Question

The $6 \, V$ Zener diode shown in the figure has negligible resistance and a knee current of $5 \, mA$ . The minimum value of $R \, \left(in \, \Omega\right)$ so that the voltage across it does not fall below $6 \, V$ is

• A. $40$
• B. $60$
• C. $80$
• D. $120$
• E. $90$

Answer 1

Answer: (C)

In Zener diode,
$\Rightarrow \frac{V_{\max} - V_{Z}}{R_{1}}\geq I_{\text{knee}}+I_{R}$
$R_{1}$ is the series resistance of $50\Omega$ and $I_{R}$ is the current through the resistance $R$
$\Rightarrow \frac{10 - 6}{50}\geq 5 m+I_{R}$
$\Rightarrow \, I_{R}\leq 80 mA-5mA$
$\because \left(I_{R}\right)_{\max}=75mA$ (the maximum current through $R$ )
So the minimum value of the resistance $R$ is
$R_{\min}=\frac{V}{\left(I_{R}\right)_{\max}}=\frac{6 V}{75 \, mA}=\frac{6000}{75}=80 \, \Omega$