The load am controlling is an inductive load, I want to feed the input of a step down transformer.The control circuit for the triacs will depend on the load you intend to drive. Some loads tolerate phase-controlled switching, others might not. If the load is a resistive heater with high thermal inertia then burst-mode control is usually appropriate.
Be aware that peak mains voltage will still be present, even if the overall RMS voltage is reduced by thyristor switching.
The output of the transformer is used for annealing of copper conductor. I.e the output is short circuited by the conductor that passes through a roller at s a speed there by heating the conductor for annealing to be achieved.What are you connecting to the secondary of this transformer?
Thanks for your response , but I don't know what point to input 0...10dc, the point that would trigger the thyristors gatesYou can try this.
You can trigger two back-to-back SCRs with a single opto-triac. You might not even need the cathode-gate diodes as most big SCR seem to have a low value resistor (10Ω or so) integrated between gate and cathode.
Although it's a resistive load, I'd still recommend a snubber. either across the thyristors or across the transformer.
For the 0-10V trigger circuit the LM317 should be set to give about 10.5V output, and the small signal transistors are your favourite small signal transistor (BC547/BC847/BC557/BC857/2N3904/2N3906 etc)
The only critical capacitor is the 47nF.
The Opto Triac is the random phase variety (obviously)
The small transformer is >3VA 12-0-12V or 15-0-15V (you may already have one in circuit), with the primary connected to the same mains as your power transformer.View attachment 245219
[EDIT] The first NPN transistor should have a 1N4148 diode with its cathode to the transistor base and its anode to the emitter, to protect its base-emitter junction from reverse polarity.
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