I am working on a project using the following circuit, which uses an Arduino type microcontroller to do pulse width dimming of AC circuitry using a TRIAC:

enter image description here

I am wondering what wattage I should choose for the 330 ohm resistor as this is not specified in the schematic. From doing research I understand that TRIAC gates only conduct current for a really brief amount of time before the device switches on, but I also couldn't find any specific guidance for picking a safe wattage value. I have a bunch of standard 1/4W through holes and I'm wondering if I can use one of those, or if I have to order a special higher wattage 330.

Is there any rule of thumb or formula I can use to calculate this kind of thing?


1 Answer 1


BTB08 datasheet says: I_gt = 100mA|max,V_gt = 1.3V|max
MOC3052 datasheet says: V_tm, Peak On−State Voltage, Either Direction = 2.5V|max

Though there are multiple factors to be considered, let's simply focus on the triggering of the output Triac. In order to trigger the Triac, its gate needs to see the current larger than the "gate trigger current". Thus the formula to calculate the worst case minimum current is:

I_gt = [V_line(t) - (V_gt + V_tm)] / R

Ex.1, With R = 330ohm: The Triac turns on when the line voltage (across A2 & A1) is about +/-29V 100mA = V_line(t) - [(1.3V + 2.5V)] / 330ohm -> V_ine(t) = 29.2V

Ex.2, In order to turn on the Triac when the line reaches 20V: R = 162ohm
100mA = [20V - (1.3V + 2.5V)] / R -> R = 162ohm

The Triac gate is protected, as long as the Triac turns on at I_gt, since the V_t1_t2 collapses to V_tm = 1.5V.

  • \$\begingroup\$ Yes but how do I calculate the wattage of that resistor? I know what the resistance needs to be but I'm not sure about the wattage. \$\endgroup\$ Aug 29, 2021 at 17:58
  • 1
    \$\begingroup\$ @EmmettPalaima , W = I^2 x R = 100mA * 100mA * 330ohm = 3.3W peak \$\endgroup\$
    – jay
    Aug 29, 2021 at 18:50

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.