How to create a zero crossing detector using a full bridge wave rectified circuit
Source link:

The circuit in this Design Idea generates a zero-crossing pulse off the ac mains and provides galvanic isolation. The falling edge of the output pulse happens at approximately 200 μsec before the zero crossing. You can use the circuit to safely stop the triggering of a thyristor gate, giving it time to properly turn off. The circuit generates short pulses only when the mains voltage is approximately 0V, thereby dissipating only 200 mW at 230V and a 50-Hz input.







simulate this circuit &ndash; Schematic created using CircuitLab



Rearranging the circuit you can get the same for your XFORMER input:







simulate this circuit