Todd Harrison did a teardown of the CSI Premier 75W and Hakko 936 solder station:
I did the teardown of the CSI 75W solder station as well as the Hakko 936. The Hakko of course had all analog circuits using operational amplifiers and potentiometers as well as a temperature sensor in the iron tip to signal back to a triac chip controller which would trigger the triac to push 24 V AC through the heater in the iron. The CSI 75W was all digitally controlled with an internal microcontroller which was reading the temp sense line and switching the triac on/off through an opto isolator IC. And because it had a microcontroller it was also able to have the programmable set points and programmable sleep and power off timers.
More details at ToddFun.com.