I'm designing a triac-based relay circuit to switch ac mains on-off to a resistive heater.
I believe the purpose of resistor R is to desensitize the main triac from noise going into triac’s Gate and improve the dv/dt of the triac. Selecting a value too high will defeat the purpose of drawing away some of the gate current, and selecting a value too low will make it hard to trigger the main triac.
Is selection of 392 ohms ok for this application?
Also, I have read about using a fused resistor for R. Is this really necessary? What failure mode would this prevent? Maybe if there is a spike of input voltage across the triac?
Are there failure modes I need to prevent where the heater could see all of line voltage and overheat?