I reverse engineered the power board of an electric heater. I think, I understand most aspects of this circuit. One thing is not entirely clear to me though: The circuit seems to have two distinct ways to detect the zero-crossing point which is needed to control the Triac:
- Via the opto-isolator (U1) that is directly driven by the primary
- Via the voltage divider R7/R8 that taps the secondary of the transformer
Both signals feed directly into a microcontroller (PIC16F886). Both pins (RA5/RB0) on the microcontroller can be configured to be inputs to the built-in ADC.
What could be the reason for having those 2 separate circuits?