Zero crossing occurs when the line voltage is changing its polarity, from positive to negative, or vice versa. What I understand is, to fire the triac properly the firing circuit and the AC line should be synchronous. Is that the reason to use zero crossing detector?
So the micro-controller takes the zero crossing as an input signal and adds a delay less than (1/50) seconds (for 50Hz electricity) to create a firing angle of the triac?
But I saw some projects that do not require this zero crossing detector. Here there is a discussion: Do I need zero-crossing detection for controlling a heater? but I still didn't understand why in some cases it is not necessary.
Could you elaborate when is necessary to use zero crossing detector in a simpler manner?
edit: Please see my related question: Confusion with TRIAC firing and zero crossing point