PNPN Trigger Diode

Another device, the PNPN "trigger diode," has an extra junction and therefore is not symmetrical. More will be explained about this configuration in the next chapter, where threshold gate circuits are discussed. 

SCRs are still being used in older equipment, but they are gradually being replaced by similar devices that work in both directions, + and -, to operate on both the top and the bottom of an ac sine wave. First of all, the PNPN trigger diode can be replaced by a "diac," which will work in either direction, + or -. Two alternative symbols are at the upper right of the Fig. 21.4, and it is sometimes called a bidirectional switch. 

---