GATE 2021 EE – Question 35
In the circuit shown, a 5 V Zener diode is used to regulate the voltage across load $R_0$. The input is an unregulated DC voltage with a minimum value of 6 V and a maximum value of 8 V. The value of $R_s$ is $6\ \Omega$. The Zener diode has a maximum rated power dissipation of 2.5 W. Assuming the Zener diode to be ideal, the minimum value of $R_0$ is ________ $\Omega$.
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Correct answer: 30
Explanation
The load current is largest when the source current is smallest, at $V_i=6$ V: $I_s=\frac{6-5}{6}=0.167$ A. The Zener must still conduct, so the load current can be at most $0.167$ A, which gives $R_{0,min}=\frac{5}{0.167}=30\ \Omega$. At $V_i=8$ V the Zener current $\frac{8-5}{6}=0.5$ A is exactly $\frac{2.5}{5}$, the maximum rating, and this is reached with no load.