0-30 Vdc Stabilized Power Supply

[audioguru2]

Hi Smit,
Your U3 isn't working properly and it and U2 have a supply voltage higher than their absolute max. You should use opamps rated for 44V like a MC34071CP, TLE2141CP or uA741A. 

 

[_ab]

Dear friends.....

I built this power supply but i failed to make it work.....I don't know why and i hope to help me!!!!!Well the
first time i gave power the output voltage was ok.and the ampere control was ok(the led was on in the fully
clockwise turn).After 1 minute the voltage was 9- 22,5- 34- 0- 12,5 and so on it wasn't steady in a few words..
Today i changed the transistor q2(2n2219)i also checked for the 2n3055 but it was ok...and i added a fuse 1A
after the transformer before the diodes.result????I blow 3 fuses and i think the transistor is dead too...The
only modification is the diodes which the text refers to 1N5402,3,4 diode 2A and i used iN5408 but i dont think
it's so important....And i also changed the tlo's...I found something but i don;t know if it is bug for sure...
check the file below and see the schematic diagram.....R13 goew to C of Q2 the pcb doesn't have such a
connection....But i used a bridge and also didn't work....I am desparate....

PLEASE HELP Panagiotis....

View attachment 38033

View attachment 38041

 

[audioguru2]

Hi Panagiotis,
Welcome to our forum. ;D

I built this power supply but i failed to make it work.....

That's too bad. We'll try to help you with it.

first time i gave power the output voltage was ok.and the ampere control was ok(the led was on in the fully clockwise turn).

The LED is supposed to indicate that the load is trying to draw more current than the setting of the pot. Fully clockwise would be about 4A or more!

Today i changed the transistor q2(2n2219)

As I have been saying in this thread, a little 2N2219 is way overloaded in this project, use a TIP31A power transistor with a little heatsink instead.

....And i also changed the tlo's...

What are tlo's?

I found something but i don;t know if it is bug for sure...
check the file below and see the schematic diagram.....R13 goew to C of Q2 the pcb doesn't have such a
connection....But i used a bridge and also didn't work....

You didn't attach a file. The pcb in the project already has R13 connected to the positive supply.

 

[_ab]

Thanks a lot for your answer!!!!!!Well tommorow i will go to buy a tip31a and i will tell you the results.
When i said tlo's i meant U1, U2, U3 = TL081, operational amplifier...
"The pcb in the project already has R13 connected to the positive supply" i can't see this connection in the
pcb could you please show it to me????Why my fuse was blow up three timew????although i didn't have a load????
only my voltemeter in paraller.

 

[audioguru2]

When i said tlo's i meant U1, U2, U3 = TL081, operational amplifier.

The TL081 has an absolute max supply voltage of only 36V. U2 and U3 in this project have a positive supply plus a negative supply that add to more than 36V. That's why I say in this thread that opamps having a higher voltage rating should be used.

"The pcb in the project already has R13 connected to the positive supply" i can't see this connection in the pcb could you please show it to me?[/quote
See my sketch of the pcb.


Q2 or Q4 or both are shorted or very leaky.

View attachment 38042

 

[_ab]

Thanks friend.... i couldn't see it :-(  .So what do you suggest me to look for????? Do you think that the transistor that you told me to change makes all this problems???? Sorry if i have become bothersome

 

[audioguru2]

Sorry, I was wrong. If Q2 or Q4 was shorted then the output voltage would be too high and without a load then there wouldn't be any current.
About the only things that could draww too much supply current is a short on your pcb, shorted or backwards C1, C7 or D11, shorted C5 or the opamps. Or any combination.

Without power, use your ohm-meter from the positive supply to the output 0V terminal or to the negative wire of C1 and you'll find the short.

 

[Smitjuh]

Hello! Audioguru!

In your last post to me, Changing U2 and U3 was a option.. well, option it was nescesarry because they where working outside their workingvoltage. Well I changed them with LM741's. But there is no difference in the working of my powersupply.

