How do Thyristors work?

Thyristors are semiconductor devices that act as switches, controlling the flow of electrical current. In configurations involving back-to-back Silicon Controlled Rectifiers (SCRs), they allow the full wave current to be conducted. Specifically, the forward SCR conducts during the positive half-cycle of the alternating current (AC), while the reverse SCR conducts during the negative half-cycle.

Each SCR is activated by a trigger pulse applied to its gate, which is a third terminal. Once an SCR is turned on, it remains in the conducting state until the instantaneous load current passing through it drops to zero. This behaviour makes thyristors particularly useful in AC applications where precise control of the current is required.

The trigger pulses necessary to turn on the SCRs are generated by a drive circuit. This circuit carefully times the pulses to ensure that the thyristor unit’s output corresponds accurately to the input control signal and the chosen firing mode. By adjusting the timing of these pulses, the drive circuit can control the exact point in the AC cycle at which the SCRs turn on, thus regulating the amount of power delivered to the load.

Thyristors are widely used in applications such as motor speed controls, light dimmers, and power supplies due to their ability to efficiently manage high voltages and currents. Their robust design and reliability make them a critical component in many industrial and commercial electrical systems.

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