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GATE 2018 EC – Question 36

Analog Circuits · Diode Circuits · 2 marks · Multiple choice

The circuit shown in the figure is used to provide regulated voltage (5 V) across the $1\ \text{k}\Omega$ resistor. Assume that the Zener diode has a constant reverse breakdown voltage for a current range, starting from a minimum required Zener current, $I_{Zmin}=2$ mA to its maximum allowable current. The input voltage $V_I$ may vary by 5% from its nominal value of 6 V. The resistance of the diode in the breakdown region is negligible.

The value of $R$ and the minimum required power dissipation rating of the diode, respectively, are

Diagram for GATE 2018 EC question 36
  1. $186\ \Omega$ and 10 mW
  2. $100\ \Omega$ and 40 mW
  3. $100\ \Omega$ and 10 mW
  4. $186\ \Omega$ and 40 mW

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Correct answer: (B) $100\ \Omega$ and 40 mW

Explanation

The load current is $5$ mA. At the minimum input of $5.7$ V the Zener must still carry $2$ mA, so $\frac{5.7-5}{R}=7$ mA and $R=100\ \Omega$. At the maximum input of $6.3$ V the current in $R$ is $13$ mA, so the Zener carries $13-5=8$ mA and dissipates $5\times8=40$ mW.