to triac or not to triac
Hi winterhunter,
actually you don't require a PID control. A PI control will do for heating purposes. Compare the desired temperature with the actual temperature. If the difference is very high there will be the "D" automatically switching the Triac at each zero crossing. As the values get closer you just cut every second control cycle and further cut more and more. You might used fixed values based on measurements (e.g. temperature rise time) If the temperature overshoot is too high, just note the overshoot and stop heating shortly before max temperature is obtained. (due to the "battery like" accumulation of heat in the heater)
The same way it is done with my temperature controlled soldering station and it keeps the temperature almost constant, although the circuit receives disturbance by the damp sponge from time to time.
Don't complicate things if you can get good results investing little time in measurements.
This circuit is not going to have the same accuracy as an auto pilot. That one has to take care of tendencies already.
Regards
Hans