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I have tested the above circuits and found that I can switch on the triac with the circuit at the top, but not with the bottom circuit.

In my understanding, the Triac can be triggered on no matter the gate is positive or negative, the main difference is I may need a larger gate trigger current (Ig). I have lowered the gate resistor value but still cannot switch on the Triac, why?

Note: I know it is not related to the optocoupler, because when I am testing it, I even shorted pin 4 and pin 6 of the optocoupler and it still does not work


3 Answers 3


Your triac is upside down in the schematic.

  • To trigger the triac you have to bring the trigger positive or negative with respect to terminal T1 (the terminal on the same side as the trigger on the schematic).
  • Your "not work" schematic shows you shorting out the trigger pin to T1 so that does nothing.
  • In your "work" schematic turning on the opto-coupler connects the trigger to the voltage across the triac. Since this is high enough the triac turns on.


simulate this circuit – Schematic created using CircuitLab

Figure 1. The WORK and NOT WORK schematics redrawn for clarity.

You are confusing operation of a trigger derived from the mains to one where a separate voltage is used. Figure 2 shows how this would work but note again that the trigger voltage is with reference to terminal T1 (the same side of the triac as the trigger pin).


simulate this circuit

Figure 2. Switching a triac with a separate trigger supply.

Finally, for reference:

enter image description here

Figure 3. Triac triggering modes and quadrant numbers. Source Wikipedia (with addition of numbers).

Note that your circuit works on quadrants 1 (both mains and trigger positive) and quadrant 3 (both negative) only.


Due to the position of the gate in the PNPN and NPNP layers of the triac the device must be triggered between the gate and T1 and not between the gate and T2


The triac is triggered by a current flowing between the gate and MT1.

Connecting the gate to MT1 does not make it turn on, rather it makes it a bit less likely to turn on.

It's like a relay coil- shorting it out will not make the relay turn on.

MT1 and gate will always be within a couple volts of each other on a (non-damaged) triac, because of the internal structure.


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