Question 53 of 209

A $8V$ Zener diode along with a series resistance $R$ is connected across a $20V$ supply (as shown in the figure). If the maximum Zener current is $25mA$, then the minimum value of $R$ will be _____ $\Omega$.

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Answer: 480

Let ${I}_{1}$ be the current through Zener diode and ${I}_{2}$ through load resistor.
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As maximum current flows through Zener diode, hence ${I}_{2}=0$.
$∴$Maximum current, $I={I}_{1}+{I}_{2}=25mA+0=25mA$
Here, ${V}_{o}$ is voltage drop across Zener $=8V$.
Since $I$ is maximum, hence minimum value of $R$
$R=\dfrac{{V}_{i}-}{{V}_{o}}=\dfrac{20-8}{25\times {10}^{-3}}=480\Omega$