In a voltage regulator circuit, an unregulated input voltage of 15 V is applied to a series combination of a 300 resistor and a Zener diode. The Zener diode has a breakdown voltage of 9 V . A variable load resistor R_L is connected in parallel with the Zener diode. What is the minimum value of R_L required for the Zener diode to maintain voltage regulation?

Step-by-step solution

For the Zener diode to maintain voltage regulation, the voltage across the load resistor (without the Zener diode connected) must be at least equal to the Zener breakdown voltage V_z . At the boundary condition for regulation, the Zener current is zero ( I_z = 0 ). The current through the series resistor R_s is: I_s = V_i - V_z R_s = 15 - 9 300 = 6 300 = 20 mA Since I_z = 0 , all the current from the series resistor flows through the load resistor, so I_L = I_s = 20 mA . The minimum load resistance is: R_L = V_z I_