Question

A 5 V zener diode is used to regulate the voltage across load resistor $R_L$ and the input voltage varies in between 10 V to 15 V. The load current also varies from 5 mA to 50 mA. Find the value of series resistance R.

Given, $I_Z\left(min\right)=20mA$

• A. $70\Omega$
• B. $90.91\Omega$
• C. $142.86\Omega$
• D. $71.43\Omega$

Answer 1

The correct option is D $71.43\Omega$

The zener current $I_Z$ will minimum when the load resistance is maximum and the input voltage will also minimum.

Thus, $I_T=I_Z\left(min\right)+I_L\left(max\right)$

By KVL, $V_{in}\left(min\right)-V_Z=I_{T}R=\left[I_Z\right(min)+I_L(max\left)\right]R$

so, $R=\frac{10-5}{(20+50)\times {10}^{-3}}=71.43\Omega$