MOV in 120/240V dual primary

[MarshallPlexi]

How can I use a MOV (Metal Oxide Varistor) with an amp that has a dual primary PT? (120V/240V)

The PT is a dual primary non-tapped with 2 120V windings in series for the 240V and the 120 windings in parallel for 120V operation. I can't seem to find a way to protect the amp in case someone switches the amp to 120V operation when they plug into a 240V outlet. Any ideas?

[Enzo]

You are thinking a MOV would protect the amp from double mains voltage? Good luck.

The MOV is used to protect a circuit from transient voltage spikes, it is not a power zener that will hold 240 down to 120. If someone switches the amp to 120 while plugged into 240, no MOV will help.

The way to protect your amp from this is to keep the monkeys who might do this away from your amp.

[MarshallPlexi]

I thought that was the point of the MOV. In that if someone applies the wrong voltage then it would short across the MOV and blow the mains fuse. Am I wrong in my understand of the MOV?

[Don Moose]

short across the MOV and blow the mains fuse. Am I wrong in my understand of the MOV?

You are. While the MOV can shunt a ton of current for a few microseconds and then heal itself ... sort of ... over some longer period, it won't hold up to sustained current.

Besides that, fuse is not a precision device - the rating is the highest current at which it WON'T (eventually) blow. It can take a "fast-blow" fuse seconds to go at even double its rated current.

What you want for this application is not an MOV but a Polyswitch Self-Resetting Fuse.

Hope this helps!