Expert Variable Bipolar Power Supply - Page 3 - Electronic Circuits Projects Diagrams Free

eng1 · 23rd October 2006, 09:21 PM

Quote:

Originally Posted by Hero999

I'm intrested about how the LM337 adds gain to the system.

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.

Roff · 23rd October 2006, 10:23 PM

Quote:

Originally Posted by eng1

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.

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?

muki55 · 24th October 2006, 12:43 AM

Adjustable Symmetrical Power Supply

Roff · 24th October 2006, 02:48 AM

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?

eng1 · 24th October 2006, 12:14 PM

Quote:

Originally Posted by Ron H

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?

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.

muki55 · 24th October 2006, 01:07 PM

Quote:

Originally Posted by Ron H

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?

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

Roff · 24th October 2006, 03:55 PM

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.

Hero999 · 24th October 2006, 10:15 PM

Quote:

Originally Posted by eng1

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.

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.

Hero999 · 25th October 2006, 09:37 PM

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.

Roff · 25th October 2006, 10:33 PM

Quote:

Originally Posted by Hero999

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.

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.

audioguru · 25th October 2006, 10:35 PM

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.

Hero999 · 27th October 2006, 10:44 PM

Quote:

Originally Posted by Ron H

With the 741's common mode input level at zero volts, you need a negative supply for it.

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

Quote:

Originally Posted by Ron H

Try a pulsed current sink for the load. I ran basically this same sim. See below.
First waveform, Ccomp=0.
Second waveform, Ccomp=100nF.

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.

Hero999 · 28th October 2006, 06:10 PM

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 .

Hero999 · 4th November 2006, 12:53 PM

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 12

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 12

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.

Roff · 4th November 2006, 02:04 PM

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

24th October 2006, 02:48 AM	(permalink)
Roff Experienced Member	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? __________________ Ron

24th October 2006, 03:55 PM	(permalink)
Roff Experienced Member	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. __________________ Ron Last edited by Roff; 24th October 2006 at 03:57 PM.

25th October 2006, 09:37 PM	(permalink)
Hero999 Experienced Member	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. __________________ What's so bad about Microsoft? Get Opera it's simply a superb browser.

25th October 2006, 10:35 PM	(permalink)
audioguru Experienced Member	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. __________________ Uncle $crooge

28th October 2006, 06:10 PM	(permalink)
Hero999 Experienced Member	Now my scope's broken! 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 . __________________ What's so bad about Microsoft? Get Opera it's simply a superb browser.

4th November 2006, 12:53 PM	(permalink)
Hero999 Experienced Member	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 12 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 12 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. __________________ What's so bad about Microsoft? Get Opera it's simply a superb browser.

4th November 2006, 02:04 PM	(permalink)
Roff Experienced Member	So did you try adding some compensation? It might get rid of the overshoot and ringing. __________________ Ron

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