Zener Diode and Emitter-Follower/Series Voltage Regulators

Almost all electronic devices and circuits need a dc source of power for their operation. While batteries and dry cells can be used as a form of dc power supply, their voltages are usually low and they require frequent replacement; additionally they are costly compared to regular dc power supplies. The domestic ac supply can be converted to dc voltage in a process referred to as rectification. This is achieved using a rectifier, filter and voltage regulator circuit. These elements combined together form a dc power supply.

Unregulated DC Power Supply

This is a power supply whose dc terminal voltage is affected significantly by the amount of load. As the load draws more current, the dc terminal voltage becomes less.

Regulated DC Power Supply

This is a dc supply whose terminal voltage remains almost constant regardless of the amount of current drawn from it. An unregulated dc power supply can be converted into a regulated supply by adding a voltage regulating circuit to it.

Key stages of a regulated DC power supply are illustrated in the block diagram below:

A typical dc power supply consists of five stages as illustrated in Figure 1.0 above.

Transformer

Used to either step up or step down the ac supply voltage to suit the requirement of the solid-state electronic devices and circuits fed by the dc power supply. It also provides isolation from the supply line which is a paramount safety consideration.

Rectifier

It is a circuit which uses one or more diodes to convert ac voltage into pulsating dc voltage.

Filter

The work of this circuit element is to remove the fluctuations or pulsations (referred to as ripples) present in the output voltage supplied by the rectifier.

Voltage Regulator

Its purpose is to keep the terminal voltage of the dc supply constant even when an ac input voltage to the transformer varies or the load varies. Normally, Zener diodes and Transistors are used to implement voltage regulation.

Voltage Divider

The purpose of this circuit is to provide different dc-voltages required by different electronic circuits. It consists of a number of resistors connected in series across the terminals of the voltage regulator.

Simple Zener Diode Voltage Regulator

The circuit in Figure 1.1 below illustrates how a Zener diode may be used directly for regulation. The output voltage remains largely constant at V_z as the input voltage and the load current vary. However, the load current is limited to a level less than the maximum diode reverse current, and the diode must pass this current when the load current is zero.

Emitter-Follower/Series Voltage Regulator

The circuit below shows how the Zener diode circuit may be combined with a transistor to deliver almost any desired level of the output current.

Zener diode D₁ in Figure 1.2 above is supplied with reverse current via resistor R₁. D₁ provides a bias voltage V_z at the base of transistor Q₁, which operates as an emitter-follower. The transistor emitter voltage is the circuit output, which is E_o = V_z – V_BE. Resistor R_E maintains a minimum transistor emitter current when the regulator load current is zero. The maximum load current that may be supplied is limited to the maximum transistor emitter current. For greater load, currents, two transistors may be connected as Darlington-connected transistors as illustrated in Figure 1.3 below. In this case Q₁ is typically a high-current device, and Q₂ is a low-current transistor to supply the base current for Q₁.

You may also read:

• A Detailed Note on the Benefits and Working of Automatic Voltage Controllers
• The Different Applications of Diodes
• Diode & Filter Applications in Amplitude Modulation (AM) Detection Circuits
• The Principle and Application of IMPATT Diode