Explain the application of Zener diode as a voltage regulator.

Zener diode as a voltage regulator (shunt regulator).

        Rs        I_L
 +o---/\/\/---+--------+
   ^   I ->   |        |
   |   I_Z    |       R_L   V_o = V_Z
  Vin        _|_ cathode up |
   |         /_\ Zener      |
 -o-----------+--------+

A series resistor R_S is connected from the unregulated DC input V_in to the Zener diode, which is reverse-biased (cathode to +) and placed in parallel with the load R_L. Output V_o = V_Z.

• \( I = \dfrac{V_{in}-V_Z}{R_S} = I_Z + I_L \).
• Line regulation (V_in changes, R_L fixed): if V_in rises, I rises; since V_Z is constant, the extra current flows through the Zener (I_Z increases) and the extra voltage drops across R_S. V_o stays at V_Z. The reverse happens if V_in falls.
• Load regulation (R_L changes, V_in fixed): I is constant; if I_L increases, I_Z decreases by the same amount, so V_o stays at V_Z.
• Condition: the diode must stay in breakdown, I_ZK < I_Z < I_ZM; R_S is chosen so that \( R_S = \dfrac{V_{in(min)}-V_Z}{I_{L(max)}+I_{ZK}} \) and the Zener power \( V_Z I_{Z(max)} \le P_Z \).

Uses: reference voltage in power supplies, regulated low-power DC supplies, over-voltage protection.