0-30 Vdc Stabilized Power Supply

[audioguru]

If  Audioguru has an ego issue, then it's only surpassed by your arrogance!!

There are more than 29,000 members here and I am the only one who fixes these lousy projects that don't work properly.

[chapz]

Hi there,

I've been reading through the posts, and have decided to build this power supply into my bench supply which I'm upgrading - the negative pumping scheme is more clever than my own design with a separate dual supply for the op amps!

However, would just like to ask a question about the R47, 10W emitter resistors for the output pass transistors. I have a couple of R22, 5W resistors handy, and believe it would be fine to use them. Since the 2N3055 transistors are parallelised, the max current through each is 1.5A. We want a 0.2V drop across he resistors, so R will be 0.2/1.5=0.13. So R22 is about right. And power wise,

P=(1.5)^2x0.22=0.495W.

Am I missing something?

Many thanks,

[audioguru]

Hi Chapz,
Welcome to our forum. ;D
The 0.33 ohm emitter resistors are used to equalize the gains of the output transistors because the 2N3055 transistor has a wide range of current gain. Members who tried 0.22 ohm emitter resistors reported that one transistor became much hotter than the other because their gains were different and 0.22 ohms wasn't enough to make them nearly the same. Try it with your transistors or test many transistors.

1.5A squared x 0.33 ohms= 0.74W. Use 2W resistors so they don't scorch the pcb.

[chapz]

Hi audioguru,

Thanks so much for your reply and advice! I will try it with 0.33 ohm as well as 0.22 and 0.47 ohm resistors and let you know what I find.

One other thing; wouldn't it make more sense to use a power FET (n-channel) for the output pass transistors? That way we wouldn't need to use emitter resistors since the drain current actually decreases with increasing temperature. So something like two parallelised IRF640's?

Your opinion would be much appreciated.  :)

[audioguru]

Hi Chapz,
An N-channel power Mosfet needs to have its gate at a much higher voltage than its source to turn on. Then this modified project would have a max voltage output of only about 26V instead of 30V.

[chapz]

Thanks so much, you've been a GREAT help! ;)

[LanDoctor]

Hi All,
I have been following this thread and am confused as to the latest revision of the circuit. I have most of the parts to build it, but want to make sure I am building the "best" version of it. Can someone please give me a link to the latest parts list and layout? I would really appreciate it. It has been frustrating trying to go through the 100+ pages to find this info.
Thanks for your time,
Donovan

[audioguru]

The improved circuit is basically the same. The transformer has a higher voltage and current rating, the opamps have a higher voltage rating , One additional output transistor is added and they each have an emitter resistor, a couple of transistors are bigger and some resistors and capacitors are bigger.

The improved version will fit on the original pcb if some of the bigger resistors are stood up and the main filter capacitor and rectifier bridge module are strapped to the chassis.

Here is the improved parts list:

[LanDoctor]

Thank you for the reply.
Is there another designation for the opa445 components. I can only get NTE components locally and it doesn't look like they have a cross referenced part available.
Thanks again,
Donovan

[audioguru]

The OPA445 opamp is the only one I found with a high enough max supply voltage rating.
It is made by the Burr-Brown division of Texas Instruments and is available through Farnell world-wide.

NTE buys ICs and sells them as replacements.

[Pawel_K]

Hi
It's my first post on this forum.

I built this power supply, but it doesn't ot work.
I changed TL081 with uA741.
I also changed 2N2219 with BD139-16.
I can regulate voltage from 0,1 to 1V. :-[

Sorry for my english.

[audioguru]

Hi Pawel,
Welcome to our forum. ;D
The BD139 is a good substitute for the little 2N2219. It should have a heatsink.

The project will be easy to fix if you measure and post some of its voltages without a load. Put the negative lead from your voltmeter on the 0V output of the project:
1) Pin 7 of all the opamps should be about +34VDC.
2) Pin 4 of U2 and U3 should be -5.6VDC.
3) Pin 6 of U1 should be +11.2VDC.
4) Pin 6 of U3 should be about +32VDC.

Then U2 amplifies the 0V to 11.2V from the voltage adjust pot by slightly more than 3 times and the voltage output of the project should go from 0V to about +28V.

[Pawel_K]

I checked voltages.

Pin 7 of all the opamps is +32,2V
Pin 4 of U2 and U3 is -7,1V
Pin 6 of U1 is +10,5VDC
Pin 6 of U3 is -4,7V
current limit diode is lighting all the time.

When the 2N3055 wasn't solder I could regulate output voltage from 0,1V to 1V.
Now after solder 2N3055 I can't regulate output voltage. It's -0,6V. :-\

[AN920]

What is the voltage on U3 (pin 2,3)?

[Pawel_K]

Voltage on U3 PIN2,3 is 2,5V

[AN920]

Sorry, I meant voltage between GND-pin2 and GND-pin3

[Pawel_K]

GND-PIN2 0V
GND-PIN3 2,5V

GND is "-" on the PCB ??

[AN920]

Yes "-"
U3 seems faulty. pin 6 should be positive as the non inverting input dominates.

[AN920]

Can you adjust the output voltage with U3 removed?

[Pawel_K]

Voltages with removed U3:
between 3pin and GND  minus 29V
between 3pin and + also minus 29V
between 2pin and + 0V
between 2pin and - 0V


Here is the opamp tester.
When I put U3 into the tester the diode flash.

[AN920]

Something strange in your circuit. Make sure you don't have any stupid mistakes, solder bridges, wiring errors etc.

[Pawel_K]

I checked PCB and it looks ok.
Diode D10 is very hot.
I checked also rest of elements and they are ok.

[AN920]

This should help you, and others to find faults in this PSU.

Voltages to expect with three different conditions.

1) PSU with almost no load (2.2k) set to 12V
2) PSU with 1A load (22 Ohm) set to 23V
3) PSU with current limit turned down to about 500mA, from condition (2). Current limit LED is on.

Measurements taken with input GND as reference and not out "-" !

[audioguru]

D10 is supposed to be reversed biased with about 1.4V across it and no current flowing. Therefore it should not be hot. Check its wiring in the circuit.

[Pawel_K]

I checked voltages with U1,U2,U3 removed.
Pin7 of all opamps is 34,4V
Pin4 of U2 and U3 is -7,2V
Pin6 of U1 is 0V
Pin6 of U3 is 35,5V
Now D10 diode is not hot.
Resistor R1 is hot.

I changed in my project:
C1 capacitor with 4700uF/50V
D11 with 1N4007
Q1 with BC547
Q2 wth BD139-16
TL081 with OP07

There are photos of my PCB:

[img width=680 height=511]http://free.of.pl/p/pawel-kolodziej/2.JPG
Please help me. ???