Q:   A zener diode shows a constant voltage of 5.6V for currents more than five times the knee current Izk is specified to be 1mA. It is to be utilized in the design of a shunt regulator fed from a 15V supply. The load current changes over the range of 0 to 15mA. Determine an appropriate value for resistance R. Calculate the maximum power dissipation to the zener diode.

Answer:

The minimum Zener current is

                                                            I_zk=5(1mA)=5mA

Load current can be 15mA, in order that the value of R, is such like that with IL=15mA, zener current of 5mA is available, thus,

                                 R= (15-5.6)/20mA

                                         R = 470W

The maximum power dissipated in the zener takes place when, IL=0, in that situation the zener current should be 20mA, thus

                                                                P=20m(5.6)=112mW