Question

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

Answer 1

Answer: 500

Voltage across $R_L=5V$
$\Rightarrow i_2=\frac{5}{R_L}$
Also voltage across $R=50-5=45$ volt
$Byv=iR\Rightarrow R=\frac{v}{i}=\frac{45}{i_i+i_2}$
$R=\frac{45}{90mA+\frac{5}{R_L}}$
Current in zener diode is maximum
when $R_L\to \infty (i_2\to 0$ and $i_i=i)$
So $R=\frac{45}{90mA}=500\Omega$