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I'm designing a triac-based relay circuit to switch ac mains on-off to a resistive heater.

The app note (AN-3004) for the opto isolator I am using, the MOC3061, suggests using a gate resistor value of greater than Vpeak/Ipeak, which would be 367V/1A, or R > 367ohms.

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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?

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