I have this circuit attached below. I have built it identical except for the 1.5M pot. All I had was a 1M pot and I am using 18 VAC instead of 12. A couple of problems is that I have to turn the pot about 3/4 of the way until I get a response from the motor. I am using this as a speed control for a model train engine operating at 18 VAC. This other problem is it operates at low speed fairly smooth but as I increase the speed the train eventually stops and the motor just hums. I have read just about everything I could get ahold of on the internet about phase control of a triac. I have not come across how to calculate the R/C circuit for the gate control. Everything you find is for 120 VAC or 220 VAC operation. Any ideas or suggestions for 18 VAC operation. Thanks.
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