Picture of Dual POS-NEG Power Supply
This is a great little power supply for breadboarding Op-Amp circuits. Here are the features:
  • 1.5 Amps a Side
  • Independent Voltage Adjustments
  • Compact
  • Easy Construction

This is a beginner electronics project so I've decided to add a lot of basic material that will hopefully assist new builders to complete this project. A lot of knowledge that many advanced users take for granted is often left unsaid. I am going to try not to do that here. If you're experienced then all you're going to need is the first couple pages of this article. If you've any questions keep reading and maybe something else I've included here will help you figuring out whatever it is that you do not understand.

Step 1: Parts

Picture of Parts

Parts the Regulator Board requires

R1, R2 220 (2)
R3, R4 2K 10 turn trimmer (2)

C1, C2 .1uF 50V (2)
C3, C4 10uF 50V (2)
C5, C6 1uF 50V (2)

D1-D4 1N400X (4) (X=2 to 7)
IC1 LM337
IC2 LM317

Heatsinks for regulators
Stranded hook up wire 3 or 6 colors
2" X 4" (51x102mm) copper clad board

Suggested Parts for Unregulated DC Circuit
  • AC Power Cord
  • SPST Switch
  • Fuse & Holder
  • Center Tapped Step Down Transformer
  • Diodes or a Diode Bridge
  • Pair of Filter Capacitors 1,500-5,000uF
  • Box, Chassis, Other Mounting Method

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mark.gray.1658 months ago

I got a question. Why are the filter capacitors so small. With all the advances in that technology, I 'd think there would be at least 'one' huge cap for both sides,

pfred2 (author)  mark.gray.1658 months ago

How would you connect one filter capacitor in this circuit configuration? This article deals primarily with the circuit on the other side of the bridge, and filter anyways. I pulled those caps out of another PSU that was doing essentially the same thing I'm doing here too. So it is safe to say this is how it is commonly done.

I understand that. I didn't see them right away on the Transformer side of the drawing.

WWC3 years ago
Hi pfred2. I really enjoy reading your Instructables, You are a wealth of information. I am trying to understand this neg voltage. You have a 24V power supply. +12v to 0. 0 to -12v. If you test between them you get 24v? If so can the 0 to -12v be used to power something all by its self or does it need the +v also? I once built a zapper from a disposable camera and it measured -450v. Had to be i guess, i was measuring from plus of the battery to the ground/out of the zapper. So i am not really clear on this neg voltage.
Goodhart WWC3 years ago
Along with Pfred's explanation, maybe a visual would help (o being ground):
Pos_neg sup.bmp
i.e. then the two extremes are rectified into a dc voltage.
WWC Goodhart3 years ago
It does help. Thank you very much for the visual aid.
pfred2 (author)  WWC3 years ago
I've a new visual aid for you:


I'll let you look it over to see if I am on the right track or not working on your circuit. I could upload my Eagle file too if that would be of any use. Once you check I've done the schematic right I'll start doing the board art.
Goodhart pfred23 years ago
A question: Is pin 8 of a UA741 connected to anything internally? On a LM741 it doesn't appear to be....assuming an 8 pin DIP.
pfred2 (author)  Goodhart3 years ago
It beats me it is Wayne's board. I'm just trying to route it.
Goodhart pfred23 years ago
I was looking at an LM741 OpAmp and it looks like pin 8 has no internal (IC) connection (I only looked it up because I was kind of familiar with the 741, and it struck me odd that all the pins were used even though I was sure one was kind of useless).
pfred2 (author)  Goodhart3 years ago
The 741 being in there did make routing the board difficult so any pin not connected would make it easier. I'll have to disconnect pin 8 and see what I can come up with.
WWC pfred23 years ago
I am thinking that would include pin 1 and the #103 cap also.
Goodhart WWC3 years ago
Yes, now if this were an LM358 or LM324, then you'd of course tie unused outputs to ground for stability. But you probably already are quite familiar with all that.
WWC Goodhart3 years ago
No i didn't know that. Thanks for tell me I went to Electronics school but that was almost 30 years ago. I only recently have really gotten back into it. So most everything i do i have to learn it. For instance this dual rail we have been going over. I only just learned this but it certainly doesn't mean i cant build one. The best project for me is one i have never built before, so i get the excuse to research it to no end and build one. For me the using the item i built is second to the research and building. Al tho i do prefer to see the end result working as opposed to not.

So dont hesitate to fill me on the details, i am sure i can use it.
Goodhart WWC3 years ago
You are welcome. There are many electronics gurus on board here at Instructables, many of them MUCH more enlightened then I am. I have only a hobbyist's background in it; never any formal training, but I had at one point some 20 years ago, gotten good enough to build many of my own pieces of (simple) test equipment; which I found quite enjoyable (the fun of putting it together and the ability to use it often afterwards).
So, yes, I understand exactly what you mean by how much fun this can all be. I still have yet to learn to use programs like Eagle, etc.
As most can tell, I have been using PAINT or GIMP to draw most of my schematics.
pfred2 (author)  Goodhart3 years ago
Learn Eagle. It is a schematic capture program, so creating board layouts is integrated in it. Once you've drawn the schematic it keeps track of your connections so you can route your parts. I use it even if I don't etch a circuit board to help me build circuits. Then I just print out the board layout and use it as a drill guide on a blank sheet of phenolic substrate, and wire it up. I mean there is no way I'd have been able to make this without it:


Well, OK maybe I could have, but it'd have been a lot harder! I use Eagle to show me where each wire goes while I wire the circuit. I pick a trace and it shows me everywhere it is connected to.

