Electronic – Why is zero crossing detector is used in some AC control circuits

1. Consider replacing the optocoupler chosen, with one that is designed for direct interface to a mains power line. For example, Fairchild Semiconductors MID400 is one such part.
2. Use a much lower LED current. While your opto may specify a fairly large (20mA to 100 mA, even 1 A in some cases) drive current for the LED, many will work fine at as little as 1 to 2 mA: The part's transfer curve should provide enough information to confirm this.
3. A capacitor can be used as an impedance element for dropping the input AC voltage, instead of the resistor. As the voltage drop is almost entirely pure reactive, there is hardly any thermal dissipation to be concerned about. You do however require "X" rating capacitors, designed for use in AC mains circuits.

You cannot smoothly dim a normal incandescent light bulb with zero-crossing control of normal 50/60Hz single-phase mains- the filament (even a really fat high-power one) will visibly flicker to an objectionable degree with even a small number of possible levels of control. Similarly, radiant heating is problematic with zero-crossing control because the temperature can change significant within a small number of cycles. In those cases, phase control is typically used.

Normally for zero crossing control we would like a cycle length of several seconds or more (up to maybe 30-60s), so that the number of half-cycles is at least in the low hundreds. That limits the applications to those where the low-pass filter formed by the heat capacity of the various elements will smooth out the power, so generally those applications with a time constant in the 1minute + range.

Phase control has problems that zero-crossing switching does not have (more EMI, there may filtering required for EMC compliance, undesirable audible noise from lamp filaments, nonlinear response power-vs-trigger angle). On the other hand, zero crossing switching of high current loads can cause visible light flickering (otherwise independent lights that happen to be powered from the same mains circuit).

For phase control or for zero crossing switching you need zero crossing detection. In the case of zero crossing switching, the micro can delegate that job to the triac driver and just tell it roughly when it wants the triac on or off, and the driver and triac will respond with some latency depending on when the zero crossing happens to hit.

There's a third alternative- the simplest- random switching, where the triac just switches on whenever it is asked to (and switches off at the zero crossing, since that's all it can do).

If you implement a zero-crossing detector for a micro and drive the triac with a non-zero-crossing opto (or use a random switching SSR) then you can select any of the three options with firmware.