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0-30 Vdc Stabilized Power Supply


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One more thing that i tried is that i changed the current 10k linear pot with a 100k pot not linear and i managed to regulate the current from 0 to almost 1A. But this is also not right. The led turns on ONLY when i regulate the current to zero and the voltages drop to -0.06 only then.

Hi Giova,
The LED should light EVERY time the circuit is regulating the current. The output voltage of U3 goes low which turns-on Q3 and the LED and pulls-down the output voltage through D9. U3 is in a feedback loop with the output-current-measuring resistor R7 so its output voltage drops just enough to regulate the output current.

Use the original 10K current setting pot again and measure the voltages at U3 with a high-input-resistance voltmeter:
1) Connect the meter's negative wire to the negative output terminal of the project.
2) Without a load, measure the voltage at the negative terminal of C1. It should measure 0V at any voltage or current setting.
3) Without a load, measure the voltage at pin 3 of U3. It should be about positive 1.7V with the current pot at maximum and drop to 0V smoothly as the current pot setting is reduced.
4) Without a load, measure the output voltage of U3 (pin 6), It should be positive 20-something volts most of the time and drop whenever the circuit current-regulates. Add a load to the PSU and adjust the output voltage to within its rating. Turn the current pot up and the PSU's output voltage should be at your setting and the output voltage of U3 should be at positive 20-something volts. Now turn the current pot down and the circuit should current-regulate according to the setting of the current pot and the voltage pot.
5) Add a resistive 2A or 3A load (don't use a lightbulb) to the PSU, set the output voltage so that it should draw 2A or 3A and turn-up the current pot. You should measure a positive voltage at pin 3 of U3. Turn down the current pot and the voltage at pin 3 of U3 should also drop and reach 0V when the circuit should current-regulate at a pot setting of 2A or 3A.
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I'm an 'old' newbie recently retired. Am returning to my electronics hobby after 30 years! I made a 1A current adjustable psu many years ago but it got the boot in the many moves around the world.
I've followed this thread for some time and was close to buying a kit of parts for the project from my UK source when I read of all the mods.
Will there be a chance to purchase a pcb which incorporates all the changes? - 3A is fine for me, I don't need 5A. (I don't need the full kit then)
I have sourced all the components here except the Op Amps. I cannot find MC34071AP or TLE2141CP or certified equivelents. Any advice please.
Best wishes to a great forum and the very knowledgeable contributors

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Hi Woody,
Welcome to our forum and back to electronics.
There are not many mods so the PCB in the SmartKit should be fine for the modified circuit. You would need a new rectifier bridge module to be mounted on the chassis and a new Q2 to be mounted the same or on its own or on the main heatsink. The kit's heatsink may not be big enough and would need drilling to mount the second 2N3055. A new Q1 could have its legs twisted around to fit the PCB and a new bigger R7 could be stood-up from the board.
Farnell in the UK should stock those ICs like their divisions in the USA and Canada do. Their high voltage ratings are important if you use a 30V/4.2A transformer that will allow the project to provide 30VDC at 3A.
Maybe another member can provide just the PCB for you.

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  • 4 weeks later...

Hi There,]

I have built this project but I am still having a few problems with the current limiting segment!! The unit is not Limiting the current at 3amp but instead at 5amp. what happens is that it pops the 2N2219 and U1 burns after about 5 seconds at this current. I have tested all the components in the circuit, even went as far as to check the tolerances of the V and A pots and the PCB. R16(and all the other resistors ) measure within spec. Could you please offer some advice or help?



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Hi everyone,
I was about to embark on building this project and thought I'd wonder over to the forum to see how other people's projects turned out. After reading through the posts i've noticed alot of modifications made is someone able to post a list of all these mods made so I dont get confused into one post. Also I wish to incorparate the LED display digital volt meter project one to display volts and one to diplay current, will i need a seperate transformer with bridge rectifier to run a 7805 and 7905 regulator to supply the displays with the necessary +/- 5v or can I just wire them straight to the output of the exsisting rectifier

Thanks in advance any advice

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Welcome to our forum.
We have been saying here that the 2N2219 is too flimsy for this project. Use a strong TIP31A mounted on a heatsink.
I don't know why U1 is burning. It is the voltage reference that doesn't have much load, operates within its supply voltage rating (unlike the other 2 original opamps), and has nothing to do with current regulation.
U3 is the current regulator. If you used the original TL081, maybe it doesn't like having a total supply voltage beyond its absolute maximum rating of 36V and is damaged the moment you turn on the project. The same applies to U2.
What are the ratings of your transformer?

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At first, hello to erveryone.

I was able get 2 toroidal core transformers with 120 Watts from some old devices. So I think the power itself should be enough to build 2 power supplys.
Unfortunatley, the transfomers have a secondary voltage of only 20.5 VAC. I wonder if that would be enough to get a maximum of 24 VDC when the schematic of this project is used.

I also could aquire 2 capacitors of 50v/10.000

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Hi Thomas,
Welcome to our forum.
1) If your transformer's output is only 20.5V when loaded, then this project's max output voltage when loaded will be only about 20VDC.
2) Your big filter caps are fine.
3) 20.5V times the root of 2 equals nearly 29V. The transformer's 120W rating divided by 29V is 4.14A. Therefore the project would be able to supply about 4.1A output when the transformer is at its limit, but the little diodes and transistors would probably burn when the project is shorted or set to a low output voltage.
The 29V peak from the transformer is dropped to about 27V by the rectifier bridge, and ripple voltage would drop it further to about 26.5V. The voltage drops across the opamp, series resistor to the little 2N2219 and drops across the transistors further reduce the project's output voltage.
4) The 33.05V peak from the unloaded transformer would provide about 31.7VDC for the positive supply for the opamps. When you add their 5.6V negative supply, then they will be operating with a supply voltage that exceeds their 36V total absolute max rating.

