Designing a +10V Supply with Transistors: A Practical Approach

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SUMMARY

The discussion focuses on designing a +10V power supply using transistors, with specifications of Iout ranging from 0-100 mA and Vin between 20-25V. The participant references "The Art of Electronics" for practical guidance and applies key transistor equations, including Ibase * beta = Icollector. The main challenge identified is calculating resistor values while considering voltage drops across components, particularly with varying input voltages affecting zener current. The conversation emphasizes the importance of understanding dynamic resistance in zener diodes for stable output.

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Homework Statement


Design a +10V supply with the following specs:
Iout =0-100 mA
Vout =10V
Vin=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 author 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


Ibase * beta = Icollector
Iemitter = Ibase (1+beta) = Ibase + Icollector
Iemitter=Ibase +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 Icollector=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 IBASE + Izener=11mA, asnd the use V/I=R? if so Ic an figure everything else out I belive.

Thank you
 

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You are on the right track but is 25V the worse case? What does the zener current fall to if the input is 20V instead of 25V.
 

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