It is used to give a trigger pulse to initiate the thyristor to the state of conduction.
It is an Operational amplifier, which converts the sine wave to a square wave. The output of the zero-crossing detector is fed to an integrator.
An Operational amplifier with a feedback capacitor is said to act as an integrator. The output of the integrator is similar to that of a saw tooth waveform.
Pulses obtained from the control circuit magnitude is less, hence it is insufficient to trigger the high current rated thyristors. This can be overcome by feeding the control circuit to a pulse amplifier which strengthens the weak control signals and later it is fed to the thysristor gate cathode terminals.
Thyristor anode-cathode voltage is high, where as gate circuit voltage is low. Hence necessary isolation must be given to the control circuit (gate circuit) and power circuit. This can be done by using an isolation transformer, whose turns ratio is 1:1 unlike the power transformers.