go_away

### Author Topic: The art of electronics : regulated power supply question  (Read 703 times)

0 Members and 1 Guest are viewing this topic.

• Jr. Member
• Posts: 33
##### The art of electronics : regulated power supply question
« on: May 31, 2011, 11:34:43 AM »
Hi so I am trying to figure out what is going on in a diagram in the art of electronics.

Here it is below.

What I am trying to make is a similar circuit but with a 13V output, 20V input and a 6.8V zener with 120mA instead of 100mA load.

I am having trouble with the mathematics around which resistors to pick to achieve these values.

Is there any chance someone could walk me through what values to use and why for the spec above? I am lost

By the way the transistors are meant to be ZTX300's but its the principles and formulas i need help with most.

#### waltr

• Supreme Robot
• Posts: 1,938
##### Re: The art of electronics : regulated power supply question
« Reply #1 on: May 31, 2011, 12:09:45 PM »
To change the output voltage to 13V just change the resistor divider (R2 & R3) to obtain 5V on the base of the transistor (Q2).
Q2's base will always be about 0.7V greater than the emitter, so 4.3V (1N4731 zener) plus 0.7V (base-emitter drop) equals 5V.

If you use a 6.8V zener then Q2's base will be at 6.8 + 0.7 = 7.5V so recalculate the resistor divider. Do you know how to calculate a divider?

I'm not sure why the circuit is marked as being limited to 100mA. Post the figure number that is in the Art of Electronics book so I can read what is said about this circuit. A 2N3055 should be able to pass much more current if properly heat sunk.

Have you built and played with this circuit yet?
What did the circuit do?

• Jr. Member
• Posts: 33
##### Re: The art of electronics : regulated power supply question
« Reply #2 on: May 31, 2011, 12:26:50 PM »
Hi thanks

At the minute I am simulating this circuit, it is a college assignment question (I know I am cheating but I have put a lot of effort into this and still got nowhere ) I was able to do the first question which didn't have feedback but when it asked for this circuit I got confused.

The figure number is 2.75 on page 105.

#### waltr

• Supreme Robot
• Posts: 1,938