One-way thyristor, also known as common thyristor, is composed of four layers of semiconductor materials, with three PN junctions and three electrodes: the electrode led by the first layer of P-type semiconductor is called anode A, the electrode led by the third layer of P-type semiconductor is called control electrode G, and the electrode led by the fourth layer of N-type semiconductor is called cathode K.. P1N1P2N2 four-layer three-terminal device based on silicon single crystal, started in 1957. Because its characteristics are similar to vacuum thyratron, it is internationally known as silicon thyratron, abbreviated as thyristor T, and because the thyristor was originally used for static rectification, it is also called silicon controllable rectifier element, abbreviated as silicon controlled SCR. In performance, SCR not only has unilateral conductivity, but also has more valuable controllability than silicon rectifier (commonly known as "dead silicon"). It has only two states: on and off. Silicon controlled rectifier can control high-power electromechanical equipment with milliampere current. If it exceeds this power, the average current allowed to pass will decrease due to the significant increase of switching loss of components. At this time, the nominal current should be degraded. Silicon controlled rectifier has many advantages, such as: controlling large power with small power, and the power amplification is as high as hundreds of thousands of times; Very fast response, switching on and off in microseconds; Contactless operation, no sparks, no noise; High efficiency, low cost and so on. Weaknesses of SCR: poor static and dynamic overload capability; Easily misled by interference. Silicon controlled rectifier can be classified into bolt shape, flat plate shape and flat bottom shape.