Answer:

A device which maintains the output voltage of an ordinary power supply constant irrespective of load variations or changes in input a.c. voltage is known as a voltage regulator.  The figure shown below is  a simple series voltage regulator using a transistor and zener diode. The circuit is called a series voltage regulator because the load current passes through the series transistor Q1 as shown in Figure . The unregulated d.c. supply is fed to the input terminals and the regulated output is obtained across the load. The zener diode provides the reference voltage.

Operation. The base voltage of transistor Q1 is held to a relatively constant voltage across the zener diode. For example, if 8V zener voltage is used, the base voltage of Q1 will remain approximately 8V. Referring to Figure ,

$ V_{out} = V_Z − V_{be}$

(i) If the output voltage decreases, the increased base-emitter voltage causes transistor Q1 to conduct more, thereby raising the output voltage. As a result, the output voltage is maintained at a constant level.

(ii) If the output voltage increases, the decreased base-emitter voltage causes transistor Q1 to conduct less, thereby reducing the output voltage. Consequently, the output voltage is maintained at a constant level.

The advantage of this circuit is that the changes in zener current are reduced by a factor β. Therefore, the effect of zener impedance is greatly reduced and much more stabilised output is obtained.

1) Line Regulation -

In this type of regulation, series resistance and load resistance are fixed, only input voltage is changing. Output voltage remains the same as long as the input voltage is maintained above a minimum value.

Percentage of line regulation can be calculated by $=\dfrac{\Delta V_0}{\Delta V_{in}}\times 100$

where $V_0$ is the output voltage and $V_{in}$ is the input voltage and  $\Delta V_0$ is the change in output voltage for a particular change in input voltage $\Delta V_{in}$.

For a good power supply , the percentage of line regulation should be very small.

2) Load Regulation -

In this type of regulation, input voltage is fixed and the load resistance is varying. Output volt remains same, as long as the load resistance is maintained above a minimum value.

Percentage of load regulation $=\dfrac{V_{NL}-V_{FL}}{V_{FL}}\times 100 $

where $V_{NL}$ is the null load resistor voltage (ie. remove the load resistance and measure the voltage across the Zener Diode) and  $V_{FL}$ is the full load resistor voltage.