expensive and sensitive electronics.
> That is correct, but an under sized, under rated MOV could cause a fire
> by itself.
>> I
>> had always thought of MOVs as sort of "sacrificial" devices, intended to
>> blow up under the worst power surges,
> They are.
>> and mounted inside a metal
>> cabinet so that when they do blow up nothing else is damaged.
> Unfortunately many omitted this part.
>> I would
>> never mount an MOV where it could set fire to something else.
>>
> Many did. <:-)) MOV's are, or used to be, rated by their breakdown
> voltage. So people were purchasing 130V MOVs for 120 V AC lines
> forgetting, or not knowing, the peak voltage is 1.414 times the RMS.
>> DE N6KB
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|