TRIAC Motor Control Circuit

Here is the circuit I am using to control the motor. L2 is meant to represent the motor and C3 is the capacitor that comes with the motor. It is a single phase AC motor found here.

This circuit is taken from LTSpice. This particular simulation is designed to simulate firing the TRIAC at 50% power, at each peak of the sine wave. In the simulation I am seeing motor current ringing which causes the TRIAC to remain on for several AC half cycles.

In the real circuit, I only start to see this behaviour at power level of 65% or more. At 65% the TRIAC fires in the right place for the first half cycle, but for the next half cycle the TRIAC doesn't turn off due to non-zero current, and so 100% of the power is delivered to the motor for that half cycle. After that I guess current does go to zero because the TRIAC turns off. Then it fires again in the next half cycle and the process repeats. So the total average power delivered to the load is not 65%, it is (65 + 100) / 2 = 82.5%.

This is not stalling the project. We can manage it. I am just wondering if any of you have any suggestions for additions to be made to the circuit to help reduce this ringing for more reliable TRIAC control of the inductive load.

  • \$\begingroup\$ Of course, you should add a RC snubber across the triac. Have you found an answer here: electronics.stackexchange.com/a/596404/82111 ? The ST note tells you to use a Rgk, and I did also a calc for you , for suitable components values. \$\endgroup\$ Dec 2, 2021 at 20:34
  • \$\begingroup\$ @MarkoBuršič no difference with or without Rgk. In simulation trying to find source of excess current that was burning my R1, I found big reduction by removing second snubber across triac (39ohm, 10nF) from the ST datasheet. Does R1C1 not act as a snubber across the triac already? \$\endgroup\$ Dec 3, 2021 at 2:25

1 Answer 1


Does R1C1 not act as a snubber across the triac already?

Yes, but R1 is too high for triac snubber, actually R1C1 is the snubber for opto triac, also. You should use something like 47 ohm and 22nF, or even your old snubber 39 ohm 10nF.

enter image description here

R1 is the key. Using high R value is good for optotriac (high immunity to dV/dt) but not for triac. Decreasing R1 is good for the triac (high immunity to dV/dt).

Fitting a Rgk also helps.

By the way, your LTSpice circuit is wrong: the motor is not composed by inductor and capacitor in series, this is just an auxiliary winding, there shall be another winding in parallel of them.

  • \$\begingroup\$ Isn't R2C1 the snubber for the opto? The application notes suggest R1, R2, C1 and Rgk are sufficient. \$\endgroup\$ Dec 3, 2021 at 21:04
  • \$\begingroup\$ I have fitted Rgk in the simulation and it doesn't help the current ringing. I'll try it on breadboard just in case, but what I am really asking for is any tips for best practices when designing a motor controller with a triac to reduce the current oscillations to help the triac turn off faster. I've mostly done DC circuits, SMPS and firmware in the past. \$\endgroup\$ Dec 3, 2021 at 21:06
  • \$\begingroup\$ @humanistscience Upload the LTSpice file somewhere, post the images of the simulation. There is no way to understand those oscillations without viewing the simulation result. \$\endgroup\$ Dec 4, 2021 at 8:03

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