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

Explanation

For a good load regulation, the current flow through the zener diode must be larger than the current flow `I_(L)` through the load resistance `R_(S)`.
Suppose `I_(Z)=5I_(L) and I_(L)=4mA`, so `I_(Z)=5xx4=20mA` hence total current through `R_(S)` is,

`I_(S)=I_(Z)+I_(L)`
`=20+4`
`therefore I_(S)=24mA`
`rArr ` Voltage drop across `R_(S)`.
`DeltaV_(S)=V_(L)-V_(Z)`
`=10-6`
`=4V`
`rArr ` Resistance `R_(S)=(DeltaV_(S))/(I_(S))=(4)/(24xx10^(-3))`
`therefore R_(S)=166.66Omega`
`therefore R_(S) ~~167Omega`
`rArr` The value of the carbon resistance `150Omega` is closest to 167`Omega`. So, series resistance of 150`Omega` is appropriate.
`rArr` Note that slight variation of the value of the resistor does not matter, but the current `I_(Z)` should be sufficiently larger than `I_(L)`.