Question

A zener regulator has an input voltage in the range 15v-20v and a load current in the range of 5 ma-20 ma. if the zener voltage is 6.8v, the value of the series resistor should be

Answer 1

I am also looking for this answer .
I have a book Arihant solutions for  previous JAM paper , if this helps you , please elaborate me too.

I max = (Vmax - Vz)/ Rs
I min = (min - Vz)/ Rs
V max = 20 V , Vmin = 15 V , 
Imax = 20mA , Imin = 5 mA
so Rs is 390 ohm.
I have tred comfirming this and by other means too. but i didnt understand.

Answer 2

Answer:24 ohm

Explanation:

Input voltage=5V

Zener current=20-5=15mA

Zener voltage=6.8V

Rs need to be such that Iz should be larger than load current

Let Iz=4× load current =60mA

=> Total current through Rs= 75mA

=>Voltage drop across Rs=6.8-5=1.8V

=Rs= V/I

=1.8/75×10^-3

=0.024×10^3

= 24ohm