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Here is a typical TRIAC SBS trigger circuit to control a resistive load. This example is from the BS08A datasheet but there are plenty of others.

What is the purpose of the D1,D2 and R3? It appears to me that there should be never current from the SBS gate through D2. (That is, D2 is never forward biased.)

enter image description here

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The purpose of D1, D2 and R3 is to reset the capacitor if it never triggers. This reduces the "snap-on" effect in cheap dimmers where there is a hysteresis in the behavior at low settings. D2 is forward biased by C1.

The reset only occurs on alternate half cycles, but that's enough.

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    \$\begingroup\$ Thank you, that makes sense and is quite obvious now when said. Never thought of how the circuit would behave if the trigger would 'dud'. I'm fixing a 2kW short planer with this circuit where TRIAC (BTA12-600B) has failed. Heatsink screw had loosened up and probably overheating killed the TRIAC. \$\endgroup\$ Mar 3 at 14:59

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