Small-scale Benchtop Power Supply




This is a short instructional for modifying a DC laptop brick to make an adjustable voltage output using the LM317 IC.

For schematics, please google "LM317 datasheet." I will only describe the build process in general terms.

Step 1: Mini Benchtop Power Supply

This instructional modifies a laptop DC brick to make a slick, breadboard-friendly power supply for all your low-powered projects.

It contains one thru-put, and two, independently adjustable voltage outputs. The circuit uses an LM317 for each of the adjustable outputs. Best of all, it outputs thru a breadboard power strip!

You will need a multimeter in order to accurately adjust the voltages. But you already have one, don't you? You can see my "pen" multimeter hanging in the back.

Step 2: What You Need:

Some of the things you will need are

1) Laptop power supply, or any other suitable "power brick." The one I used is actually a regulated 12V 1.25A power supply that was originally for a VOIP phone.
2) A couple of LM317 IC's.
3) resistors, caps, and pots/trimmers to build the LM317 circuit. Because I had 500R trimmers handy, I used 50R for the fixed resistor. This provides for adjustment from 1.25V up to just above 12V. If you have any question, try Google.
4) A suitable enclosure.. such as this nifty case that used to house a travel alarm clock.

To do this MY way you will also need:
1) a glue gun!
2) some machined pin IC sockets and 26-27 awg solid core wire
3) a breadboard power bus strip.

Step 3: OK, Here's One Way to Do It.

First install the connectors (optional):
I put three "machined pin" IC-socket connectors on the front and one in the back. First, hold a bit of protoboard over where you want the connector. Then tape it in place. Now, using a 1/16" bit, drill a hole for each pin, using the protoboard as a guide. Now put a dab of hotmelt glue over the holes. Then stuff the connector in.

Optionally, you could prep your power brick in the same fashion.

Next, you can put your lm317's and filter caps in. I hotmelt glued them to the case before soldering them in with jumpers. *Disclaimer - the IC's will get hot and probably melt the glue under high loads. But I doubt it would melt plastic... then, again, I'm not an electrical engineer. You might want to use protoboard for this part.

I did use protoboard, copper up, to hold my trimpots. I then glued the board to the bottom of the case before soldering the connections.

You can also see that I layed down a bit of copper tape as a ground strip underneath the front connectors.

I also added a switch to turn off the LM317's. When not used, they will still draw a wee bit of current. If you are just using the "thru-put," you might wanna turn them off.

So what you are looking at is this... two trimpots adjust voltage of two LM317's. The connector on the back and the one in the middle of the front are continous with positive rail (This is because I designed this to be a stand-alone component - input was supposed to go in the back connector, with thru-put to the center front connector, if you see what I mean). The lateral connectors on the front connect the LM317 circuits. Almsot done!

Step 4: Finishing Touch

Not satisfied, yet, I decided to use a piece of breadboard power strip for the outputs. Here's the how-to:

1) Take one of those removeable power bus strips
2) Saw it to the appropriate length
3) Remove the foam backing
4) Using a small screwdriver, catch hold of the clips from the back and pull them out
5) Clean off the clips with acetone (to remove the residual foam tape)
6) You will see that each row of 5 holes basically has it's own clip, but there is a small bit of metal connecting them together. Disconnect each row of clips as needed. In my design, I partioned the power rail into three partions, one for each output. The ground bar is continuous.
7) Now solder a bit of 26-27AWG solid core wire to each clip. Then you can cut a small notch in the plastic strip where the wires will exit, if you are anal. Then put the clips back in. Inspect for shorts.
8) Now hotmelt glue the power bus on.
9) Clip and strip the leads and insert directly into the machined pin sockets... Ahh, perfect fit.
9.5) Oh, in case you didn't know: An easy way to modify the power bar markings is by wiping bits away with a q-tip and acetone.



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    14 Discussions


    3 years ago on Introduction

    This is an awesome project but I have to agree with what others have already said, it is lacking alot of the main things that would make it a great project, I love what it is but would love to see how you got there, not just beginning and end but the path between both points.


    5 years ago on Introduction

    nice presentation but for sure this is nothing! No schematic provided, no details nothing. Its like someone baking cookies, you will need sugar, flower, cinnamon butter and its baked!!!! hahahahahha no way mister isn't instructable!


    10 years ago on Introduction

    i am just new here. could you provide a schematic of the design? thank you.


    11 years ago on Introduction

    I don't see how this is an instructable. Don't get me wrong it's a nice idea and a useful one at that but I don't see how this instructed anyone on how to do anything.

    2 replies

    Reply 11 years ago on Introduction

    This instructable provides the idea to adapt a common power brick into a multiple variable output powersupply with a universal breadboard output bus. If you take 5 minutes to read an lm317 datasheet, then you have all you need to know in this instructable. I believe I even added a link here, somewhere. But where does it end? Do I need to tell you how to solder, read, do simple mathematic calculations? I made this from spare parts... some of which I can't provide links or part numbers for. But the principle applies to a wide range of power supplies and applications.


    12 years ago

    Good idea... I like how small it is. Heatsinks on those regulators would be a good idea though.

    1 reply

    Reply 12 years ago

    you can get away with out them they just use up space. unless your going to draw alot of current you dont really need them if the start getting hot i would go out and buy some though


    Reply 12 years ago

    i agree howm i sposed to make it if he dosent even provide pin-pin diagram he better hurry up with it though... and what is a 'power brick' is it just a transformer 110/240 > 12


    Reply 12 years ago

    Yes, a power brick, or "floor-wart" is just a rectified and filtered transformer, such as for a laptop power supply. The different between this and a "wall-wart" is that rather than plugging directly into the wall, it has a socket for a standard AC power cord. I have simply opened it up and removed the DC output wires. Then I routed power to two LM317 circuits. The final result is just a glorified transformer with it's original output, plus two adjustable outputs, and instead of a mess of output wires, posts, or jacks, I use a single, partitioned breadboard bus. This means I could theoretically hook up 20 small circuits up to it, at three different voltages with no special connectors. The strip also makes adjusting the outputs easier, since my multimeter's probes fit into a breadboard socket.


    12 years ago

    I used RCA jacks for my power supply's outputs. They work rather well and are easy to connect/disconnect. I've got more then enough cables to hack if I ever need to use more power jacks :)

    1 reply

    Reply 12 years ago

    I considered that. I have a dozen-odd salvaged RCA connectors in the parts bin. But I decided that special cables or sockets were a part of the equation that I wanted to remove. Sometimes, you don't want to solder an RCA jack (or half an RCA cable) to a project, where you might just clip a couple wires, tin the ends, and plug it in. And anyways, there isn't any space on this thing for RCA jacks... although, with enough hot melt glue, I firmly believe that anything is possible! :)