# Help with circuit

Discussion in 'The Projects Forum' started by toppytop, Sep 4, 2009.

1. ### toppytop Thread Starter Member

Aug 1, 2009
10
0
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.

Last edited: Sep 4, 2009
2. ### CDRIVE Senior Member

Jul 1, 2008
2,223
99
Welcome to AAC!
You forgot to post the schematic though.

3. ### toppytop Thread Starter Member

Aug 1, 2009
10
0
Here it is. Crashed as I posted.

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4. ### CDRIVE Senior Member

Jul 1, 2008
2,223
99
That's a typical thyristor circuit but it's late and I will have to pick this up in the morning. Someone else might get back to you before me though.

5. ### toppytop Thread Starter Member

Aug 1, 2009
10
0
If someone could just help me to understand how to calculate for the values of the pot, capacitor, resistor portion of the circuit to correctly fire the SBS/triac according to the mains voltage, it would be appreciated. That is my biggest hang up. Thanks

6. ### toppytop Thread Starter Member

Aug 1, 2009
10
0
Thank you, that is the type of info I need. The train is 18 VAC. I have looked at numerous diagrams and some say they are suitable for brush type motors by adding a snubber circuit and/or inductor. Is zero crossing something that can be added to my circuit?

7. ### CDRIVE Senior Member

Jul 1, 2008
2,223
99
Could you please elaborate a bit further. I spent a bit of time trying to Spice the failure that your experiencing but I couldn't make it happen. From the start, I could not fathom why the train stalls and buzzes when increasing the speed to a certain point. I could understand this if you were reducing the speed to a certain point.

As far as a zero crossing detector is concerned; I see no purpose for it in this type of circuit as Triacs intentionally chop out sections of the pos & neg going slopes starting at zero until trigger voltage is reached. See attached spice.

BTW, the Diac that I used is a macromodel that I built because my Diac models have a bug. Unfortunately, I couldn't find a Bilateral Switch in my library. Which is why I spiced this with Diac.

There is a possibility that since your load is inductive (mine is not) you do need a snubber. Maybe back EMF is affecting the charge of C1 causing the problem you describe.

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8. ### CDRIVE Senior Member

Jul 1, 2008
2,223
99
OK, I changed the 30 Ohm load resistor to a 500mH inductor with a DC resistance of 30 Ohms. Yes, it caused major problems do to spikes. I then connected a 470 Ohm to the Anode of the Triac in series with a 5uF cap connected to GND. This cured the problem but you will probably need to play with these values.

Last edited: Sep 7, 2009