If I turn my Potentiometer P2 wholy clockwise, and I put a ballast on my output, my powersupply gives, lets say: 10V 1,5A.

If I turn back  P2 (counter-clockwise) noting happens. Until the last part of the turnig cylcle... then the voltage fals down with about a 0,5V.. current remains the same.

 

[audioguru2]

Changing U2 and U3 was a option.. well, option it was nescesarry because they where working outside their workingvoltage. Well I changed them with LM741's. But there is no difference in the working of my powersupply.

An LM741C also has a max supply voltage of 36V. Some 741A opamps are rated for 44V.

If I turn my Potentiometer P2 wholy clockwise, and I put a ballast on my output, my powersupply gives, lets say: 10V 1,5A.

If I turn back

 

[_ab]

well i looked everything you told me and it still not working....The parts you told me to look are ok!!!!i also replaced the 2N with the tip31
I didn't understand how to find the short with my ohm-meter....Could you please post a schematic with the way
to do it(sorry)and also when i changed again the 3 tlo81 the same problem it was working for 30 seconds and
then the voltage went for a walk......i am joking 32volt the p1 was dead and the p2 when i turned it fully
clockwise the led lit... :-( 

 

[audioguru2]

Hi Ab,
It is good that your project worked, but bad that it failed again after only 30 seconds.
1) Now the output voltage is high at 32V and P1 doesn't control it, right?
2) Before it blew fuses but now it doesn't, right?

U2 controls the output voltage. U2 might burn-up if the collector and emitter of Q1 are reversed. It also might burn-up Q1.

U1 might burn-up if P1 is wired incorrectly or has a resistance much too low.

When you turn P2 clockwise and the LED lights, then the operation of P2 is backwards, check its wiring.

View attachment 38057

 

[logitec]

hii
i have a question
82 Ohm
0.47 Ohm
1.5K
3.9K
for what they are used in this project and can i change those to some thing else more common resistors- even the shops dont have them is their stoke

and does it makes big difference if i put polyester condinsator instead of the ceramic ones?

 

[audioguru2]

Hi Logitec,
These are very common resistor and ceramic capacitor values.
What country are you in? www.farnell.com have electronics parts offices in many countries.

Are you making the original or the fixed and improved project?
I recommend that you use parts on this list:

View attachment 38058

 

[rodrigo.j.p]

i'm from brazil sorry my english, but i built the power supply and in the out i have only 0 to 9,3V i test all but i don't know what i can do help me please

 

[rodrigo.j.p]

sorry for my english i'm from brazil and i built the power supply, but in the output i have only 0 to 9,3V please help me..

 

[audioguru2]

i built the power supply and in the out i have only 0 to 9,3V i test all but i don't know what i can do help me please

Hi Rodrigo,
Welcome to our forum. ;D
Your project should be easy to fix.
1) Check that the collector and emitter pins of Q1 are not reversed.
2) Check that the voltage feeding P1, the voltage adjust pot, is 11.2V.

Please tell us:
1) Which opamps did you use?
2) What is the positive supply voltage across C1?
3) What is the negative supply voltage across D7?

 

[Gruber]

hi again.
after some time i managed to build the supply with some little differences - i used 1W R1 and 741CN / LM1458 instead of OPA445 since i cant find those OPA's.
It was quite fun ti run it for first time since i forgot to put R4 and there were no change at output
But i noticed some "issues" - R1 is heating too much and i think if it will work for long time can eventualy overheat.All other elements seemed to be cool at first run.
The negative suppy from D5,D6,D7 and so is producing around minus 5-6V and dont know if it should be that way.The voltage acros U1 U2 and U3 is around 42V,41V and 40V for each of them - not 44V as stated here before,which causes me to think there is a mistake somewhere around my circuit.
I use ~28V transformer rated 3,5A for idle and get around 36V after C1.The supply can regulate voltage from 0,00 up to 35,6V i think.
The strange thing is that after putting some heavy load at output (reostat) and at full load (theoreticaly) it produced 27.somethingV and 2,75A with one 2N2055 (no parallel).The transformer is rated for 3,5A at 26V and i think it should be capable of doing at least 3A at 26V but...The current voltage seemed strange for me...in idle mode when using P1 for voltage regulator it changes from 0 up to 35V,but with load it didnt changed,it did when using current regulator P2.But when turning current regulator to reach max voltage the current increases to max ratings too - which i believe could burn a little load.

