Variable Bipolar Power Supply - Electronic Circuits Projects Diagrams Free

Hero999 · 27th September 2006, 03:21 PM

Here's the plans for a power supply I'm intending to build. By my claculations it should provide me with +-15.7V@1.5A. The mains transformer I'm using is a 15V-0-15V torroid rated to 2.67A which is slightly over-rated which is perfect as the regulators might not current limit untill 2.2A.

Anyway, I just thought I'd ask you lot if you can think of any improvements before I etch the PCB and build this.

Hero999 · 1st October 2006, 07:23 PM

Not that anybody cares but I've nearly built this project. The PCB is finished and the componants are all soldered in place. The circuit works quite well the -V supply is alway within <10mV of the +V supply which is as close as fu*k is to swearing. My work's calibrated meter would show 0.01V difference on the 0-20V reading and 0.001V on the 0-2000mV setting, this is probably due to the tollerence in the meter rather than the circuit. I did consider using a better op-amp than the uA741, but then I thought I have no need to as it's only acting as a DC amplifier and the regulator IC will handle all the load transients, besides I've got loads of them in my junkbox.

I've drilled all the holes in the box and mounted the terminal posts. All I need to do is drill the holes for the power switch, LED, voltage adjustment pot, and power connector. When I've completed it I'll post some pictures providing I can find my digital camera.

JimB · 2nd October 2006, 04:32 PM

Quote:

Originally Posted by Hero999

Not that anybody cares ....

Well as I write this, your circuit has had 34 views, so various people have taken time to look at it, including me when you first posted the circuit.

You asked if there were any improvements to be made, I could not think of any, and presumably no one else could either.

So, take heart, you have built it, it worked in the way you expected first time, that sounds like a success to me.

Award yourself a few brownie points

JimB

Hero999 · 2nd October 2006, 08:18 PM

Point taken, it's had 35 veiws now.

I've made the hole for the power connector and drilled the hole for the potentiometer. Progress has been slow because I'm doing it at work. Hopefully I'll have it finished by the end of the week.

Hero999 · 14th October 2006, 02:36 PM

I've finnished this thing last week but I've only got round to giving it a full test and I'm still quite satisfied even though it isn't quie as good as I first thought.

At the maximum output voltage the -V input is about 25mV lower than it should be, while at the lowest setting it's spot-on. This is because the inverting amplifier has a slightly higher gain than one which I is due to the componant tollerances. Even though I haven't bothered with a proper heat sink (I've mounted each regulator on a piece of copper and fixed it to the metal case which is being used as the heat sink), it performs quite well and the box doesn't get too hot even when short circuited.

I hope to get my digital camera back soon (I'd lent it to my brother), as soon as do I'll post some pictures.

Dr.EM · 14th October 2006, 05:18 PM

It shouldn't get hot short circuited as the chips shut down all together in that condition. Running at near the max (1A or so) it will do, but shouldn't overheat if your heatsinks are suitably sized. I', interested in the pics

. I like the design, where they track each other. Mine has 2 controls and thats awkward, mabye I should change it to this design.

Hero999 · 14th October 2006, 05:59 PM

Quote:

Originally Posted by Dr.EM

It shouldn't get hot short circuited as the chips shut down all together in that condition.

In theory yes, but in practice it tends to oscillate at a low frequency. The chip gets too warm so it shuts down, it cools down so it turns back on agian. You're still right though I should test it at its maximum load.

Quote:

Originally Posted by Dr.EM

Running at near the max (1A or so)

The datasheet says it sould shutdown between 1.5A and 3.4A but I haven't tested it.

Quote:

Originally Posted by Dr.EM

it will do, but shouldn't overheat if your heatsinks are suitably sized.

There isn't a propper heatsink, it just dumps the heat into the case. Performanse will depend on the voltage, at lower voltages the current will be limited by the power dissipation and cooling of the chip, at higher voltages ripple might be a limiting factor.

Quote:

Originally Posted by Dr.EM

I', interested in the pics

. I like the design, where they track each other. Mine has 2 controls and thats awkward, mabye I should change it to this design.

It can be handy to have separate controls if you want to have a different negitive supply (which probably isn't that common). I decided that this wasn't an issue for me since I already have another monopolar LM217 power dupply if I need a different positive/negitive supply.

Anyway, you've convinced me I need to do some more performance testing at the highest and lowest voltage settings.

I have one more trick with an op-amp and a uA714 that can make the LM317 go all the way down to 0V. I'll post if you like, but I don't really need it.

Hero999 · 14th October 2006, 08:49 PM

Oh well I'll post it anyway.

eblc1388 · 17th October 2006, 12:45 PM

Have you done the ultimate test yet?

A direct short circuit between +ve and -Ve at maximum output voltage, done several times with a few seconds in between. If the power supply survives, then it has passed the test.

Hero999 · 17th October 2006, 06:35 PM

Yes, that was my first test and of course it survived, the LM317 & LM337 just shut down.

When I've done the other tests I'll post the results.

Roff · 18th October 2006, 03:34 AM

Nice job, Hero!
Have you looked at the outputs with a scope? Have you done transient load tests? I would be wary of marginal loop stability (poor phase margin) in the negative regulation circuitry. If it is a problem, a cap (10nF?) from the 741 outout to its inverting input might help. If it's not a problem -

Hero999 · 18th October 2006, 08:01 PM

That's all in the pipeline. I didn't think the loop stability would be a problem because the LM337's gain is in effect unity, remember it's only subtracting Vref from the output of the 741 - it's acting like an emmiter follower.

