In a given circuit diagram, a 5 V zener diode along with a series resistance is connected across a 50 V power supply. The minimum value of the resistance required, if the maximum zener current is 90 mA will be ___ $$\Omega$$.

For minimum series resistance $$R$$, the load current is zero ($$I_2 = 0$$), forcing all source current through the Zener diode:

$$I = I_1 = I_{z,\text{max}} = 90\ \text{mA} = 90 \times 10^{-3}\ \text{A}$$

Voltage drop across the series resistor: $$V_R = V_i - V_z = 50 - 5 = 45\ \text{V}$$

Using Ohm's law: $$R = \frac{V_R}{I} = \frac{45}{90 \times 10^{-3}} = \frac{45000}{90} = 500\ \Omega$$