A Zener diode is used in a voltage regulator circuit to provide a constant voltage of 12 V to a load. The unregulated input voltage V_ in fluctuates between 16 V and 24 V . The series resistance in the circuit is 40 . If the Zener diode requires a minimum 'knee' current of 4 mA to maintain breakdown, what is the minimum value of load resistance R_L for which the output voltage remains regulated?

Step-by-step solution

To maintain regulation, the Zener diode must remain in the breakdown region, which requires a minimum current of I_ z,min = 4 mA . The most critical condition for losing regulation occurs when the input voltage is at its minimum and the load draws maximum current. The minimum input voltage is V_ in,min = 16 V . The total current available from the source through the series resistor R_s at this minimum input voltage is: I_ s,min = V_ in,min - V_z R_s = 16 - 12 40 = 4 40 = 0.1 A = 100 mA Applying Kirchhoff's current