Question

Explain how a Zener diode maintains a constant voltage across a load.

Answer 1

Principle: In the breakdown region of a Zener diode, for widely changing Zener current, the voltage across the Zener diode remains almost constant.

                     Zener diode as a voltage regulator

R_s - current-limiting resistance,

R_L - Load resistance,

I_z - Current through the diode;

I_L - Load current

Electric circuit: The circuit for regulating or stabilizing the voltage across a load resistance R_L against change in load current and supply voltage is shown in the above figure. The Zener diode is connected parallel to load R_L such that the current through the Zener diode is from the n to p region. The series resistance R_s limits the current through the diode below the maximum rated value.

From the circuit, I = I_z + I_L

and V = IR_s + V_z

= (I_Z + I_L)R_s + V_Z

Working: When the input unregulated de voltage V across the Zener diode is greater than the Zener voltage V_z in magnitude, the diode works in the Zener breakdown region. The voltage across the diode and load R_L is then V_z. The corresponding current in the diode is l_z.

As the load current (I) or supply voltage (V) changes, the diode current (I_z) adjusts itself at constant V_z· The excess voltage V − V_z appears across the series resistance R_s·

For constant supply voltage, the supply current I and the voltage drop across R_s remain constant. If the diode is within its regulating range, an increase in load current is accompanied by a decrease in I_z at constant V_z.

Since the voltage across R_L remains constant at V_z, the Zener diode acts as a voltage stabilizer or voltage regulator.