Those are the problems,or at least i think that there are some problems...

 

[audioguru2]

R1 is heating too much

That's why the mod'd project uses 2W.

The negative suppy from D5,D6,D7 and so is producing around minus 5-6V and dont know if it should be that way.

-5.6V is normal.

The voltage across U1 U2 and U3 is around 42V,41V and 40V for each of them - not 44V as stated here before,which causes me to think there is a mistake somewhere around my circuit.

At 28VAC with no load on the project, the peak voltage of your transformer is 39.6V, which is reduced to 38.2V by the rectifier bridge. When you add the -5.6V negative supply, some of the opamps get a total supply voltage of 43.8V.

I use ~28V transformer rated 3,5A for idle and get around 36V after C1.The supply can regulate voltage from 0,00 up to 35,6V i think.

I don't think so.
The transformer's output at full power is 26VAC, has a peak voltage of 36.8V but is squashed at the peak by the massive current pulses from the bridge rectifier, and the main filter cap C1 will have 1V of ripple for additional losses resulting in a positive supply of only about 32.3V.
At full load U2 has about a 1.5V loss, R15 up to a 3V loss if it is the original 1k, Q2 plus Q4 could have up to a 4V loss and R7 has a 1.4V loss at full current.
The total of the circuits losses results in a max regulated voltage of only 22.4V. Maybe you are lucky and your transistors have much more than their guaranteed min current gain. That's why the mod'd project uses a 30V transformer. 

The strange thing is that after putting some heavy load at output (reostat) and at full load (theoreticaly) it produced 27.somethingV and 2,75A with one 2N2055 (no parallel).The transformer is rated for 3,5A at 26V and i think it should be capable of doing at least 3A at 26V

The transformer is rated at 26V x 3.5A= 91VA. The transistors heat with up to 29.7W of power (at max output voltage) which must come from somewhere. Your transformer will be overloaded about 18% if your projects load draws 3A.

The current voltage seemed strange for me...in idle mode when using P1 for voltage regulator it changes from 0 up to 35V,but with load it didnt change

Sure the max output voltage will change with a few amps into a load. It will drop to as low as 22.4V.

when turning current regulator to reach max voltage the current increases to max ratings too - which i believe could burn a little load.

First set the voltage pot for the max voltage for the load. Then set the current pot which causes the output voltage to reduce when it is regulating the current. The output voltage should never exceed the setting of the voltage pot.

 

[Smitjuh]

Hi audioguru,

Here I am again  I changed my op-amps to 44V rated ones (op37 from Analog) But With the last op-amps I could regulate from 0-32V, but now It is only possible to about 24V. And when the supply is powered for some time, the voltage decreases slowly. down to 22V.. When currents are flowing, it all go's wrong and my voltage drops dramatically. My LED still don't lights at any time...

 

[audioguru2]

Hi Smit,
Your problem is caused by your super-high-speed opamps. They are de-compensated for high speed, so can't be used in circuits with a closed-loop gain less than 5 so that they don't oscillate at a very high frequency.

They oscillate in this project:
1) U1 has a gain of 2.
2) U2 has a gain of 1 at high frequencies.
3) U3 has a gain of 1 at high frequencies.

Use "normal" opamps that are compensated for a gain of 1 and a 44V supply rating. ;D

View attachment 38074