"Harvey Babb" wrote :
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.
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Harvey , Tnx for the useful info . I was interested in the spike voltage
suppression level of the MOV, which is required since the heater element is
for AC . In the event of DC a reversely connected diode would have done the
trick .

Frank GM0CSZ / KN6WH