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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)? |
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Answer» Solution :For a good load regulation, the current FLOW through the zener diodemust 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)`. |
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