Bidirectional thyristor is a silicon controllable rectifier device, also known as bidirectional thyristor. This device can realize contactless control of alternating current in the circuit and control large current with small current, which has the advantages of no spark, fast action, long life, high reliability and simplified circuit structure. From the appearance, bidirectional thyristor is very similar to ordinary thyristor, and it also has three electrodes. However, except for one electrode G, which is still called the control electrode, the other two electrodes are usually called the main electrodes Tl and T2 instead of the anode and cathode. Its symbol is also different from that of ordinary thyristor, which is drawn by connecting two thyristors in reverse together, as shown in Figure 2. Its model is generally represented by "3CTS" or "KS" in China; Foreign data are also represented by "TRIAC".