About SCR thyristor

The international common name of thyristor (SCR) is thyristor, abbreviated as thyristor in Chinese. It can operate under high voltage and current conditions, with advantages such as high voltage resistance, large capacity, and small size. It is a high-power switching semiconductor device widely used in power and electronic circuits.

thyristor are divided into unidirectional and bidirectional thyristor. Unidirectional thyristor has three lead out pins: anode A, cathode K, and control electrode G. The bidirectional thyristor has three output pins: the first anode A1 (T1), the second anode A2 (T2), and the control electrode G.

Only when a forward voltage is applied between the unidirectional thyristor anode A and the cathode K, and the required forward trigger voltage is applied between the control electrode G and the cathode, can it be triggered for conduction. At this point, A and K are in a low resistance conduction state, and the voltage drop between anode A and cathode K is about 1V. After the unidirectional thyristor is turned on, even if the trigger voltage of controller G is lost, as long as the positive voltage between anode A and cathode K is still maintained, the unidirectional thyristor continues to be in a low resistance conduction state. Only when the voltage of anode A is removed or the polarity of the voltage between anode A and cathode K is changed (AC zero crossing), can the unidirectional thyristor transition from a low resistance conduction state to a high resistance cutoff state. Once the unidirectional thyristor is cut off, even if a forward voltage is reapplied between anode A and cathode K, it still needs to be reapplied between control electrode G and cathode K to conduct. The conduction and cut-off states of unidirectional thyristor are equivalent to the closed and open states of switches, and can be used to make contactless switches.

Regardless of whether the polarity of the voltage applied between the first anode A1 and the second anode A2 of the bidirectional thyristor is positive or negative, as long as there is a triggering voltage with different polarity between the control electrode G and the first anode A1, the conduction can be triggered to a low resistance state. At this point, the pressure drop between A1 and A2 is also about 1V. Once the bidirectional thyristor is turned on, even if the triggering voltage is lost, it can still maintain the conduction state. Only when the current of the first anode A1 and the second anode A2 decreases and is less than the maintenance current, or when the voltage polarity between A1 and A2 changes and there is no triggering voltage, the bidirectional thyristor will be cut off. At this time, only by reapplying the triggering voltage can it conduct.