This component is designed to pass current in one direction only. It can be forced to conduct in the opposite direction if the reversed potential exceeds its breakdown voltage, but this mistreatment is likely to cause damage.

By comparison, the diac and triac are designed to be bidirectional.

The SCR has three leads, identified as anode, cathode, and gate. Two functionally identical versions of the schematic symbol are shown in . Early versions sometimes included a circle drawn around them, but this style has become

obsolete. Care must be taken to distinguish 1 discrete semiconductor > thyristor > SCR SCR 1 between the SCR symbol and the symbol that

represents a programmable unijunction transistor (PUT), shown in Figure 1-2.

Figure 1-1. Two functionally identical schematic symbols for an SCR (silicon-controlled rectifier). The symbol on the left is more common.

Figure 1-2. The symbol shown here is for a programmable unijunction transistor (PUT). Care must be taken to distinguish it from the symbol for an SCR.