This was one of those projects I knew I had all the parts for and it just took being bored one night to decide to actually do it. I decided I wanted to make my own power supply, not just a modified computer power supply, but entirely from scratch. This may not have been the best idea since it’s entirely based on linear regulators (I did put heatsinks on them, but no ventilation). Anyways, here it is:
I should emphasize that I purchased none of the parts to build this project myself, they were all either donated, salvaged, or found.
The base circuit for this I got from here and I modified it slightly to suit my needs. I decided that I wanted 5, 12, 3.3, and a variable voltage output (at this point I realize I could have done all of this with a computer power supply, but too bad!). The second place I got inspiration was this EEVblog post, I decided I needed some more equipment and built some. Now, some design decisions based on parts on hand.
First, it only goes up to about 14vDC. it was the best transformer I had on hand at the time (although before rectification I read it as about 12vAC, I have no idea why it’s higher once rectified).
Second, I have fine and coarse knobs since I couldn’t find my 10 turn pots (they had ball bearing based planetary gears).
Third, I opted not to have power cut switches for the regulator inputs (space concerns) but I do have output switches. I originally wanted to cut power to the regulators because they would just be dissipating heat otherwise.
<where my schematic would be if I bothered to make one>
OK, now that that is out of the way I can explain a bit of my design process. I took the base circuit from Raijuu.net and modified it to suit my needs. I didn’t put in a fuse since I didn’t have one on hand (well, I didn’t have a panel mount holder). The power connector, rectifier, and power switch are from a computer power supply, the transformer came out of some old piece of equipment, probably a radio, or amp, or something (it was in my ‘transformers drawer’), the large filter capacitor is of unknown salvaged origin.
As you can probably already see the entire thing is put together with computer power supply wiring and ethernet cable (how else would you know I built it?).
Up to this stage we have a 14vdDC power supply with a switch, now let’s get some useful voltages. The 5v and 12v legs of the power supply were built exactly according to the Raijuu.net schematic, except I calculated the resistor for 25mA (1.2v red LED) and I put a switch between the output and the LED. The 3.3v leg was basically the same exact circuit as the Raijuu.net variable leg except with r1 and r2 fixed.
The variable portion of this build was the same as the 3.3v leg, I had some pots lying around from an old equalizer whose transformer blew and figured a 1k and 5k in series would constitute decent fine and coarse pots. I called that a 6k r2 and set r1 toso the max voltage was just below the point where the lm317 started freaking out about not enough input voltage.
That basically covers the entire circuit, the enclosure is a standard radio shack project box I got from a friend’s box of un-finished projects (I think), the screws for this did not come with the box, I assume the originals were pan head and these are not.
The banana jacks came off an old electronic educational board (the sort of thing that exists so you can build circuits with just banana cables).
The project was compressed onto this proto board as much as possible, but the LEDs and resistors were done hanging.
This has been a simple overview of my power supply (I intended on posting this a long long time ago, but I got sidetracked thinking I was going to make a schematic).