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I wonder if the resistance of the heating element changes significantly with temperature - with a resulting change in current. If so, the current could be used as an indication of temperature and drive a controller that would be set to maintain a specific flow of current. While the element temp may not exactly indicate tip temperature it's a start and an improvement over simply varying the input voltage or waveform with no feedback.
The conventional method as indicated by the electronics master has one serious drawback, that it slows down the normal rate of heating. thus after switching on the power, one has to wait for a long time to start work. This can create a lot of annoyance, especially when one is is a hurry.
To overcome this, I have designed a ckt which sloves this problem in a simple & inexpensive way can be used with various loades of up to 80W.
It has 2 transistors, one acts as a voltage regulator and the other drives a relay acting as a timer.
"Danieljm" if you let me know your e-mail, I can submit the design & full details.