Design a 10V regulated supply for load currents from 0 to 10

Design a +10V regulated supply for load currents from 0 to 100mA; the input voltage is +20 to +25V. Allow at least 10mA zener current under all(worst case) conditions.

A. what power rating must the zener have?

B. With the same specifications use a sener emitter follower. Calculate worst-case dissipation in transistor and zener.

C. from part B: What is the percentage change in zener current from no-load condition to full load?

D. compare the two circuits from part A and part B

Thanks for your help :)

Solution

worst case Zener current = 10mA and max input voltage = 25V)

yield a worst case power in the Zener of: 0.010 A * 25V = 0.250 watts.
So it needs a power rating of at least a quarter watt.

Because the Zener can be subject to the max input voltage, not only the input minus the output voltage. Its resistance changes to maintain the 10V in the output by dropping 25V - 10V = 15V. But... since the circuit has to be designed to allow for a zero load in the output due to any of several failures, the Zener must be designed to drop the full input voltage, not just the input minus the output voltages. But your teacher could have in mind the Zener\'s operating load in which case it would need to be rated for 0.150 watts.