Transistor Series Voltage Regulator : Circuit Design and Its Operation

In a power supply system, a regulator is an essential component, used to control the output power in power electronics. The power electronics can be defined as the control as well as the conversion of electrical power in the part of electronics. A voltage regulator generates a stable output for the variations in input or load. There are different types of voltage regulators like Zener, series, shunt, fixed positive, IC, adjustable, negative, dual tracking, etc. This article discusses an overview of the transistor series voltage regulator.

What is a Transistor Series Voltage Regulator?

The series voltage regulator can be defined as a regulator which has the limitations like high dissipation, less efficient, and the transistor voltage and Zener diode voltages are affected once the temperature rises.

Circuit Design of Transistor Series Voltage Regulator

This voltage regulator circuit design is shown below. The following circuit can be built with a transistor as well as a Zener diode. In this circuit, the load current flows through the Q1 series transistor. So this is the reason to call this regulator a transistor-series-voltage-regulator. When the unregulated DC supply is given to the input terminals of the circuit then we can get the regulated output across the load. Here Zener diode provides the reference voltage.


The transistor series voltage regulator working is when the voltage at the transistor’s base voltage is held to the stable voltage across the diode. For instance, if Zener voltage is 8V, the transistor’s base voltage will stay approximately 8V. Therefore, Vout = VZ – VBE


The operation of this transistor can be done in two cases like when the output voltages increase and decrease.

  When the Output Voltage Decreases

When the o/p voltage decreases in the circuit, then the BE voltage will be increased and causes the transistor to perform more. As a result, the output voltage will be maintained at a stable level.

  When the Output Voltage Increases

When the o/p voltage increases in the circuit, then the BE voltage will be decreased and causes the transistor to perform less. As a result, the output voltage will be maintained at a stable level.

Advantage/ Disadvantages

The advantages of this series voltage regulator are listed below.

  • The main benefit of this voltage regulator circuit is, the changes within Zener current are decreased through a factor ‘ß’. Therefore, the Zener impedance effect will be decreased extremely & we can get an additional stabilized output.

The series voltage regulator disadvantages are listed below.

  • The adjustments within Zener current are decreased to a significant amount; the amount produced is not completely stable. This occurs due to both VZ & VBE reduce by the rise in room temperature.
  • It is not simple to modify the o/p voltage because no such resources are provided.

Thus, the Zener RPS (regulated power supply)efficiency turns into extremely low as the load current is high. Beneath these conditions, a transistor-like Zener controlled is used frequently to maintain o/p voltage stable. Fundamentally, the transistor voltage regulators which are controlled by Zener are classified into two types namely series voltage regulators and shunt voltage regulators. Here is a question for you, what is the main function of the voltage regulator?