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, since wide variations of the process variable are unacceptable in such an application.Control elements such as variable transformers have been used ever since the beginnings of automation, as they permit a continuous variation of the electrical power. A variable transformer is, however, very expensive, subject to wear, and only permits slow adjustment.This publication is intended to clarify the operating principles of electronically controlled power units, which are free from wear and have a very high rate of adjustment of the output level. The descriptions of the power units are generalized, but in some places they refer specifically to thyristor and IGBT power units from JUMO.