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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.
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.
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Signing UpStep 1WHY 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.
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.
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Would you be confident buying a power supply where the company takes shortcuts like that? Take RaidMax for example. Have a look inside one of their PSUs and then take a look at a high-end Antec, Silverstone, PCP&C, etc. and see what the differences are. Switching power supplies are extremely complicated. Good design and careful attention to detail are marks of a good, reliable supply. Marks of a bad supply are those that use too little copper to carry too many electrons, causing a meltdown of the insulation and internally shorting the supply. Even with short circuit protection and all the other protections that are built in, do you really want to deal with that as a consumer?
I have been using this as my main home PC for over a year, now. No problems. :)
But all seriousness aside, I barely know which end of a transistor is up. The most technical Instructable I have is how to make a 5 transistor PIC programmer.... and the electronics aren't really very exciting. It's basically just using transistors as signal buffers. Analog electronics is largely fascinating voodoo to me. :)
All you have to do is click on my name , up there, to see a list of all my Instructables and even all comments I have left for other people. I'm sure you can find at least 5 of my Instructables that are even worse than this one!