Top circuit:
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Signal is applied to PIC_T1 which "turns on" the photo-triac gate. Photo-triac gate conducts and "turns on" the gate of T1 (or T2).
R48 and C17 "pass some of the current from L1 which is fed to the photo-triac and they also set a time constant for the "turn on" time of T1. T1 senses the zero crossing of the AC signal and conducts for the length of time determined by the time constant of R48 and C17.
Someone else correct me if I'm wrong. I am sometimes
Regards,
Pld4me