There are many applications where it is desired to control (or regulate) the power fed to a load
e.g. to change the speed of a fan or motor. So far we have been using electrical methods to exercise such a control. However, electrical methods do not permit a *fine control over the flow of power in a system. Moreover, there is a considerable wastage of power. In the recent years, such semiconduc- tor devices have been developed which can exercise fine control over the flow of large blocks of power in a system. Such devices act as controlled switches and can perform the duties of controlled rectification, inversion and regulation of power in a load. The important semiconductor switching devices are :
(i) Silicon controlled rectifier (SCR) (ii) Triac
(iii) Diac (iv) Unijunction transistor (UJT)
The silicon controlled rectifier (SCR ) has already been discussed in the previous chapter. There- fore, we shall deal with the other three switching devices in the following discussion.