In a Zener regulated power supply , a Zener diode with `V_(Z) = 6.0` V is used for regulation . The load current is to be 4.0 mA and the unregulated input is 10.0 V . What should be the value of series resistor `R_(S)` ?

Explanation

The value of `R_(S)` should be such that current through the Zener diode is much larger than the load current . This is to have good load regulation . Choose Zener current as five times the load current , i.e., `I_(Z) = 20 mA` . The total current through `R_(S)` is , therefore , 24 mA . The voltage drop across `R_(S)` is `10.0 - 6.0 = 4.0 V` . This gives `R_(S) = (4.0 V)/((24 xx 10^(-3)) A) = 167 Omega` . The nearest value of carbon resistor is `150 Omega` . So , a series resistor of `150 Omega` is appropriate . Note that slight variation in the value of resistor does not matter , what is important is that the current `I_(Z)` should be sufficiently larger than `I_(I) = `