Introduction: Easy ATX Bench Top Power Supply.
There have been a few good writeups and Instructables on this subject, recently. This picture I found on dutchforce.com finally inspired me to make my own. http://www.dutchforce.com/~eforum/index.php?showtopic=20741
Not being familiar with the inner workings of an ATX power supply, I applied one of my favorite hacking methods... I ported all the lines to a tidy little color-coded row, where I can mess with them at my leisure.
This also allowed me to bypass a lot of hard work, and resulted in a very compact design that is easy to further adapt and modify.
Step 1: WHY ARE THERE SO MANY DARN WIRES???
Ok, relax. There's a ton of redundancy in the wiring here. For the life of me, I'll never figure out why they need so many wires in this stupid power supply, especially when so many of them go to the same place.
1.There is a green wire that goes to the 20/24 pin ATX connector. When it's pulled to ground, it turns on the supply. Unless it's held low, the only DC power that comes out of the thing is a low current 5V standby power from the purple line.
2. There is a grey "Power Good" line. I can't find a lot of info on this, but several people suggest you should put a small load on it, like an LED and resistor. Mine appears to work fine without doing that, and the voltage measured on this line is 4.7V or so.
3. There may or may not be a brown line, which is the 3.3V feedback line, which should be attached to one of the orange 3.3V lines. On my supply, this wire was already in continuity with 3.3V output on the pcb itself. So I wonder why they even bother using this wire, cuz it goes into the ATX connector, sharing a pin with a 3.3V line, anyway... more redundancy.
4. There may or may not be a small thin red and/or yellow wire, which are the +5V/+12V feedback lines, which should be attached to the respectively colored +5V/+12V power line. Mine had just a the small red wire.
There are several red, yellow, and orange large diameter output wires. You can remove them all but one of each color, unless you are going to keep long lengths of this wiring and can't afford a miniscule voltage drop from this already relatively poorly regulated type of high output supply, then there's really no point in connecting big bunches of them together, like many other people have done in their own version.
Anyways.. those are the basics. The only other thing to add is that some supplies need a minimum load on the 5V line before the output voltage (of the 12V line) becomes stable.
I experimented with the 12V output on my power supply, using a 1 ohm piece of resistance wire. This was done with and without an 80 ohm load resistor between 5V and ground.
Without load: The 12V output when open circuit was 13.06V. The output with the resistance wire attached and glowy hot was 11.53V. The spec on the supply states 15A output. So this seems perfectly acceptable to me.
With load resistors between 5V rail an ground: The 12V when open circuit was 13.06V. With resistance wire attached was 11.55V. The difference was statistically insignificant, with my low quality multimeter.
After a deeper investigation, I found out why the load resistor makes no difference on my supply: There is already a resistive load built in. Even without the load resistor, there is an 8 ohm resistance between 5V rail and ground! So no, my power supply isn't magically efficient... but at least that's one less part to worry about. I also found that the 3.3V line was loaded with a 10 ohm resistor. I actually opened it up to take a look and I spotted both of these power resistors inside the supply. I also took some pictures while I was in there, but I had an irritating flash card reader problem, and I'm too annoyed to do it over again.
Step 2: Walkthrough:
First off, unplug the supply. Then hack off all the wires, leaving a few inches hanging out of the power supply. If it has been plugged in within the last day or two, be sure to bleed the capacitors off. There are many tricky ways to do this.. but you can easily do this without even opening up the supply. Cut the green wire. Turn on the power switch, if there is one. Then touch the green wire to the chassis and wait till the fan stops moving.
Open the chassis. If you want to remove some of the extraneous wires, you can either cut them off or desolder them. I desoldered mine. If you choose to desolder them, you'll need to remove the pcb. Remove the screws and lift the pcb, carefully. Then touch a conductor between the contacts of the big high voltage caps, just to be sure they're completely bled. Be sure to use only one hand while doing this, so you aren't forming a circuit that goes near your heart.
I left only a single wire for each output, and two for ground. You can then solder up the voltage sensing lines and/or the green line, right now, as outlined in the previous step. Or if you aren't sure exactly how to connect these yet, don't worry about it. You can just port over all the lines onto the outside of the power supply and figure it out, later.
Step 3: Output Connectors
One popular type of connector to use for the power output is a binding post. These handy connectors screw up/down over a post with a hole in it. If you buy a breadboard, they often come with a set of these binders integrated on the backboard. I have never liked them, and I have removed and discarded them from all of my breadboards.
Another popular type of connector is the banana plug/jack. I don't have any of these, either.
One could alse use RCA jacks. If one had any.
I used the universal connector: solder.
I took some half ounce copper pcb material and cut to size with a jigsaw so it fits over the side of the chassis, next to the hole where the wires come out.
I drilled four screw holes, so that it affixes firmly to the chassis.
Then I took out a tape measure and marked off a spot for each wire
Mark off your lines with a marker
Remove copper with a carbide tipped hand-etching tool
Test "pcb" with a continuity tester
Cover the connections with epoxy, leaving some exposed pad for soldering connections to. This serves to keep the wires from falling off when you solder other large wires to the solder pads.
I added a thinner copper board over top of this pcb as a "scratch pad." I can remove and replace this "scratch pad" by loosening the screws and cutting any soldered jumpers. This provided a good place for my initial testing, and I'll use it for fabbing up ideas I have for additional control circuitry. Eventually, I may end up making a cover panel with some standard output jacks.
Step 4: The End
Well, I know that I didn't add much new information or near as many pics as I wanted, due to the aforementioned card reader malfunction. But at least I did some actual testing and uncovered one reason that some supplies may not require loading of the 5V output... So probe the resistance between 5V rail and ground on your supply. It may be good-to-go right out of the box, like mine was. And if you really wanna know what's going on, pull out the multimeter and do some testing. There' s no substitute for checking and knowing things for yourself.