Why is zero crossing detector is used in some AC control circuits?

Question

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

Answer 1

Zero cross switching can be used so that the switch action occurs when there is no voltage across the load and thus no current through the load. This prevents fast rise time current flow as what would occur if the switch closed or opened when the voltage was high. Eliminating this can:

1. Lower stress in semiconductor components that are used for the switches.
2. Reduces stress in capacitors that may be in the circuit so they are not asked to charge and/or discharge at a very high speed.
3. Reduce EMI (electromagnetic interference). EMI can occur when voltage sources are switched to and from their loads and resultant current transients can cause large amounts noise of noise be induced into nearby circuits. Zero switching eliminates this.

Answer 2

You are correct in stating that zero crossing detection is used to synchronise the firing angle of a TRIAC (or other switching device) to achieve dimming or speed control.

The answer from @Michael Karas explains why you might want to do zero crossing switching, which is completely different to doing dimming etc using phase control, which by definition cannot use zero crossing switching.

For controlling a heater you could use cycle times much longer than one cycle, ie on for 10 secs and off for 20 secs. In this case you would probably want to use zero crossing switching for the reasons Michael stated.