The basic structure and types of thyristors

A thyristor is a controllable diode, available in both unidirectional and bidirectional types, each with three electrodes. Unidirectional thyristors have a cathode (K), an anode (A), and a control electrode (G); A bidirectional thyristor is equivalent to two unidirectional thyristors connected in reverse parallel, forming T1 and T2 poles, as well as a control pole (G).
Working principle of unidirectional thyristor
Thyristors (unidirectional thyristors) belong to NPNPN five layer devices. During operation, its anode A and cathode K are connected to the power supply and load to form the main circuit, while its gate G and cathode K are connected to the control device to form the control circuit. When the thyristor is subjected to a forward anode voltage and the gate is not subjected to voltage, the anode current Ia ≈ Ic0 (the reverse leakage current of the J2 junction), and the thyristor is in a forward blocking state. When a current Ig flows from the gate G under a positive anode voltage, a sufficiently large Ig flows through the emitter junction of the NPN transistor, increasing the current amplification factor a2, generating a sufficiently large collector current Ic2 flowing through the emitter junction of the PNP transistor, and increasing the current amplification factor a1 of the PNP transistor, generating a larger collector current Ic1 flowing through the emitter junction of the NPN transistor. This strong positive feedback process quickly proceeds, and the thyristor is in a forward conducting state. After the thyristor is turned on, if the power supply voltage is continuously reduced or the circuit resistance is increased to reduce the anode current Ia to below the holding current IH, due to the rapid decrease of a1 and a2, when 1- (a1+a2) ≈ 0, the thyristor returns to the blocking state.
Working principle of bidirectional thyristor
Bidirectional thyristor is a type of silicon controlled rectifier device, also known as a bidirectional thyristor, that can achieve contactless control of alternating current in circuits. In addition to the control electrode G, the other two electrodes of the bidirectional thyristor are collectively referred to as the main electrodes T1 and T2. Its forward and reverse characteristic curves have symmetry, so it can conduct in any direction. When the voltage of the control electrode G and T2 relative to T1 is negative, T1 is the anode and T2 is the cathode; Vice versa. Under specific conditions, applying an appropriate trigger signal to the control electrode will cause the bidirectional thyristor to conduct, and after conducting, it can conduct bidirectionally. By controlling the timing and magnitude of the trigger signal, the conduction angle of the bidirectional thyristor can be controlled, thereby achieving control over the current and voltage in the circuit