0-30 Vdc Stabilized Power Supply

[audioguru2]

An LCD voltmeter, ammeter project has already been done and was posted in our Projects Section only a few days ago.
http://www.electronics-lab.com/projects/test/022/index.html

 

[rufinus]

Yes, thank you, audioguru, but i already saw it. It is based on a chip from Atmel plus it is a surface mount version of chip, which will be difficult to mount for beginners. I work with PICs and program in PicBasic.
There is no sourse code for development and in general schematics and project are not very clear, at least for me.

 

[audioguru2]

I would just buy two LCD voltmeter modules and add resistors for them to display voltage and current. The current meter should measure the voltage across R7 which indicates the current without reducing the output voltage.

 

[rufinus]

I just sampled some TC7107 from Microchip 
I have some spare LED 7-segment displays, so i will try them first. But i still like the idea with PIC  8)

 

[spy_id]

Guys

You have been awesome supporting this project. I have built this PSU (see attached) but need some help!

I have a step-down transformer with 2x75VA secondary windings. If I connect them in series I get 34.0v AC and 52.0v rectified. This I think is too much for the OPA445s, in fact I know it is because 2 of them blew up after I connected it  ;D

As a test for the circuit, I disconnected one of the secondary windings and replaced my dead OPA445s with 60p TL081s. With P1 cranked up, I measured [email protected] across pins 3 and 4  8)

With the 2 windings connected, the reference voltage is ok - 5.6v appearing across D8 - and 11.v at pin 6 of U1. U2 is not happy though, he gets an input voltage at pin 3 proportional to P1's setting but nothing at pin 6.

What's my best bet? Upgrade R1? If so, what to?

Thanks fellas
spy

View attachment 39726

 

[roujhan]

Hello All.
I am new to electronics, can anybody help me about some questions?!
First i want to know how the voltage in output of U1 gradually increases till D8 turn on?
And my second question is about the current limiter; in fact i read the explanations but i wonder if some body explain more about how it work...
I'm sorry for my stupidily questions...

Many thanks,

 

[audioguru2]

I have a step-down transformer with 2x75VA secondary windings. If I connect them in series I get 34.0v AC and 52.0v rectified.

52VDC is much too high for many parts of the circuit.

This I think is too much for the OPA445s, in fact I know it is because 2 of them blew up after I connected itThe OPA445 has an absolute max supply voltage of 100V. The factory tests them at 80V. Your circuit gave them a total supply of only 52V + 5.6V= 57.6V but had them dissipate too much heat and they melted. The heat is caused by the supply voltage times their output current.

U2 is not happy though, he gets an input voltage at pin 3 proportional to P1's setting but nothing at pin 6.

Then either U2 is broken or its output is shorted. With a 52V positive supply, maybe Q1 is turned on which would short the output of U2 to ground. Q1 is turned off by R14. The vaue of R14 was 1.5k for a 35VDC positive supply, then was changed to 1.2k for a 43VDC positive supply. R14 should be about 1k for your 52VDC positive supply to make sure Q1 is off during operation of the circuit.

What's my best bet?

Get a 30V/4.3A 130VA transformer.
Get a much bigger heatsink and mount it on the outside of the case.

 

[audioguru2]

First i want to know how the voltage in output of U1 gradually increases till D8 turn on?

U1 doesn't have a negative supply so its output can go only positive.
R5 and R6 provide positive feedback so that the output voltage is double the voltage at pin 2.
The output voltage begins a little positive and the positive feedback increases it until the zener diode conducts at 5.6V, making pin 2 also 5.6V and then U1 makes its output double at 11.2V.

And my second question is about the current limiter; in fact i read the explanations but i wonder if some body explain more about how it work...

U3 is used as a comparator. It compares the voltage across R7 due to load current, with the voltage setting of the current-setting pot.
When the voltage across R7 exceeds the voltage from the pot, then the output of U3 goes low which causes D9 to conduct and pull down the voltage at the input of U2. Then the output voltage decreases, reducing the load current until the voltage across R7 equals the voltage at the current-setting pot.View attachment 39728

 

[roujhan]

Thanks audioguru .
But I thought R7 and pot connect to the same pin of U3 and can't compare with each other  (?)
And what about C8 role here?!
Thanks again...

 

[audioguru2]

But I thought R7 and pot connect to the same pin of U3 and can't compare with each other

 

[roujhan]

Thanks alot audioguru .
But I think I'm right. see, R7 connect to R17 and then P2... and the other pin of opamp connct to pin 4 through R21...
please answer and help me!!!
Thanks In advance, audioguru .

 

[audioguru2]

Hi Roujhan,
U3 measures the voltage across R7 which is caused by load current, and subtracts the offset voltage provided by P2.

 

[roujhan]

ok, i understand now.
Thanks for your reply, audioguru .

 

[12345678]

hello

 

[audioguru2]

Hi Markus,
Welcome to our forum. ;D
I think the -5.6V supply in your project is weak. It collapses when the 4mA of current in R20 and in U3 turns on Q3. Check the parts in the negative supply circuit.

 

[12345678]

thank you for the answer.
but i think thats not the problem.
i tryed something and i think its an ofset problem with u3.
the polarity from u3 pin3 against pin2 turns around if i turn p2 to min. at this point when it turns around the output voltage is negativ.
and that should not be.




sorry the last time i learned english was in before 20 years 

 

[audioguru2]

I tried something and I think its an offset problem with u3.
The polarity from u3 pin3 against pin2 turns around if I turn p2 to min. at this point when it turns around the output voltage is negative.
That should not be.

Maybe you have a bad opamp. The typical input offset voltage of an OPA445AP opamp is only 1.5mV and only 5mV is its max. The voltage across R17 when P2 is turned all the way down is 5.6mV.

Connecting a 1N4148 diode across C8 might fix it, cathode to pin 6 of U3.

I still think your negative 5.6V supply is too weak.

 

[12345678]

U1-U3=UA741CN @41V PIN 4-PIN 7 
42V is the max.
i will try the 1n4184.

 

[12345678]

with 1n4148 accross c8 it was a little bit better.
not -0,6 at the output but -0,4.

now 1n4148 is not over c8 but i changed r17 to 900 ohm.
now my min current at output is +2 mA and not - 5 mA.
so it works correct but its not perfekt.

 

[12345678]

these are my parts for my power supply

nice and big  ;D ;D ;D