I measured 550F putting a diode in line with a 700F iron. I would put the
circuit in a box with a socket and small piece of plugged cord, in line,
near the transformer so can be removed easily. And a trembler sensitive
enough to sense the bench being kicked.
Not that I will be making such a unit as I switch off my iron and have
dozens of tips left over from a batch bought 20 years ago. Tap wood, both
switch and element have survived about 20 years together in this same iron,
repeatedly switched on and off. Had to replace the mains switch once though.


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Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/