The difference between bidirectional thyristor and unidirectional thyristor

The difference between bidirectional thyristor and unidirectional thyristor

Controllable silicon, also known as thyristor, is a commonly used semiconductor device that can control the magnitude of current like a gate. Therefore, thyristors also have functions such as switch control, voltage regulation, and rectification. There are many types of thyristors, and the thyristors used in high-voltage circuits mainly include unidirectional thyristors and bidirectional thyristors. The common symbols for the appearance of thyristors are shown in the figure above. There are two types of thyristors: unidirectional and bidirectional. Can you distinguish which one is unidirectional and which one is bidirectional?

Pin function difference: Unidirectional thyristor abbreviated as SCR, bidirectional thyristor abbreviated as TRIAC. The pin symbols for unidirectional thyristors are K G、A， Among them, G is the gate and control electrode, A is the anode, and K is the cathode. The pin symbols for bidirectional thyristors are T1 T2、G， Among them, G is the gate terminal, and the other two terminals are the main terminals, represented by T1 and T2, because they can conduct bidirectionally without distinguishing between the cathode and anode. Its characteristic is that when the voltage of the G and T2 poles relative to T1 is positive, T2 is the anode and T1 is the cathode. On the contrary, when the voltage of the G and T2 poles relative to T1 is negative, T1 becomes the anode and T2 becomes the cathode. Due to the symmetry of the forward and reverse characteristic curves, bidirectional thyristors can conduct in any direction.

Difference in working state: If a unidirectional thyristor is used in a DC circuit, once the trigger signal is turned on and a certain amplitude of current is maintained, the thyristor will remain in the on state. Only by disconnecting the power supply once can it be turned off. If used in an AC circuit, during the period when it is subjected to forward voltage, if a trigger signal is received, it will remain conductive until the zero crossing of the voltage arrives, and will automatically turn off due to the absence of circulating current. During the period of withstanding reverse voltage, even if a trigger signal is sent, the thyristor remains in the cutoff state due to the reverse voltage between A and K.

Regardless of whether the polarity of the voltage applied between the first anode T1 and the second anode T2 of the bidirectional thyristor is forward or reverse, as long as a trigger voltage with different positive and negative polarities is applied between the control electrode G and the first anode T1, it can trigger conduction in a low resistance state. At this time, the pressure drop between T1 and T2 is also about 1V. Once a bidirectional thyristor is turned on, it can continue to maintain a conducting state even if the trigger voltage is lost. Only when the current of the first anode T1 and the second anode T2 decreases and is less than the holding current or when the voltage polarity between T1 and T2 changes and there is no trigger voltage, the bidirectional thyristor will cut off. At this time, only by re applying the trigger voltage can it conduct.

Application difference: Unidirectional thyristors are widely used in controllable rectification because they only conduct when the anode voltage is greater than the cathode voltage, and turn off when the control voltage is applied to the gate. This is similar to the unidirectional conductivity of diodes. It should be noted that once it is turned on, the gate signal cannot turn it off, and it can only be turned off by removing the load or reducing the voltage at both ends. When a bidirectional thyristor is triggered by a pulse when a forward or reverse voltage is applied, it will conduct. The conduction has a switching function, and the controllable conduction angle has a voltage regulating function. Therefore, bidirectional thyristors can perform dual functions of switching and voltage regulation in AC circuits. Controllable silicon is currently used in various fields, and the soft starter for electric motors is a typical application of controllable silicon.