question_answer

The value of the resistor, \[{{R}_{S}},\]needed in the dc voltage regulator circuit shown here, equals : [JEE Main Online Paper (Held On 11 April 2015)]  

A) \[({{V}_{i}}-{{V}_{L}})/n\,{{I}_{L}}\]

B) \[({{V}_{i}}+{{V}_{L}})/n\,{{I}_{L}}\]

C) \[({{V}_{i}}-{{V}_{L}})/(n+1)\,{{I}_{L}}\]

D) \[({{V}_{i}}+{{V}_{L}})/(n+1)\,{{I}_{L}}\]

Correct Answer: C

Solution :

  Voltage drop across zener diode is \[{{V}_{L}}\]so voltage drop across S \[{{K}_{s}}\] \[{{K}_{Rs}}={{V}_{i}}-{{V}_{L}}=\left( n+1 \right){{I}_{L}}{{R}_{s}}\] \[{{K}_{s}}=\frac{{{V}_{i}}-{{V}_{L}}}{\left( n+1 \right){{I}_{L}}}\]