Consider the following circuit. Zener diode has zener voltage

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Consider the following circuit.
Zener diode has zener voltage V_Z = 16 V at a minimum i_Z of 15 mA. If V_b = 24 ± 3V and R_L varies from 250 Ω to 2 kΩ, find the value of R_S to maintain regulation—

1. R_S > 55·92
2. R_S < 123·04
3. R_S < 63·29
4. R_S > 26·8

Correct Option: C

The given circuit (i.e., fig)
Given, V_Z = 16V, I_Z(min) = 15 mA
V_b = 24 ± 3V gives V_b(min)
= 21V & V_b(max) = 27V
R_L varies from 250Ω to 2KΩ
RS =?

I_L(min) =		V_Z	=	16V	= 8 mA
		2KΩ		2KΩ

I_L(max) =		V_Z	=	16V	= 64 mA
		250Ω		250Ω

The minimum value of R_S to maintain the regulation is given by expression

R_S ≤	V_b(min) – V_Z
	I_ZK + I_L(max)

or R_S ≤	21 V – 16 V
	15 mA + 64 mA

or R_S ≤	5 V
	79 mA

or R_S ≤ 63·29Ω
Hence alternative (C) is the correct choice.