P.S. It looks a lot nicer on the top:


Goodhart pfred23 years ago
I know, I have to take some time and learn it......time is not really on my side these past 2-3 years however :-)
WWC Goodhart3 years ago
Why yes it does. Between pin 1 and pin 8 there a 100p cap. It must just be hard to see because when pfred just now sent me the art work for the board the cap is connected correctly. Check the link a few posts down from pfred with the jpg.
Goodhart WWC3 years ago
No, I mean when I look at a pin out for the 741....pin 8 need not be connected to anything as it has no connection inside the IC itself.
WWC Goodhart3 years ago
WOW! You are very observant. The reason is, I substituted the LM741 for the LM301 that was in the original diagram.

Now would be a good time for some exotic explanation. But i really didnt catch it. I am impressed you discovered this. Did you know this from before?

Either way the PCB will be ok as is, i believe.
Goodhart WWC3 years ago
I've used the 741 so often that something didn't "look right" about the layout, so I double checked myself before posting. I tend to see the odd ball stuff that some miss (and sometimes miss the obvious LOL).
pfred2 (author)  Goodhart3 years ago
I checked if Wayne had the polarity right, he seemed to be having some troubles with that on the negative side, but that was it. Now that you bring it up I am going to remove that trace from the board I am fooling around with.
WWC pfred23 years ago
Ya looks great to me

I was trying to find those resistor symbols you used. The Eagle library is quite extensive as you know. Ever time i am looking for a part, i find many more parts i haven't seen b4. Sometimes its hard to find the part i already used.

With more use i will get more familiar with it.
I will keep at it.
pfred2 (author)  WWC3 years ago
While you keep at it have a look at this:


It is the schematic you said was right routed. I'm sure it could do for a little more tuning but I think I am getting close.
WWC pfred23 years ago
I am now working on controlling relays with my parallel port.
pfred2 (author)  WWC3 years ago
Probably more than you need but this is a board I took some time to design:


I took it to the OK I'll make it stage.
WWC pfred23 years ago
I have seen that board of yours before and have made a BOB for my system. I have taken a couple of ideas from yours to include in mine. I plan to make an Instructable on it some time.

I have been working on my parallel port relay project. I will include a screen shots and 3 pics. It toggles an LED to be able to see it is working. just the first stages of this project, but working. One relay anyway. Its hard to see but there is only 3 resistor, one transistor and an opto coupler in there. Oh and a relay.

Not in a big hurry for that board from you. I know how it is playing with computers. I have 19 pc plus a server that controls all of them.
pfred2 (author)  WWC3 years ago
Whenever you mix inductor coils with electronics you have to put a reverse diode across them to shunt the current kickback that comes out of the coils when they turn off. Well, you don't have to, but it is an awfully good idea to.


Looking forward to your article when you submit it.
WWC pfred23 years ago
You have a lot better of a compact design then i come up with. How many time did it take you so you didnt have a jumper?

I dont know what else you could do, it looks really good to me.
pfred2 (author)  WWC3 years ago
I see a lot of places I can improve it. I didn't hand tweak and move anything yet. That was all auto-routed. I'll see if I get to working on it a bit later today. I should have named some signals too. That helps me when I am eyeballing the circuit. Using the eyeball tool in Eagle that is.

I've been sort of wrapped up playing with my new toy over here though lately. Shiny new computers are hard for me to resist.
pfred2 (author)  WWC3 years ago
Those resistors are in res-us in the rcl library I believe. I've been using Eagle for a while. Plus hit their website and get more parts libraries. I hate having to make them. Though I've done that too. You know I didn't find the big part in this schematic in their library:


I'm on a new, to me, computer right now. I am just in the process of setting it up, just so long as the tree guys out in front of my house quit knocking my power out! I don't have Eagle setup on this machine or anything yet.

Now that you've said I'm on the right track with your schematic I'll give it a shot routing the board. I'm not adding any component values though. I'll leave that chore to you when I'm done. I don't mind drawing them, it is the details that weigh me down.
Goodhart WWC3 years ago
BTW: also take note of pfred2's new post after my example; it will help even further.
Goodhart WWC3 years ago
YW !
pfred2 (author)  Goodhart3 years ago
What happens after being rectified and filtered is a lot more boring. The bridge chops off the top, and bottom of the wave depending on which direction is being crossed, and the filter capacitors fill in the differences. So you end up with this:
Goodhart pfred23 years ago
Exactly !
pfred2 (author)  Goodhart3 years ago
I forget not everyone sees all of this stuff in their heads like I do. I guess because I've seen illustrations, if not oscillograms of the phenomenon happening.
Goodhart pfred23 years ago
Some of us are more "visual" then others, tis the way the brain is wired. I tend to see "geometry" and "trig" better then symbolic things like algebra or calc.
Goodhart pfred23 years ago
It may be more boring, but it is definitely an integral part of what happens.
pfred2 (author)  WWC3 years ago
Positive, negative power can be a bit difficult to wrap your head around until you've had a little practical experience with it. But essentially ground is declared halfway up the potential scale. A lot of amplifier circuits work with dual voltage power supplies. There is in fact a 24 volt difference between -12 and +12 volts if you call -12 ground.

The transformer is center tapped, with the center tap ground. It is really like two circuits mirroring each other on either side of ground. Although in the case of my supply I could set one for say +12 Volts and the other for -9 Volts if I wanted to.

In use I usually would use them both the same, but opposite values.
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