In the single-phase half-wave controllable rectifier circuit, when the thyristor is triggered, the current in the positive half cycle of the circuit begins to flow, and there is a 90-degree phase difference between the voltage waveform and the current waveform. Because the thyristor is triggered late and the current rises slowly, the current waveform lags behind the voltage waveform by 90 degrees. When the current decreases to near zero, the thyristor will cut off the current, and the current waveform will come to an abrupt end and start again in the next cycle. Therefore, the waveform of single-phase half-wave controllable rectifier circuit requires 90 degrees, which is also to ensure the normal operation of thyristor, shorten the flow time and reduce the risk of thyristor over-current burning.