Power control is useful when soldering. It allows to keep the temperature of a soldering tip at the optimal level, reducing its destruction and extends its life.
I found in the Web two circuits of microcontroller power regulator by L.I. Ridiko. These devices are control the power by changing the number of half cycles through mains soldering iron on the second.
The higher power set, the more half-cycles will go on a soldering iron and the less will be a pause between them. Because the switching occurs in moments of transition of line voltage through the zero,
electrical noise, caused by the regulator, will be minimal.