[Boatanchors] MOVS (was: Article on changing from 2 to 3 wire power cords)

Alex [Temple Boatanchors] temple.boatanchors1 at miwww.com
Tue Aug 6 20:58:58 EDT 2019

At 05:50 PM 8/6/2019, you wrote:
>         MOVs have tendancy to blow up. i wont use them. Russ wb3fau

I build in MOVs wherever I can into older electronic equipment that 
does not have them, or home brew stuff, and never had any problems. 
Without knowing more details about your particular situation, if your 
MOVs keep blowing up it is because they are trying to do  their job 
protecting equipment absorbing overvoltage surges but you have not 
provided a means of overload protection for them (fuse, circuit 
breaker, etc) which in the case of big voltage spike or surge might 
cause the MOV to go short circuit while trying to take the hit and if 
there is no fuse it will blow itself apart trying to absorb the 
additional inbound energy of the surge/spike.

In my experience 130V MOVs and the diameter of about a nickel to a 
quarter are more than adequate to operate inside equipment that will 
run off 120-125V AC mains, and will chug away quietly protecting 
transformer primary windings, etc. Perhaps you have selected the 
wrong MOV voltage specification for your particular area AC mains 
voltage, or maybe you need a higher energy capacity device able to 
withstand more surges.

BTW MOVs don't have an unlimited lifetime and will wear out over time 
they clamp and dissipate incoming surges and spikes, but each event 
will cause some cumulative wear and eventually will reach a point 
where the MOV might either go completely short circuit and blow the 
hopefully existing main AC input fuse and isolate the protected 
equipment  from any damage. Or go open circuit and blow up if no fuse 
protection is in place between the mains input and the MOV. So never 
install them without adequate overload protection in place, otherwise 
you might get taken out with a nice bang during a heavy voltage surge 
overload event, but your equipment might still get damaged all the same. 

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