This is a functional diagram of a LM317. You can obtain the one for the LM337 with simple changes.
We're used to see two resistors (R2, R1) that form a feed-back network and set the output voltage.
If we don't use the standard configuration and we add an op amp the circuit is more complicated, especially as far as the loop gain is concerned.

Attached Images

pos_adj_reg.png (7.7 KB, 91 views)

 

But can you think of a way to make dual tracking regulators without using a scheme similar to the one we have been talking about?

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Ron

 

Adjustable Symmetrical Power Supply

Attached Files

~$justable Symmetrical Power Supply1.doc (162 Bytes, 117 views)

 

My computer wants me to guess what version of Word (or whatever) you used. At 162 bytes, I can't imagine it is a schematic. Is it? How about posting it as a GIF or PNG?

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Ron

 

Yes. You can use a voltage reference and a dual op amp. The first section amplifies the ref. voltage with a variable gain (non-inverting). Its output is inverted by the second amp (gain = -1, with 1% resistors).
These equals voltages (with opposite polarities) are connected to the adj pins. Resistors between output and adj pin of each regulator are still necessary (you made me aware of this some time ago) but they don't set the Vadj; they only set a minimum load current for the regulators (1.25/R).
The op amps are used in a closed loop configuration. If they add a pole, it is beyond the unity gain frequency of the regulators and doesn't affect stability. The LF411/2 have low offset, low drift, high bandwith, high output current.
The positive output can track the negative one with good precision in my opinion. The error is mainly the mismatch between the internal references of the regulators, but this could be be easily eliminated because it is constant.

 

sorry sir for inconvenience here's the files in png and gif.

Attached Images

	adjustable symmetrical power supply.png (32.8 KB, 157 views)
	part list.GIF (27.9 KB, 78 views)

 

Most of the dual pots I've seen have tracking specs around 20%. I've never actually tried to build supplies with them, but I do have an op amp version. I have an old plugboard which had built-in +5, +15, and -15 volt supplies. I modified the ±15 supplies by changing the trimpot on one to a pot with a knob on it, then added the op amp to make them track.
A dual pot costs more than an op amp, the op amp version has better tracking with good stability.

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Ron

 

I have looked at the internal diagram and I appreciate that is has some gain and phase shift. What I don't get is why the load doesn't cause negitive feedback and cancel the change at Vadj. I visualise this as being a unity gain stage in the feedback loop of a very large gain stage.

I've tried simulating this myself. I didn't have a model for the LM317 so I've made one using a uA741 with a few other componants and it works. However Ron's model will be far more acurate than mine so I accept his results over mine.

The only thing I can think of is I'm probably wrong. Maybe the LM317 acts as an open loop gain stage which is sure to cause oscillation. Perhapps the uA741 is over- compensated to such a high degree that it cancels this effect out so my circuit works as does my simulation.

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Here's the simulation of my model of the LM317. Is is acurate enough for this? The only thing that's missing is the current and thermal overload protection circuits.

I couldn't upload this for some reason so I've uploaded it somewhere else.

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With the 741's common mode input level at zero volts, you need a negative supply for it.
Try a pulsed current sink for the load. I ran basically this same sim. See below.
First waveform, Ccomp=0.
Second waveform, Ccomp=100nF.

Attached Images

	LM317 equiv with op amp feedback sch.PNG (23.9 KB, 65 views)
	uncomp.PNG (27.9 KB, 25 views)
	comp with 100nF.PNG (11.4 KB, 18 views)

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Ron

 

The LM317 has a high gain compensated opamp inside. The compensation causes 90 degrees of phase shift. If it is in the negative feedback loop of a similar opamp like a 741 then the phase shift adds to 180 degrees at high frequencies and the combination will oscillate.

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Uncle $crooge

 

Of course I do. I knew that! I've made typical shoolboy error there!
I'll try that.

It5 still works according to the simulation, there's some spikes when the switch turns off and dips when it turns on but there isn't any ringing.

Anyway I don't trust simulators. I've brought it home from work now and I'll test it properly tomorrow.

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I just got round to testing it when I turned the scope on, it made a buzzing sound, a gree line appeared on the screen briefly then it stopped working.

I'll start a new thread about this .

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Well I still haven't got round to looking at the scope but I took my power supply into work and tested using a scope that works.

I couldn't see any oscillation on either the -V or +V supply but both behaved pretty poorly under transient conditions. I connected a MOSFET to a 12hm: resistor on the output and drove the gate with a 50kHz waveform. I treid to make the leads as short as possible but there was significant ringing on the output when the load was turned on and a big glitch when it turned on. I would say that the noise at worst was about 1Vp-p. I performed this test on both the positive and negitive lines and got similar results, as expected noise on the +V line gets transferred to the -V line but not the other way round.

I was impressed with the current output though, I managed to set the supply upto +-12V with 12hm: connected from + to - and the output didn't drop at all. 2A isn't bad at all for a pair of regulators only rated for 1.5A.

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So did you try adding some compensation? It might get rid of the overshoot and ringing.

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Ron