The main distinction between the Bidirectional Thyristor & Unidirectional Thyristor
The thyristor has unidirectional(one way) type and bidirectional (two way) type, use a different symbol, the unidirectional thyristor has 3 PN-junctions in total, 2 PN-junctions from external layer P electrode and N electrode, an anode and cathode, 1 PN-junction from middle P electrode as the grid.
Unique feature of unidirectional thyristor: when the anode connect with reverse voltage, or the anode connect with forwarding voltage but the grid no extra voltage, it still no turn on, but when the anode and grid connect with forwarding voltage in same time, it turns on, and control voltage lost control on it, no matter with control voltage or without, no matter electrode of control voltage, the unidirectional thyristor will keep turn on, if user wants to cut off it, the only method is to reduce the anode voltage to a critical value or reverse.
Bidirectional thyristor’s pin according to T1, T2, G sequence from left to right. The trigger pulse and time change on the grid G also can change its conduction current flow.
The distinction with the unidirectional thyristor, if the bidirectional thyristor’s G grid trigger pulse electrode has changed, its conduction direction will follow its electrode changing, so the user can control the current load, but the unidirectional thyristor only conducting from anode to cathode after trigger, so there are Bidirectional Thyristor & Unidirectional Thyristor.