I have a few concerns.

1k2 is too high for R1 in the LM317 and LM337, I think you've got the decimal point in the wrong place, use 120R.

With a 12V transformer you might have trouble getting 12V out at 1.5A.

Like my circuit, yours doesn't have any compensation so it might be unstable and oscillate under certain conditions.

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The 1k2 resistor and 10K pot are taken from the LM311 datasheet
(page 9 2nd pic, attached):
"1.2V-20V Regulator with minimum Program current"
The original parts are: 1k2 resistor, 20K pot.
So I guess it's not a problem.
Or could you recommend a smaller resistor and pot for 1.25-12V?

You are right the 2x12V transformer is might be too small.
I didn't include any feedback compensation because I don't know how to calc them and this PSU its only for hobby purpose.

Can I load this PSU through + - rails only?
If I can then how much current can it provide (1x1.5A or 2x1.5A)?

Do you have any further simple ideas how to improve the schematic?

Thank you!

Attached Thumbnails

 

 

The circuit in the datasheet will only work if there's a 4mA load connected to it, you've obviously missed the part where it says "*4mA minimum load current".

I got away witout using a compensation capacitor but you might not be so lucky. If you have any trouble, then don't bother calculating it, just try connecting 100nF between the output of the op-amp to the inverting input.

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The LM117 is shown. It is a premium IC and costs more. Its minimum load current is 5mA but the LM317 that you used has a minimum load current of 10ma.
Your 10k resistor on the output isn't a 10mA load and if you used a 100 ohm output load resistor for 10mA when the output is 1V then the current will be 120mA when the output is 12V.

The problem is that the output voltage will rise when the load current is low.
so use a 120 ohm resistor with the LM317.

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Uncle $crooge

 

I recalculated to 240 Ohm R and 2k2 potentiometer so the output voltage will be 1.25V - 12.7V.
Now I don't have to worry about the minimum load current?

Thank you!

 

240 ohms is used with the more expensive LM117.
120 ohms is used with the LM317.

You might not find a 1.1k pot so use a 100 ohm resistor and a 1k pot for an output voltage of 1.25V to 13.75V if your input voltage is as high as at least 16.25V.

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Uncle $crooge

 

I found a better: 100 Ohm + 10 Ohm (series) and 1k pot: 1.25V - 12.61V.

What difference makes 120 Ohm instead of 240 Ohm in an LM317 circuit?
There are a lot of LM317 sample circuits in the datasheet with 240 Ohm resistor.
Thanks!

 

Whose datasheet are you looking at? The National datasheet has LM117 in the examples. LM117 has a minimum load current of 5mA. LM317 has a minimum load current of 10mA.

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Ron

 

With a 240 ohm resistor and an LM317 that has a max minimum load current then the output voltage will rise without a load. The voltage regulation will be useless.

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Uncle $crooge

 

I had a National 96' pdf, I downloaded the newest 07' one.
Sorry for that, I will use 120 Ohm.
Thanx!

 

I did a simulation with the LM317 and a TL074 opamp driving the adj pin with a 120R resistor between the output and adj. I got very good results (simulation wise) and a constant 10.5ma flow from the ADJ through the TL074 over a 0-24v regulation.

Does this 10.5ma count as a load current ? If I omit the 120R resistor then the circuit still simulates without a problem but the current flow through the ADJ pin is almost non-existent.

 

The 317 is not smart enough to know (or care) where the output current goes, so yes, it counts.
I ran a sim with the LM317 model I have, and swept the value of Rref (the resistor from out to adj). The reference voltage started to change drastically when Rref was greater than about 1k. With an op amp in the feedback loop, this probably won't cause a problem unless the op amp saturates at the negative rail.

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Ron

 

I have never checked out this project till now, and I would have to say that it looks really good hero999! The Dual rails is pretty cool.

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I am looking for a schematic for an independent/tracking dual power supply 0 to 30 Volts 0 to 3 Amps. Can anybody help me. please

 

What has this power supply thread got to do with stepper motors ?