The series regulator derives its name from its control element. The output voltage. VO, is regulated by modulating an active series element, usually a transistor that functions as a variable resistor. Changes in the input voltage. VI, will result in a change in the equivalent resistance of the series element identified as RS. The product of the resistance. RS, and the load current. IL creates a changing input-to-output differential voltage, VI – VO, that compensates for the changing input voltage. The basic series regulator is illustrated in Figure 5-3, and the equations describing its performance are listed below.
Series regulators provide a simple, inexpensive way to obtain a source of regulated voltage. In high-current applications. However, the voltage drop which is maintained across the control element will result in substantial power loss and a much lower efficiency regulator.