CBSE Class 12-science Answered

STUDY OF ZENER DIODE REGULATION:LINE REGULATION: an easy but important project:

Properties of zener diode

Zener is a semiconductor diode which is specially used for the regulation of the DC supply. Just like the general diode this has also a PN junction but its electrical characteristics are different from the characteristics of general purpose diode. Its basic structure and the symbol are given below –

Symbol and characteristics of zener diode

          To obtain the characteristics of zener diode, the graph is plotted in I and III quadrants. In I quadrant the anode is positive and the cathode is negative. In this condition the current through the zener forward biases it, at a very small voltage (about 0.6V or 0.7V). The current through it steeply increases as shown in the above diagram. However in III quadrant, when anode is negative and cathode is positive. In this case the diode works in breakdown region and the voltage on its cathode remains always equal to the zener voltage irrespective of the change in the supply voltage. In this condition, the current through the zener is almost parallel to y-axis and current is called as the avalanche current or Zener current (Iz).

Formula –

Specifications of components –

Procedure –

1)    Switch on the circuit and keep its line voltage at 170VAC, with the help of dimmerstat.

2)    Be careful while using the dimmerstat, as it carries dangerously high voltage.

3)    Measure the output voltage of the circuit in this position of dimmerstat.

4)    Repeat the above procedure as per given in observation table.

5)    Plot a graph of line voltage versus output DC voltage. 

6)    Calculate percentage line regulation using the given formula. 

Brief theory – the zener diode is used to regulate the voltage across a load when there are variations in the power supply voltage or load current. The zener regulator circuit is connected between the output of the bridge rectifier with filter and the load.

            The zener regulator consists of a current limiting resistor (R_s) connected in series with the input voltage of power supply (V_in) and the zener diode is connected in parallel with the load. The zener diode is reverse biased. it is selected with a breakdown voltage (V_z) equal to the voltage desired across the load. The value of series resistor can be calculated using the formula –

            As long as the (V_in) is greater than (V_z), the zener diode operates in the breakdown region and output voltage remains constant.