Question

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

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

Answer 1

Answer: (C)

In Zener diode,
$\Rightarrow \frac{V_{\max }-V_Z}{R_1}\ge I_{\text{knee}}+I_R$
$R_1$ is the series resistance of $50\Omega$ and $I_R$ is the current through the resistance $R$
$\Rightarrow \frac{10-6}{50}\ge 5m+I_R$
$\Rightarrow I_R\le 80mA-5mA$
$\because {\left(I_R\right)}_{\max }=75mA$ (the maximum current through $R$ )
So the minimum value of the resistance $R$ is
$R_{\min }=\frac{V}{{\left(I_R\right)}_{\max }}=\frac{6V}{75mA}=\frac{6000}{75}=80\Omega$