Hello everyone!I was given to make a project consisting in a linear series voltage regulator which has the following requirements:
-variable input voltage between 21 and 24 volts
-variable output voltage between 16 and 18 volts
-load resistance of 1.4kohms
-overvoltage and shortcircuit protection
Now,I have never designed a circuit before and I don't understand exactly how this should be done.Somehow I managed to come up with some kind of schematic by making some research and asking a few collegues but now I am stuck.I used the OrCAD software to simulate the schematic.

I started from the block diagram of a voltage regulator and replaced it the following way:
R7,R8,R10(potentiometer) represent the sampling circuit
-Q16,Q17 represent the control element
-Q12,Q13 represent the comparator circuit(and I'm guessing R4,R5 and R3 are part of it as well)
-D1 represent the voltage reference
-R11,R13 represent the load
From what I understood J1 and R2 represent a current source but I don't know what it does in the circuit

Also,I was given a set of components to use in my circuit, which I attached here
Perhaps I'm losing myself in details but I really want to understand how this voltage regulator should be made.
I hope it is understandable what I wrote above and I'm waiting to hear your ideas

 

From what I understood J1 and R2 represent a current source but I don't know what it does in the circuit

Let's start with that. How well does J11 conduct as the current across R2 increases?

 

I do not see any overvoltage and shortcircuit protection in your diagram?
Try read this
http://www.learnabout-electronics.org/PSU/psu22.php
http://www2.eng.cam.ac.uk/~dmh/ptialcd/csink/csink.htm

 

I do not see any overvoltage and shortcircuit protection in your diagram?

It's true.I didn't work on that because I don't have a regulated voltage at the output yet and I was thinking I should do that first.

 

I don't have a regulated voltage at the output yet

So, your circuit does not work in the simulation?

 

No,it gives me this at the output

 

What is the D1 diode current for Vin=21V?

 

If I'm reading right,the current on D1 at Vin=21 is about -4.3mA...I put the graph here as well

 

Remove D1 and J1 and replace D1 with a 6.8 voltage source. And see the resoult.

 

Remove D1 and J1 and replace D1 with a 6.8 voltage source. And see the resoult.

The output is the same

 

Hello,

Here is the schematic for direct view to others:



What happens when you lower the value of the voltage divider R7, R10 and R8 by a factor of 10?

Bertus

 

Remove D1 and J1 and replace D1 with a 6.8 voltage source. And see the resoult.

This is homework. The student should be allowed and encouraged to do the work.

Schematic with some whitespace removed:

 

Hello!I came back after I tried to check what mister Bertus said:

What happens when you lower the value of the voltage divider R7, R10 and R8 by a factor of 10?

and it improved the output a lot.I made some changes in the schematic trying to make the output better but it still doesn't seem quite right,it doesn't start increasing from 0V and it doesn't reach 16V or 18V "directly".Also when I set the potentiometer R10 to 0 the output voltage increases and when i set it to 1 the output decreases,souhldn't be the other way around? If anyone can give me any ideas I would be grateful and also thank you for all the help from before as well.Happy holidays!

 

t doesn't start increasing from 0V and it doesn't reach 16V or 18V "directly".

But why Vout should increase immediately from 0V? It is wrong? Do you know what voltage drop is needed to "turn-on" your output stage (Darlington pair)?
And what do you mean by saying that "it doesn't reach 16V or 18V "directly" "?

 

I made some changes in the schematic trying to make the output better but it still doesn't seem quite right,it doesn't start increasing from 0V and it doesn't reach 16V or 18V "directly".

First, some questions.

1. Why did you increase R5 from 820 to 100k?
2. Why did you change the resistor values in the voltage divider?
3. Why did you increase the input voltage?
4. Why did you decrease R2 from 330 to 680?
5. Why did you increase the output cap from 100uF to 100F?

 

But why Vout it should increase immediately from 0V? It is wrong?

I don't know,I thought that's how it should be

Do you know what voltage drop is needed to "turn-on" your output stage (Darlington pair)?

I don't know that either,this didn't even cross my mind.How can I find that?I suppose I should apply Kirchhoff to find it?

And what do you mean by saying that "it doesn't reach 16V or 18V "directly" "?

Well..,what I meant is that when it starts to stabilize it is a bit under the wanted value at the beginning.For example if I want to reach 16 volts when it starts to stabilize, at first is 15.something volts and increases until it reaches 16 volts.I'm sorry if it doesn't make any sense,I don't know how to explain or if what I'm trying to explain here is actually a problem or not in my output.

 

,it doesn't start increasing from 0V

Why do you want it to? That wasn't in the original requirements.

variable output voltage between 16 and 18 volts

 

First, some questions.

1. Why did you increase R5 from 820 to 100k?
2. Why did you change the resistor values in the voltage divider?
3. Why did you increase the input voltage?
4. Why did you decrease R2 from 330 to 680?
5. Why did you increase the output cap from 100uF to 100F?

The output capacitor is 100uF but I replaced it with an electrolytic capacitor and when I put the value I forgot the micro

When I posted my problem I received a link about current sources and there I saw a graph with the output current of a given current source and I measured my current source and saw that the graph was not the same so I replaced with different values until I got the same graph

as for the rest I wanted to see how my output would change

I guess I should make some computation on the circuit and stop guessing..

 

Every Series Voltage Regulators needs a higher voltage at the input to be able to work properly as a voltage regulator. And typical this minimum input voltage is 3V larger than the output voltage.
And this is why your input voltage is between 21V...24 volts and a desirer output voltage is between 16V... 18 volts to provide a necessary headroom voltage.
https://www.sunpower-uk.com/glossary/what-is-headroom/

 

Why do you want it to? That wasn't in the original requirements.

I thought that maybe I did something wrong,it wouldn't be that hard to make mistakes at the level I am now