The MOV (Metal Oxide Varistor) is a surge supressor whose only function
is to absorb the "kick" when the mechanical thermostat opens (or
"bounces" when closing). The one in the Weller unit (if memory serves)
clamps the voltage at about 40 volts (Transformer secondary is 24 VAC).
If it isn't working properly the thermostat contacts arc and weld shut,
causing runaway overheating.

Frank Dinger wrote:

While you're at it, check one more thing! If the problem turns out to be
welded contacts on the thermostat switch, check that the MOV across the
secondary of the transformer is still good. I had one that repeatedly
welded the contacts till I replaced the MOV and has worked for 5 years
now without a problem.

=============================
Not having such a Weller soldering iron myself ,nevertheless I am interested
in the MOV (firing) voltage against the normal operating voltage . Also , is
it DC or AC ?

Frank GM0CSZ / KN6WH