Use a 10-volt Zener diode in both cases

**Fig. 1 **

**Solution: **

(a). The design consists of choosing the proper value of resistance, R i , and power rating for the Zener. We use the equations from the previous section to first calculate the maximum current in the zener diode and then to find the input resistor value. From the Equation (E-6), we have

I Zmax = 0.533 A

Then, from Equation (E-3), we find R i as follows,

It is not sufficient to specify only the resistance of R i . We must also select the proper resistor power rating. The maximum power in the resistor is given by the product of voltage with current, where we use the maximum for each value.

P R = ( I Zmax + I Lmin ) (V smax – V Z ) = 6.3 W

Finally, we must determine the power rating of the Zener diode. The maximum power dissipated in the Zener diode is given by the product of voltage and current.

P z = V z l zmax = 0.53 x 10 = 5.3 W