| To obtain full
wave operation, so that the lamp can be provided with the full ac supply
voltage, two thyristors are required as shown.
Th1 is triggered during the positive half cycles of the ac supply, and Th2 during the negative half cycles. The waveforms
show the thyristors being triggered halfway through each half cycle and
the current through the lamp would be the sum of the two currents.
A pulse generator and a phase shift circuit is necessary to provide gate pulses which can adjust the power from minimum to maximum. Since the gate voltages have to be positive with respect to the cathodes, attention must be paid to the polarity of the gating pulses with respect each other. The high power ac circuit can be isolated from the control circuits by means of transformers or opto isolators.
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