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Transistor series voltage regulator

KlausST

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I just did an internet search.
There are many explanations, tutorials and even videos.

What information do you expect from us that is not explained in most if the search results?

Klaus
 

anditechnovire

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I just did an internet search.
There are many explanations, tutorials and even videos.

What information do you expect from us that is not explained in most if the search results?

Klaus
A lot of those site are just coping from each other. They assume that it just a simple circuit, but if you look closely, the actual process or characteristics of the transistor that makes it regulate the voltage is not explained.
So please don't hesitate, I've read through so many site.
 

betwixt

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It doesn't regulate particularly well but it is better than a Zener diode alone.

Simplest explanation: the E-B junction in the transistor is a PN junction so it has a fairly constant voltage drop. The emitter will be approximately 0.7V lower in voltage than the base. If the emitter is pulled to a lower voltage by the load current it will cause more base current to be drawn and because an emitter follower configuration has current gain, it will make the transistor conduct more and try to bring the voltage back up again.

Brian.
 

KlausST

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Hi,
A lot of those site are just coping from each other.
For sure they all tell the same. This is not the problem.
If they tell different things, then there was a problem.

It seems it's not the problem of understanding of the whole circuit, its the understanding of the bjt in emitter follower mode.
If Brian's explanation is not sufficient, then do a search for basic bjt operation.

Klaus
 

BradtheRad

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The regulating action is easier to grasp by seeing a simulation.
Supply voltage was varied 4-12 V. Load receives zener voltage minus diode drop.

series voltage regulator NPN 4v zener load gets 3_3v from supply 4-12v.png
 

danadakk

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If you look at the Zener behavior might shed some light.

It is operating in the reverse direction. You can see that in yellow region
it does not change its V by much over a wide range of current.. In the
forward direct, upper right quadrant, it just looks like a diode. Which is true
of the Vbe of a transistor when it is forward biased. There it does change its
V somewhat with current. Its still a sloppy but a relatively fixed V.

So if we write a loop equation we have Vz = Vbe + Vload, or Vload = Vz - Vbe.
Since Vz is fixed, and Vbe sorta fixed, Then Vload is also sorta fixed. When we
add the transistor (we could have used a R in its place)

1593370043024.png
--- Updated ---

So lets look at simple circuit (no transistor) -

1593385420213.png

If we sweep V1 voltage ( we see load V somewhat regulated, and zener taking current when V1 tries to go up
which would otherwise make load go up if that current went thru it instead of zener).

1593385745833.png

If we sweep load R we see load V is somewhat regulated over significant portion of load change.




1593385528763.png

Now lets do one with a Transistor instead of R2
--- Updated ---

Now look at replacing the R with a transistor. We see the Vload much tighter, flatter, less
change with input V once Vz turns on. Basically the transistor, because of its G, varies its
Vce to absorb the changing V into regulator, thereby keeping load V more stable.




1593386545600.png
Basically the transistor senses any changes in its emitter loop and amplifies the
change in load to produce - feedback. So if load V tries to go up the transistor Vbe
will be lowered then the emitter current drops, Vce will rise (its not conducting as
much) to offset it, to absorb the V change so load does not see it. Again Vz = Vbe + Vload,
or Vbe = Vz - Vload. When Vbe drops a transistors Ic and Ie drop. Look on web at a NPN
transistor characteristic curves.

1593388182584.png

The Vz is nothing more than to set a reference V to compare by transistor to make its corrections.

Regards, Dana.
 
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