Introduction: 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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