Question

In a given circuit diagram, a $5\, V$ zener diode along with a series resistance is connected across a $50 \,V$ power supply. The minimum value of the resistance required, if the maximum zener current is $90\, mA$ will be ______$\Omega$.

Answer 1

Voltage across $R_{L}=5 V$
$\Rightarrow i_{2}=\frac{5}{R_{L}}$
Also voltage across $R=50-5=45$ volt
$B y v=i R \Rightarrow R=\frac{v}{i}=\frac{45}{i_{i}+i_{2}}$
$R=\frac{45}{90 \,m A+\frac{5}{R_{L}}}$
Current in zener diode is maximum
when $R_{L} \rightarrow \infty\left(i_{2} \rightarrow 0\right.$ and $\left.i_{i}=i\right)$
So $R=\frac{45}{90\, mA }=500 \,\Omega$