A Thyristor rectifier is a power electronic device that is widely used in the conversion process from alternating current to direct current. It achieves a unidirectional flow of current by controlling the turn-on and cut-off of the thyristor, which converts alternating current into direct current. Thyristor rectifiers are characterized by high efficiency, reliability and fast response.

How thyristor rectifiers work

Thyristor rectifiers are mainly composed of thyristors, transformers, filter capacitors and load resistors. By controlling the trigger angle of the thyristor, the output voltage and current can be adjusted. There are three electrodes, namely anode A, cathode K, and gate G (also called control electrode).

Figure (1): Appearance of the thyristor

The following is the basic working principle of a thyristor rectifier:

Rectification process: The thyristor rectifier accepts an alternating current input and converts it into a one-way pulse current. In the positive half cycle, when the input voltage exceeds the trigger voltage of the thyristor, the thyristor is triggered to turn on and the current begins to flow through the load resistor. In the negative half cycle, the thyristor is turned off and the current does not flow through the load resistance. In this way, the thyristor rectifier achieves a unidirectional flow of current.

Trigger control: An external trigger signal is required for the thyristor to turn on. The trigger signal can be generated by the control circuit or by other thyristors. The provision of a trigger signal allows the thyristor to switch from the off state to the ON state, allowing current to pass through.

Filtering process: When the thyristor is turned on, the output current is in the form of pulses. In order to obtain a stable DC output, thyristor rectifiers are usually filtered with filter capacitors. Filter capacitors smooth current ripples and provide a stable DC voltage output.

Load regulation: By controlling the on-time of the thyristor, the output voltage and current can be adjusted. Increasing the on-time will increase the output voltage and current, while decreasing the on-time will decrease the output voltage and current

Table (1) Thyristor turn-on and turn-off conditions