Maximum allowable SCR gate voltage


Joined Dec 26, 2010
200μF is a pretty big capacitance, so I would expect that the capacitor is electrolytic. Is it going to be subjected to reverse voltage on the negative hal-cycles of the 24V AC? Most ordinary electrolytics would object to that!

Ron H

Joined Apr 14, 2005
The way you are trying to do this won't work. Your variable resistor will burn up, among other problems.
I ran simulations on the circuit posted below. It should work, but be aware that the maximum duty cycle is only about 25%, and this is not a square pulse (leading edge follows sine wave).
Be aware that I have no idea how your motor will respond to this.


Thread Starter


Joined Jun 2, 2011
Thanks for your effort in coming up with the motor control circuit and for running simulations on it. I will check it out with the motor when I have a chance to collect the parts, but in the meantime I would like to pursue my present scheme before I give up on it. I know it has problems, but I am not concerned about the variable resistor. I am using a rotary switch rather than a pot and I believe, through a combination of careful selection of the resistance values near the low end and selection of resistors with adequate power rating, I can avoid any burn-out problems.

I have some questions about the 0.025 ma and 1.5 volt figures that NTE gets for IGT and VGT in their test which I have contacted them about.