Power Supplies have been for a long time ubiquitous devices. At least half a dozen power supplies can be found in any household, and they keep pretty much everything running, electronics, gadgets, modems, battery chargers, tools and so on. But one might find him/herself in the situation where a power supply of a certain voltage is needed, but is not immediately available. Yep, I found myself in that situation too, quite a few times actually. The idea to build a variable miniature power supply occurred to me a couple of months ago, when I purchased a treadmill that requires a 6V, 1000 mA power supply for the console. I went through a box full of power supplies but found none to fit the requirement. I bought one the next day, but soon after I found myself in need of a 7.5V charger for a car vacuum cleaner. Then I thought that a power supply for both gadgets would be ideal. And perhaps, for few other gadgets :) .. It would save me space, and in the future would spare me from the frustration of not having what I need, immediately. So, the idea was born. Well, it wasn't really a new idea, I built few other PS in the past, but never had a miniature concept in mind. This power supply would be so small, that might even go unnoticed :). It would get a "piggy back" ride on a power adapter...
Step 1: STEP 1. Design the Circuit.
The circuit is based on a Texas Instruments LM317 Voltage Regulator. According with the datasheet, the regulator can supply currents in excess of 1.5A over a 1.25V to 37V output range. Typical application schematics are provided in the device datasheet. I reproduced the schematic in my PCB design software... See the image above ;). I used the values suggested in the datasheet for all the components.
Step 2: STEP 2. Bill of Materials
Since this is a miniature power supply, I used mostly surface mount components.It is not a must though. Three capacitors, a resistor and a trim pot soldered on a project board can definitely squeeze into the enclosure I chose.
Voltage regulator LM317AEMP ($1.78 on DigiKey)
Resistors: R1 240 ohms. Trimmer potentiometer R2 5k. ($2<)
Capacitors: C1 - 0.1uF, C2 - 10uF, C3 - 1uF. ($2<)
Box: 1551MFLBK, by Hammond Industries. ($2.44 on DigiKey)
Voltmeter: 2.6 - 30V. DigiKey PN: 1597-1236-ND ($2.50)
Step 3: STEP 3. Design the PC Board.
There are several PCB design software options, some of them free.. You can google that and choose one. Most of them are pretty straight forward, anyone would be ready to draw a schematic and do a very simple PCB layout after a couple of hours of tutorials an practice...
Basically...You need to create a schematic, associate each part with a footprint, connect the footprints according with the schematic and then generate Gerber files. These files are used to fabricate the PCB. The PCB design process is more simple than it sounds. At least for this project.. This is a schematic suitable for any beginner. I can provide the Gerber files to anybody interested to skip this step (although to me this was the most fun part). I sent my Gerber files to OSH Park, and they made me 5 boards for $10 + shipping.
Step 4: STEP 4. Build the PC Board
As I mentioned in the previous step, I sent my PCB design to OSH Park and got 5 boards for a cool $10. It took about a week to receive the boards. The picture shows a bare board as received from the PCB house. I used 0805 footprint surface mount components. In order to save space I designed a double sided board, but as you can see in the picture, I have plenty of real estate available, so I could have easily gone with a single side board. Alternatively, I could cut the board in half and use even a smaller box.
Step 5: STEP 5. Install the Board in the Box. Testing
The board fits easily in the box. I cut a rectangular opening on the front of the box (ruined few X-Acto knife blades in the process :):)) for the voltmeter. I drilled a tiny hole on the side for access to the trim pot screw. I hope you can see it in the picture attached. If not, you can trust me :). There is a tiny hole. Next...I attached the board to the box using two screws. Then I cut the cord of a power adapter (12V 1000mA), and soldered the wires into the pc board: the power adapter side to the input and the jack side to the output .... You can use rubber grommets or strain reliefs for the cables, to make it look even cooler. I know for sure I'll do it on my next power supply (yep, I love it so much, that I want to build more..)
I closed the box, and secured the bottom cover with two screws. I plugged the power adapter into a 120V outlet. Using a small flat-head screw driver I turned the trim pot to check if the voltage displayed by the on-board voltmeter changes accordingly. TA-DAAA ! My power supply works.
Step 6: STEP 6. Atach the Box to the Power Supply
Affixed the box to the power adapter using epoxy glue, but I could have very well left it hanging, or secured it with zip-ties... For my next one I'm thinking about cutting the flanges on the side of the box.
This proved to be a very versatile and incredibly useful power supply for me. I already found use for it with several electronics, and even if they came with their own PS, I prefer to use mine :))
I hope you guys like my power supply, and if you decide to go ahead and build it, please feel free to ask questions or guidance. And when it's done, send me a message to tell me how much fun you had.