Characteristics of thyristors

The international common name for silicon controlled silicon (SCR) is Thyyisty, which is abbreviated in Chinese. It can work under high voltage and high current conditions and has the advantages of high withstand voltage, large capacity, and small size. It is a high-power switching semiconductor device and is widely used in power and electronic circuits.

SCR is divided into unidirectional SCR and bidirectional SCR. The unidirectional thyristor has three lead pins: anode A, cathode K, and control electrode G. The bidirectional thyristor has three lead pins: a first anode A1(T1), a second anode A2(T2), and a control electrode G.

Only when a forward voltage is applied between the anode A and the cathode K of the unidirectional thyristor, and the required forward trigger voltage is applied between the control electrode G and the cathode can it be triggered to conduct. At this time, there is a low-resistance conduction state between A and K, and the voltage drop between anode A and cathode K is about 1V. After the one-way thyristor is turned on, even if the controller G loses the trigger voltage, as long as the forward voltage remains between the anode A and the cathode K, the one-way thyristor continues to be in a low-impedance 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) does the one-way thyristor switch from a low-resistance conduction state to a high-resistance cutoff state. Once the one-way thyristor is turned off, even if a forward voltage is re-applied between the anode A and the cathode K, a forward trigger voltage still needs to be re-applied between the control electrode G and the cathode K to conduct. The on and off states of a one-way thyristor are equivalent to the on and off states of a switch, and a contactless switch can be made with it.

Between the first anode A1 and the second anode A2 of the bidirectional thyristor, no matter whether the polarity of the applied voltage is forward or reverse, as long as trigger voltages with different positive and negative polarities are applied between the control electrode G and the first anode A1, the conduction can be triggered to be in a low-resistance state. At this time, the pressure drop between A1 and A2 is also about 1V. Once the triac is turned on, it can continue to remain conductive even if the trigger voltage is lost. Only when the current of the first anode A1 and the second anode A2 decreases and is less than the maintaining current, or when the polarity of the voltage between A1 and A2 changes and there is no trigger voltage, will the bidirectional thyristor be cut off. At this time, only when the trigger voltage is re-applied can it be conducted.