Help with circuit

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

  1. toppytop

    Thread Starter Member

    Aug 1, 2009
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    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.
     
  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.
     
  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
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