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.
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.
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.
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.