0-30 Vdc Stabilized Power Supply

[rodrigo.j.p]

the output of my power supply remains always in 30V the input+ of U2 have regulation of 0,3 - 11,2 V but your output remains in 30V





sorry for my English but i'm from brazil and my English is very bad ;D

 

[audioguru2]

Hi again Rodrigo,
Before your supply's problem was that its output voltage was too low. Now it is too high. What happened that changed it?
Is Q2 or Q4 shorted?
What is the output voltage of U2 when the voltage control pot is turned down? It is normal for it to go low but U2 will overheat if left for much time.

 

[rodrigo.j.p]

I find that the problem is in the U2 because your output is not gradual in the input - of u2 i have 10,5V and the input + i have 0V to 11,3V q2 and q4 is OK

 

[audioguru2]

Hi Rodrigo,
The input voltage to U2 should change smoothly from 0V to about 11.2V from the voltage adjust pot. Without a load, U2's output voltage should change smoothly from about 1.2V to about 36V (if the supply voltage is high enough) since Q2 and Q4 are emitter-followers and since R11 and R12 apply negative feedback to U2 so that the output stage has a gain of 3.1.

U2 has a gain of 200 thousand so its inputs should have just about exactly the same voltages. Since yours has its - input voltage too low at 10.5V then it is trying to force its output as low as it can. Replace U2 and turn off the power if it gets hot.

Which opamp did you use for U2?

 

[rodrigo.j.p]

U2 = TL-081
i replace them but the problem continued.

 

[audioguru2]

Hi Rodrigo,
Check the pins on Q2 or Q4. If its base is connected to +30V where its collector is supposed to be then the output voltage will stay at max.

 

[rodrigo.j.p]

the pins are corect

 

[audioguru2]

Hi Rodrigo,
It is time to troubleshoot your project:
1) Remove R15 (1k or 100 ohms), R12 (56k), C6 (100pF) and C9 (100pF).
2) Temporarily connect the 56k resistor where C9 was.
3) Turn on the power without a load and measure if the output pin 6 of U2 goes smoothly from 0V to 35V when the voltage adjustment pot is turned.
4) Measure the project's output voltage, it should be 0V.

If U2's output works properly then it is fine.
If the project's output has high voltage then either Q2, Q4 or both are bad.

 

[rodrigo.j.p]

i go see this but now i don't have many time 

 

[rodrigo.j.p]

i have a new project and he is perfect thanks for your help

 

[vjedlicka]

Hi all,

I am going to build this PSU. I cannot get any of the opamps mentioned in this forum (OPA445, MC34071AP, TLE2141CP)

Can I use NE5534 instead?

Thanks for advice.

Vaclav

 

[audioguru2]

The NE5534 is for a closed-loop gain of 3 or more without an extra capacitor as compensation, has diodes between its inputs and has an opposite voltage for its input offset adjustment. I wouldn't want to modify the circuit and pcb to use it.

 

[vjedlicka]

Thanks a lot for the quick response!

How about LF356N?

Vaclav

 

[audioguru2]

The LF356N also has an unusual offset adj. voltage and its max supply is only 36V.
Look at www.datasheetarchive.com for more.

 

[mix]

Hello,

I build the original 30V x 3A PSU and it works pefectly, but after some (test ;D) short circuits the output fall to 0V. Searching I found Q2 transistor (2N2218A) damaged. This happened two times and I don't know what is the problem ???.
Somebody can help me?

Thanks!

PS: sorry for English ;D

 

[audioguru2]

Hi Mix,
Welcome to our forum. ;D
When the project's output is shorted or set to a low voltage at 3A. Q2 has about 32V across it and up to 100mA through it. That is heating of 3.2W. A little 2N2218A will be at its absolute max internal temperature with about only 1.5W with the clip-on tiny heatsink recommended in the original project.

That is why I recommend using a real little power transistor like a TIP31A that has a metal tab for bolting to a real heatsink.

View attachment 38441

 

[MP1]

Hmmm....Interesting.   

57 pages of posts regarding changes needed for this project and a guy posts that his original circuit is working fine except when he shorted it out.

MP

 

[audioguru2]

I bet it doesn't produce a regulated 30V at 3A. Probably only 2.5A at 26V with lots of ripple like many of the complaints.
It is rated for zero volts to 30V at 3A, so it shouldn't melt over that range.

 

[MP1]

So you are saying he is a liar or just stupid?

I build the original 30V x 3A PSU and it works pefectly, but after some ...

I have to take his comments for face value. When he says it works perfectly for him, I have no doubt. I think your comments are rather arrogant in this regard. Especially since I doubt you have even built it.

MP

 

[audioguru2]

MP,
I am not arrogant!
Mixos, the owner of this site built this project and got together with Staigen, our expert power supply member. They reported the problems that Mixos had with his project in the topic about modifying it for a 5A output. The problems that Mixos had are the same problems that other members had with the ones they made:
1) At an output current of only 2.1A the ripple voltage increased with an output voltage of only 25V and higher output voltages were not possible.
2) The transformer, rectifiers, Q2 and Q4 were overloaded and got very hot.
3) The opamps were operating at a supply voltage higher than their absolute max rating.
4) Mixos's project broke, stopping further testing.

So I recommended improvements to fix these problems.
Unfortunately, Mixos and others gave up with the project since it didn't work properly and wasn't reliable.