For simple applications, switching devices such as contactors or solid-state relays can be used to control electrical power. The electrical power in a process can be regulated by varying the ON and OFF times of these devices. But in many processes this provision of energy in large blocks will cause significant variations in the process output. As an example, it would not be possible to control lighting levels simply by using such two-state on/off switching elements. Neither could good temperature controllers be implemented in this way.
