In the given figure, what is the voltage needed to maintain 25V across the load resistance `R_(L)`, if Zener diode required a minimum current of 20 mA to work satisfactorily ?

Explanation

Given load resistance `=5 k Omega =5xx10^(+3) Omega`
Voltage needed to maintain current across `R_(L)` is
`V_(L)=25V`
`:. ` Current through load `R_(L)` is
`I_(L)=(V_(L))/(R_(L))=25/(5xx10^(3))=5mA`
Zener current `I_(Z)=20 mA`
Current through R is `I=I_(Z)+I_(L)`-
`rArr =l =20 +5`
`rArr l=25 mA`
Series resistance `=300 Omega`
`:.` Voltage drop across R is `V_(R)`=IR
`=25xx10^(-3) xx300=7.5 V`
`:. ` Input voltage needed `V=V_(R)+V_(L)`
=25+7.5
=32.5 V