Question

A Zener regulator has to handle supply voltage variation from 19.5V to 22.5V. Find the magnitude of regulating resistance, if the load resistance is 6KΩ. The Zener diode has the following specifications: breakdown voltage =18V, (IZ)Min=2µA, maximum power dissipation=60mW and Zener resistance =20Ω.

(a) 0 < R < 500Ω

(b) 77.8 < R < 500Ω

(c) 77.8 < R < 100Ω

(d) 18 < R < 500Ω

This question was addressed to me in an internship interview.

This is a very interesting question from Voltage Regulation Using Zener Diode in portion Application of Diodes of Electronic Devices & Circuits

Answer 1

Correct answer is (b) 77.8 < R < 500Ω

Easy explanation: (PZ)MAX/rZ=(IZ)MAX2 . So, (IZ)MAX =60m/20=54.8µA. IL=VO/RL=18/6000=3mA.

RMAX=(VMin-VZ)/[( IZ)Min+( IL)MAX]=(19.5-18)/(2µ+3m)=500Ω.

 RMin=(VMAX-VZ)/[( IZ)MAX+( IL)Min]=(22.5-18)/(54.8m+3m)=77.8Ω.