ZENER DIODE VOLTAGE REGULATOR

                         A Zener diode can readily be used as voltage regulator element to maintain constant voltage at the output.

Basic principle:

                        When a zener diode operates in zener breakdown region (under reverse bias condition), the voltage across it is constant for a large change in current through it. Therfore as long as the input voltage is greater than the zener voltage, it operates in breakdown region and maintain constant voltage across the load resistor even there is a change in input voltage or in load current.

Construction:

                        A simple zener diode voltage regulation is shown in figure. A current limiting resistor (R) and a suitable zener diode (D) are connected in series across a unregulated power supply in between filter and load. The zener diode is reverse biased so as to operated in the breakdown region. The regulated output is obtained across the load resistor (RL) which is connected in parallel R is selected such that the current flowing through the zener diode (I) is about 50% of its maximum current. The current flowing through the resistor R is given by formula I=IZ+IL., where IZ is the zener current and IL is the load current.

Working:

                      When the input voltage (Vin) is constant and greater than zener voltage (Vz) the current flowing through the resistor R is also constant. When the load current increases, the zener current decreases by the same amount. Therefore the zener current flowing through the limiting resistor is kept constant and hence output voltage across the load resistor is constant. Vo = Vi-IR. Suppose yhe load current remains constant and the unregulated input voltage (V1) increases, then the total current (I) drawn from the supply will increase and hence the zener current increases by the same amount. As a result the voltage across R and compensates the increase in supply voltage. Therefore the voltage across RL remains constant. Similarly when the unregulated input voltage decreases, the total current drawn from the supply will decrease and hence zener current decreases by the same amount. As a result the voltage across the resistor R decreases and compensate the decrease in supply voltage. Therefore the output voltage remains constant.

                       Thus the zener diode voltage regulator keeps the output voltage constant irrespective of the variations in either the input voltage (V1) or the load current (IL).

Advantages :

1. It is small and light weight.

2. It provides voltage regulation over a wide range of current.

Disadvantages:

1. It has poor voltage regulation.

2. It has low efficiency.

3. Regulated output voltage is determined by the zener breakdown voltage and can not be easily changed.

4. It is used only when there are small variations in load current and output voltage. 

 

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