Designing a +10V Supply with Emitter Follower Circuit: A Practical Approach

[irishhockey]

Homework Statement

Design a +10V supply with the following specs:
I_out =0-100 mA
V_out =10V
V_in=20-25V (unregulated supply w/ ripple)

The circuit will have the diagram attached.

The book I am using is The Art of Electronics, which is very "practical" the authoppr says things like why worry about calculating something to 1 in 100, when you will then reach for a 10% resistor or 20% cap.

Homework Equations

I_base * beta = I_collector
I_emitter = I_base (1+beta) = I_base + I_collector
I_emitter=I_base +0.6V

The Attempt at a Solution

I start at the output, 10V and 0-100mA. The Voltage at the base and also zener must be equal to collector voltage +0.6V. The current in the base is 1/100 the collector current, and output current is 100mA(max) so I _base = 1mA and I_collector=100mA. The zener also requires a constant 10mA, which is needed to keep the dynamic resistance under control. My issue is now the resistors.

So, I know that the resistoprs have 25V on the side facing the power supply, and for R, the voltage on the base/zener side is 10.6V. Does this mean I can say the voltage drop across the res is 14.4 V (using 25V and planning for worst case)? If so, can I just say I_BASE + I_zener=11mA, asnd the use V/I=R? if so Ic an figure everything else out I belive.

Thank you

 

[rude man]

The worst case is when power supply = 20V.

If you assume 25V and compute R your way, what would be the consequence of a "soft" zener voltage if the supply dropped to 20V?