Roff · 18th October 2006, 08:07 PM

Quote:

Originally Posted by Hero999

That's all in the pipeline. I didn't think the loop stability would be a problem because the LM337's gain is in effect unity, remember it's only subtracting Vref from the output of the 741 - it's acting like an emmiter follower.

Yes, but it adds another pole in the loop, which would degrade your phase margin.

eng1 · 18th October 2006, 08:39 PM

Another thought: if you use the 120 ohm resistor the current is quite high: about 10 mA... an LF411 can be a good replacement for the ua741?

Hero999 · 20th October 2006, 11:50 PM

Quote:

Originally Posted by eng1

Another thought: if you use the 120 ohm resistor the current is quite high: about 10 mA...

The 120ohm resistor keeps the LM337 happy because it's this minimum recommended no load current for the LM337.

Quote:

Originally Posted by eng1

an LF411 can be a good replacement for the ua741?

I know there are plenty of better alternatives to the uA741 (take the TL071 for example), but why would there be any advantage in using them? The uA741 is perfect for this application, using a better op-amp won't increase the performance much. High frequency response isn't improtance since the LM337 will handle load transients, CMR isn't important since the tollerances in the resistors will add a far greater error, noise doen't matter since the ripple from the mains will exceed any noise generated in the op-amp and it's configured for unity gain so negitive feedback will get rid of it anyway, I could go on. . .

Quote:

Originally Posted by Ron H

Yes, but it adds another pole in the loop, which would degrade your phase margin.

I don't think so. Imagine you subsitute the LM337 for a PNP transistor, it wouldn't effect the phase margin since it's phase shift is practically zero and it's gain is slightly than zero. Exactly the same is true with the LM337.

The only reason I'm using a LM337 for the negitive supply is because it will shutdown if overloaded, it could just as easilly be a normal transistor.

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1st October 2006, 07:23 PM
Hero999 Experienced Member	Not that anybody cares but I've nearly built this project. The PCB is finished and the componants are all soldered in place. The circuit works quite well the -V supply is alway within <10mV of the +V supply which is as close as fu*k is to swearing. My work's calibrated meter would show 0.01V difference on the 0-20V reading and 0.001V on the 0-2000mV setting, this is probably due to the tollerence in the meter rather than the circuit. I did consider using a better op-amp than the uA741, but then I thought I have no need to as it's only acting as a DC amplifier and the regulator IC will handle all the load transients, besides I've got loads of them in my junkbox. I've drilled all the holes in the box and mounted the terminal posts. All I need to do is drill the holes for the power switch, LED, voltage adjustment pot, and power connector. When I've completed it I'll post some pictures providing I can find my digital camera. __________________ What's so bad about Microsoft? Get Opera it's simply a superb browser.

2nd October 2006, 08:18 PM
Hero999 Experienced Member	Point taken, it's had 35 veiws now. I've made the hole for the power connector and drilled the hole for the potentiometer. Progress has been slow because I'm doing it at work. Hopefully I'll have it finished by the end of the week. __________________ What's so bad about Microsoft? Get Opera it's simply a superb browser.

14th October 2006, 02:36 PM
Hero999 Experienced Member	I've finnished this thing last week but I've only got round to giving it a full test and I'm still quite satisfied even though it isn't quie as good as I first thought. At the maximum output voltage the -V input is about 25mV lower than it should be, while at the lowest setting it's spot-on. This is because the inverting amplifier has a slightly higher gain than one which I is due to the componant tollerances. Even though I haven't bothered with a proper heat sink (I've mounted each regulator on a piece of copper and fixed it to the metal case which is being used as the heat sink), it performs quite well and the box doesn't get too hot even when short circuited. I hope to get my digital camera back soon (I'd lent it to my brother), as soon as do I'll post some pictures. __________________ What's so bad about Microsoft? Get Opera it's simply a superb browser.

14th October 2006, 05:18 PM
Dr.EM Experienced Member	It shouldn't get hot short circuited as the chips shut down all together in that condition. Running at near the max (1A or so) it will do, but shouldn't overheat if your heatsinks are suitably sized. I', interested in the pics . I like the design, where they track each other. Mine has 2 controls and thats awkward, mabye I should change it to this design. __________________ Angry!? I'm absolutely electrolytic! Will have to make do with myspace now I guess...

17th October 2006, 12:45 PM
eblc1388 Experienced Member	Have you done the ultimate test yet? A direct short circuit between +ve and -Ve at maximum output voltage, done several times with a few seconds in between. If the power supply survives, then it has passed the test. __________________ L.Chung

17th October 2006, 06:35 PM
Hero999 Experienced Member	Yes, that was my first test and of course it survived, the LM317 & LM337 just shut down. When I've done the other tests I'll post the results. __________________ What's so bad about Microsoft? Get Opera it's simply a superb browser.

18th October 2006, 08:01 PM
Hero999 Experienced Member	That's all in the pipeline. I didn't think the loop stability would be a problem because the LM337's gain is in effect unity, remember it's only subtracting Vref from the output of the 741 - it's acting like an emmiter follower. __________________ What's so bad about Microsoft? Get Opera it's simply a superb browser.

18th October 2006, 08:39 PM
eng1 Experienced Member	Another thought: if you use the 120 ohm resistor the current is quite high: about 10 mA... an LF411 can be a good replacement for the ua741? Last edited by eng1 : 18th October 2006 at 10:07 PM.