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Im new here. But I am interested to build this PSU.
I already have PCB. But before I will buy material I
checked shop for material, but they dont have got "OPA445AP".
If there is any other IC that could replace OPA445AP.
I would also like to know if it is better to build thid PSU with original parts or this modified parts that audioguru made them (http://www.electronics-lab.com/forum/attachments/0_to_30V_PSU.txt)


PS: sorry for my bad english :)

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Hi audioguru,

First of all thanks for your quick help.

There are a few things I'm not getting.

If the absolute max. rating of the opamps is exceeded by using a 20.5V transformer, hoy can you use a 24V one? Surely a 24V transformer has a even higher voltage if unloaded?
And where do you get 31,7V from? I understood that the 33,05V are the absolute max. by using this transfromer, zero load and a ideal filtering? Okay, there are also tolerances in the line...

The little diodes could be replaced by bigger ones. Either bridges (which I also have good ones) or (my favourite) Schottkys. I don't see a big problem there. And I could use more 2N3055.

Is there a way og getting more output voltage from the transformer by adding a few more secondary coils to the core?

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Hi Tedy,
Where on earth are you that you can't get common Texas Instruments/Burr-Brown opamps? You could try MC34071P opamps that are rated at 44V or another TLExxxx listed in this thread. With a 24V transformer unloaded, the total supply to the opamps will be about 42.4V. The little 2N2219 in the original project should be replaced with a heatsinked TIP31A. Use a 10A rectifier bridge module boltad to a metal chassis or heatsink.

Hi Thomas,
1) That's right, the kit and project operate 36V-rated opamps at about 42.4V if the project is unloaded and has its suggested 24V transformer. I guess they like to gamble.
2) The 33.05V peak from the unloaded transformer minus 2 diode drops in the rectifier bridge equals 31.7V.
That's right, I forgot. If the line voltage is 5% high, then the opamps' supply is also high.
3) I recommend using a 10A rectifier bridge module bolted to a metal chassis or heatsink.
4) I have never seen a schottky bridge module. Dual ones are nice.
5) For a 3A project, I recommend two 2N3055 transistors. For a 5A project, as described in the other thread, I recommend three 2N3055 transistors.
6) Good idea, if the transformer has room for more windings, add another secondary winding and connect it in series with the 20.5V one.

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Hi audioguru,

I haven't seen a schottky bridge module either, but I could build it easily with 4 single diodes. But since I have complete "normal" modules here, I think I will use them instead. They look as they could stand >10A. Very stable things which can be bolted.

Adding a few more secondary coils to the transformer works nice. I need ~2 coils to get more 0,5VAC.
If anyone else wants to do the same: check that the direction of the windings is the same as the secondary coil. If not, you'll get even less voltage.
Could there be a problem with core saturation or the higher secondary resistance of the coil? Surely less secondary current? Something else I wouldn't dream about?

So I could do some "pimp up my trafo" to 24V and gamble with the opamps. I'm going to buy a few more, so I can replace the burned ones. Doesn't look that bad anymore :)

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Hi Thomas,
It is great that you can add a winding to the transformer to step-up its voltage. You aren't changing it much so don't worry about core saturation of additional temperature if the wire is the correct size. The project's max output current must be derated according to the transformer's power rating.

You said that you are going to gamble with operating the original TL081 opamps beyond their voltage rating. Somebody else in this thread said, "If your power supply isn't reliable, then it isn't worth having." Somebody else said, "You have spent months building a circuit. You connect it to this unreliable power supply which fails and suddenly gives max output voltage and ruins your circuit!"

How high a regulated output voltage do you need? With a 24V/4.2A transformer the modified project will give about 25VDC/3A. I recommend a 30V/4.2A transformer and those high-voltage opamps to get 30VDC/3A out.

Hi Tedy,
If you can't get the ICs then how are you going to build it?

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Hi audioguru,

I think you have a point there. Using higher rated opamps would be a much better solution. Even if the tl081 won't go bang at the first time, they will fail at some time later because of getting too much voltage for a long time.
Okay, so I need to have high-voltage opamps. I'm going to read the other thread about this project in order to get more informations about the neccessary opamps and other needed changes. It's the one about getting 5A Output current from this project, isn't it?

Thanks for your great help :)

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Hi Dosster,
Welcome to our forum.
I shop at www.newarkinone.com and www.digikey.com .
I entered Hammond Power Transformer into Newark's search, selected Torroidal type and found a deluxe 30V/4A transformer with dual primaries and secondaries. They have less expensive ordinary ones too.
Shipping will be expensive since the thingy isn't made of feathers! ;D

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Hi audioguru

So, when those (expensive :o) opamps are used, the only thing to change in the project would be the layout, probably more 2N3055 with each one serial resistor and a big rectifier bridge, right? Getting 5A is impossible with my transformers but I'm sure I don't need that much current.

P.S.: I intend to go with TI's friendly sample policy. Did anyone manage to register by using netscape browser 7.02? I tried it several times but without success. Nothing happens when "register" is clicked after filling out the form. ???

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