Question

The value of power dissipated across the Zener diode $(V_z=15\text{V})$ connected in the circuit as shown in the figure is $x\times {10}^{-1}\text{watts}$.


The value of $x$ to the nearest integer is

Answer 1

Across $R_1$, potential difference is $22\text{V}-15\text{V}=7\text{V}$


Current flowing in resistance $R_1$ is
$i=\frac{7}{35}=\frac{1}{5}\text{A}$
Current flowing in resistance $R_2$ is
$i_1=\frac{15}{90}=\frac{1}{6}\text{A}$

From Kirchhoffs junction law,
$i_2=i-i_1$
$\Rightarrow i_2=\frac{1}{5}-\frac{1}{6}$
$\Rightarrow i_2=\frac{1}{30}\text{A}$

Therefore, power across Zener diode:
$P=V_2{i}_2$
$\Rightarrow P=15\times \frac{1}{30}$
$\Rightarrow P=0.5=5\times {10}^{-1}\text{W}$

$\therefore 5$ is the required value.