Fast spot welding machine circuit using thyristor switch

Fast spot welding machine circuit using thyristor switch

. In the figure, sQ is the limit switch . When the upper welding arm (driven by the motor M) moves downward,
it is pressurized once (so is closed), the relay KA is closed once, and the thyristors vl and V2 are triggered to conduct once. Adjust the potentiometers
RPi and RPz to make the positive and negative half-cycle waveforms output by Vl and Vz symmetrical.