The following diagram shows a Zener diode as a voltage regulator. The Zener diode is rated at $V_z=5

Question

The following diagram shows a Zener diode as a voltage regulator. The Zener diode is rated at $V_z=5 \mathrm{~V}$ and the desired current in load is 5 mA . The unregulated voltage source can supply upto 25 V . Considering the Zener diode can withstand four times of the load current, the value of resistor $R_s$ (shown in circuit) should be $\_\_\_\_$ $\Omega$.

Professor PC Thomas Classes · Answer

To protect the Zener diode under maximum input voltage ($V_{in,max} = 25 	ext{ V}$), the series resistor $R_s$ must limit the total current $I_s$. The maximum Zener current is $I_{z,max} = 4 	imes I_L = 4 	imes 5 	ext{ mA} = 20 	ext{ mA}$. The total source current is $I_s = I_{z,max} + I_L = 20 	ext{ mA} + 5 	ext{ mA} = 25 	ext{ mA}$. Applying Kirchhoff's laws, $R_s = \frac{V_{in,max} - V_z}{I_s} = \frac{25 	ext{ V} - 5 	ext{ V}}{25 	imes 10^{-3} 	ext{ A}} = \frac{20}{0.025} = 800 \Omega$. (Note: 800 $\Omega$ is the technically correct answer, which was not among the original given options).