Hi,

If you are doing a phase angle control then make sure you have a EMI/RFI filter between your zero cross sensing path. Using an opto-coupler is a good choice and the LED currents are not that much sensitive to the line noise (third harmonic injection by your thyristor control module). On the photo-transistor side use a constant current source followed by a schmitt trigger buffer. You can also use a low pass filter there. This gives you a rock-solid sync. If you are not doing a phase angle control then make sure you are firing the thyristors right at the zero cross. I use a sustained fence-pulse circuit (using the HPWM module of PIC). And for low current applications I use opto-triacs (both zero cross and non zero cross depending on the application). As far as the field experience goes , phase reversal are common problems that drive the synchronized trigger algo crazy. Post your schematic if possible.