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'[ee]: voltage regulator (circuit)'
2003\01\30@224505 by rad0

picon face
I know that many of you have the book, The Art of Electronics,
this question deals with the circuit from fig 2.75 on page 105.

a zener diode and two transistors, along with 3 1K resistors.

I thought I would try to make this circuit, but I couldn't get the
exact components, and I thought great, this will be a learning
experience, LOL

this is pretty stupid, but maybe some of you will help out

Q1 - 2N3055
Q2 - 2N3904
D1 - 1N4731 4.3V
R1,2,3   1K


I substituted a TIP3055 for the Q1 2N3055
are these even close?

a MPS3904 for the Q2 2N3904
??

and a 1N4733 5.1V  for the 1N4731 4.3V

my input voltage is 15vdc and I get 5.5vdc output

what I'm really wanting to know is, what is the most important
component for determining the output? is it the diode? if so, why?

I rekon if these transistors are just variations of the ones called for
in this circuit, then the resistors are the same, the diode must be
the deciding factor?  but the circuit claims to regulate 12 to 25 v input
to 10 v output, so my 5.5v out is way off from this, so does a difference
of a 5.1V zener versus a 4.3V zener make this much of a difference?

thanks

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2003\01\30@230203 by Spehro Pefhany

picon face
At 09:43 PM 1/30/2003 -0600, you wrote:
>I know that many of you have the book, The Art of Electronics,
>this question deals with the circuit from fig 2.75 on page 105.
>
>a zener diode and two transistors, along with 3 1K resistors.
>
>I rekon if these transistors are just variations of the ones called for
>in this circuit, then the resistors are the same, the diode must be
>the deciding factor?  but the circuit claims to regulate 12 to 25 v input
>to 10 v output, so my 5.5v out is way off from this, so does a difference
>of a 5.1V zener versus a 4.3V zener make this much of a difference?

No, maybe something else is wrong. Is it possible you've got the
'3055 connected wrong?

  .---.
 |  O  |
 .-----.
 |     |
 |1 2 3| TIP3055
  -----
  | | |
  B C E

That's about the output voltage you'd get if you swapped B
and C. (this can permanently reduce the beta of the transistor).

To a first approximation, the base of Q2 draws no current, so
the voltage there is just half the output voltage. In your case, it
should be about 5.7VDC. The voltage on the emitter of Q2 should
be about 5.1V. And the output voltage should be about 11.5V.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam@spam@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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2003\01\31@005241 by rad0

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----- Original Message -----
From: "Spehro Pefhany" <.....speffKILLspamspam.....INTERLOG.COM>
To: <EraseMEPICLISTspam_OUTspamTakeThisOuTMITVMA.MIT.EDU>
Sent: Thursday, January 30, 2003 10:00 PM
Subject: Re: [EE]: voltage regulator (circuit)


{Quote hidden}

reward"

--double checked the transistor, it looks OK
turned the zener diode around and I get 11.08 volts, this must be it.

how would you change this circuit to get different voltages?

R1 is only to protect the Q1, correct?
So, R2 and R3 must be the 'controlling' values for this circuit,
up to the limits of the resistors, is this kinda how this circuit works?

I'm going to try some different values for R2,3

thanks

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2003\01\31@024802 by William Chops Westfield

face picon face
   --double checked the transistor, it looks OK
   turned the zener diode around and I get 11.08 volts, this must be it.

OK.  I wouldn't have expected 5.5ish volts with the zener backward (more
like 3.something), but the voltage drop of zeners in the forward direction
isn't something that gets a lot of documentation.


   how would you change this circuit to get different voltages?

You can change either the Zener OR the voltage divider R2/R3.  The
circuit actually acts to regulate the voltage at the base of Q2 to
Vzener + Vbe(q2)  This is 5V for the circuit shown in the book.
R2/R3 form a voltage divider that divides the output voltage in
half (for R2=R3=1k), so the output voltage for the circuit in the
book is 10V.  In your case, Vz = 5.1V, so the regulation point is
about 5.8V, so the output should be about 11.6V...

Halve R3, and the output voltage should be 3x the Vz+Vbe value.

BillW

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