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Electronics Mains heating element - triac control

Discussion in 'Modding' started by g0th, 7 May 2007.

  1. g0th

    g0th What's a Dremel?

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    I need to throw together a circuit which uses a pot and triac to provide adjustable control of a heating element rated at 150W 240V AC.

    Does anybody know where to find a suitable circuit?

    Would an ordinary light-dimmer circuit be appropriate or not?

    http://www.electronics-project-design.com/Light-Dimmer-Circuit.html

    Will this circuit do the job?

    If i use this circuit, can I remove R2 and R3, change the pot to 47K (or 100K ?), use a BT137 triac, and change C1 to 100nF?

    Is the inductor really needed? Can I get rid of it?

    Cheers.
     
  2. cpemma

    cpemma Ecky thump

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    C3 and R2 drop off a large fraction of the mains voltage. R2 will act as a current-limiter when the circuit is first switched on (when C3 is empty so acts as a short) if the pot is near zero. I suspect R3 is there to give a more linear fading effect with lighting, like using a log pot.

    Compare this simpler circuit but note it's for 110/120V only.

    The heater is presumably a non-inductive load, so a light dimmer would be OK.

    With a more powerful heater, a very-low-frequency PWM controller could be used with a zero-crossing opto-triac on the end triggering the power triac.
     
    Last edited: 7 May 2007
  3. g0th

    g0th What's a Dremel?

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    OK, great, thanks for that.

    I'll replace C1 with a series RC network as described, and include the 100uH inductor

    Would a logarithmic pot be a better choice than linear?

    How are the values of C2, C3, R1, Pot resistance, etc related - how are they chosen?

    Cheers.
     
  4. cpemma

    cpemma Ecky thump

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    C3 is picked to set the maximum current the pot needs to handle, from its reactance (1/2pi.F.C) at mains frequency. Pot then sets a delay before the voltage at the diac is high enough to strike it, maximum delay will need to be a mains half-cycle for maximum dim. I think then C2 discharges through the diac (at a lower voltage than C3) to give a strong short pulse to the triac.

    There's another similar circuit (but no values) for this kit using a preset in parallel with the control pot, may be to fine-tune the pot to give a minimum speed. :confused:
     

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