I am using a triac circuit to convert a 3 speed motor into a fully variable speed motor. The triac circuit I’m using is one I lifted from another motor which is already a fully variable speed motor.

The characteristics of the 3 speed motor are different than the variable speed motor, and my question is how can I adjust the snubber circuit R and C values to better match the characteristics of the 3 speed motor.

More detail as follows:

The three speed motor is a PSC 120V, 60Hz, 1.1 Amp motor. There is a dial to adjust speed between Low, Medium and High. The main motor winding has three different taps that the hot wire connects to, one each corresponding to the Low, Med and High speed settings. When I have set this fan up for variable speed control with the triac circuit, I have the dial switch set to the highest speed setting.

The variable speed motor is a PSC, 120V, 60Hz, 2.5 Amp motor. This motor is controlled by a triac circuit, with a variable resistor to adjust the speed of the fan. The control circuit for this fan is shown in Figure 1. I am using the same circuit to control the 3 speed fan – everything the same, except a different motor. The Triac is an ADT25C80H.

Figure 1: Triac control circuit

Figure 2 shows the voltage profile across the variable speed motor for a half cycle at lowest speed. You can see that in the first quarter cycle, which in an ideal world would be fully off, the voltage gets no higher than 25V (each division on the chart is 50V).

Contrast this with Figure 3 showing the voltage profile across the 3 speed motor for half a cycle at lowest speed. You can see here that in the first quarter cycle when the voltage should ideally be off, the voltage across the motor actually gets quite high – approaching 60V before the triac switches on. This has caused the motor to spin at a higher RPM at lowest speed than I was expecting. I’m guessing this is due to the snubber circuit R and C values not being optimized for this lower power fan (1.1 Amp vs 2.5 Amp).

Figure 2 and 3: Half cycle at min speed for variable speed and 3 speed motor

So back to my question: How should I adjust the snubber circuit values (R2 and C2 in Figure 1) for the 3 speed motor to get the first quarter cycle of Figure 3 looking more like that in Figure 2 (i.e. closer to being “off”, not approaching 60V)? From what I have read about snubber circuits, the 18 ohm resistor (R2) in my triac cct is on the lower end of what is typically used, and the 0.47uf capacitor (C2) on the higher end. Should I use a higher value resistor? lower value capacitor? if so, by how much?

Pls let me know if there is additional information I can provide that will be helpful (note: I don’t have a meter to measure the inductance of the 2 different motors).

Thanks in advance for the help.


1 Answer 1


Belated update on this: I replaced the original triac chip with a snubberless triac chip to see if I could get away without an RC snubber. The answer is that I can, and it now works better (less oscillations on voltage across the motor) than the original triac + snubber.

  • \$\begingroup\$ indeed, it should work without RC snubber, because when Triac turns off, then R1, R4 and C1 combination will themselves act like snubber. \$\endgroup\$ Jul 2, 2020 at 14:58

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