One of the manual temperature control circuits

This circuit adopts single-phase bridge half-control mode. The trigger circuit is a resistor-capacitor phase-shift bridge, which inputs control signals to the thyristors vl and Vz
through the diodes VDi and VDz in the positive and negative half cycles respectively . Adjusting the potentiometer RP can shift the phase of the triggering sine wave, thus changing the conduction angle of the thyristor, so that the voltage applied to both ends of the electric heater EH can be changed to achieve the purpose of temperature regulation.     In the figure, Ri and Rz are the current limiting resistors of the thyristor control electrode.     This circuit is greatly affected by the fluctuation of grid voltage and is only suitable for occasions with low requirements.