There are tons of Instructables on how to hack a discarded ATX (computer) power supply unit (PSU) into a lab bench power supply. This is not one of them.
My observation is that most makers, hackers, and mad scientists don't follow instructions very well. We like to improve on what already exists and fit the product to our needs. My objective here is to give you all the information that you might need in order to hack an ATX PSU to meet your needs. This isn't so much an Instructable as it is an Informable. This is meant as a reference guide and so I've tried to avoid extensive explanations. If you want a greater depth of information, see Step 6.
Table of Contents
"Introduction" ~ <-- You are here.
"Step 1" ~ Some background information and suggestions for projects
"Step 2" ~ A list of standard wire-colors and functions in an ATX PSU
"Step 3" ~ A chart of wire-associations on ATX connectors
"Step 4" ~ A chart of wire-associations on other connectors (molex, floppy, PCI-e power, etc.)
"Step 5" ~ Tips, tricks, warnings, anomalies, and conspiracy theories
"Step 6" ~ Sources & further reading
Requisite Disclaimer: Electricity is scary (where fear is equal to voltage x amperage / [stupidity + EtOH]). If you're not afraid of electricity, then there's a good chance that you don't understand it and should do more research before messing with it. Everything in this Instructable is correct to the best of my knowledge, but please use your own good judgement. I am not a licensed electrician, professor of physics, PhD engineer, or any other remotely-credible title. You should clearly not trust anything that I have written. By using this information, you are agreeing that I am not responsible for any of your actions or their results, even if I totally am.
Step 1: Background Information & Suggestions
So, you want to hack an ATX PSU, but you don't really know where to start? There are basically two common hacks. Either,
A. Open the PSU; cut the connectors off the wires; drill holes in the existing housing; insert binding posts, switches, etc.; and attach the wires appropriately (See the photos for my version of this method).
B. Build a box with the requisite binding posts, switches, LEDs, etc.; wire them to ATX and Molex connectors; and plug your creation in to an unmodified PSU.
Method B is more work, but allows you to easily swap out the PSU when you burn it out doing something dumb. (Did you see that pretty power supply box in the photo? it's now a paperweight. With binding posts.) It also gives you more room to add functionality. And you get to build a box! Birdseye maple and dovetails, anyone?
One piece of vocabulary before we continue: A "rail" generally refers to a PSU's output of a particular voltage. In the context of ATX PSUs though, "rail" refers to each output that has a separate group of current-regulating circuits. This nuance is why it makes sense to talk about having multiple +12V rails in some PSUs.
Step 2: Wire Colors & Functions in the PSU
Upon opening a PSU, you will find that it is a jubilee of wires in the worst way possible. Thankfully, the colors are (typically) standardized as shown in the chart. Your PSU may not have all of the wires on the chart, depending on the age and output of your unit. See the notes in the chart.
Remember, you should probably only be messing with the wires that would be accessible without opening the PSU's case (i.e. wires that exit the PSU and end in connectors). If a wire is completely internal to the PSU (i.e. starts and ends its run inside the PSU's enclosure), you probably don't need or want to change it. The chart only shows the wires that leave the enclosure.
Whatever nefarious plans you have for these wires, realize that each pin in an ATX connector is rated to a max of 6 amps. It might be a prudent assumption that other portions of the circuit are similarly rated. So, for instance, if you plan to use all 20+ amps that the +5V rail can throw, you should probably connect as many of the red wires as practical to whatever you're powering. That might mean sticking a bundle of wires on your binding post (if you're using option A from Step 1), or it might mean taking connections from the ATX connector and a few molex connectors (if you're using option B). Either way, the more wires, the better.
You should also know that a PSU (because it is a "Switched-Mode Power Supply", or SMPS) requires a minimum load in order to provide consistent voltage. Most people suggest taking one of the +5V (red) wires and wiring it to ground (black) through a 5 or 10 ohm, 10 watt resistor. For some PSUs, the fan is just enough load to get them to power up without the resistor, or they may have a resistor built in. Given that not having a large enough load will detract from a unit's reliability in unpredictable ways, I would strongly suggest using an additional resistor (or use your +5V to power a tiny USB hot-plate to keep your coffee within spilling-distance of your sensitive electronics--that's fine too). If you want to waste the minimum amount of power, or if your unit behaves erratically, you can usually look up your PSU's minimum loads for each rail on its spec sheet. From there, you can probably figure out which rails need resistors, and how big those resistors need to be.
Step 3: The ATX Connector
The two common ATX connectors are 20 and 24 pins. There are also 20-pin connectors with adjunct 4-pin connectors that can be mated together to function as a 24-pin connector. 24 pins is the newer standard.
Chances are that if you have a PSU with a 24-pin connector you will not have a -5V rail.
You can also buy adapters to use a PSU with 20 pins on a 24-pin motherboard connector or vice-versa. These adapters are also a good source of a pre-wired connector if you don't want to scavenge one off of a motherboard and then spend an eternity soldering wires to each pin. (Mmmmm, de-soldering and re-soldering 20+ connections. Have you gotten your California-recommended dose of lead today?)
If you are planning on connecting to your PSU through the connectors (i.e. "method B"), you will need to use more than just the ATX connector to safely use the PSU's full power. As mentioned in "step 2", a conservative estimate would be to use at least one supply wire per 6 amps carried (based on wire-gauge-to-amperage tables, as well as the max rated amperage of a molex connector).
Image Credit: Please note that the supplied graphical chart is not my work. It is reproduced here for informational purposes only from http://www.smpspowersupply.com/connectors-pinouts.html This site is also linked to in References & Further Reading (step 6).
Step 4: Other Connectors
In addition to the ATX connector, PSUs will have some or all of the following additional connectors:
~ 4-pin molex connectors (aka "Peripheral Connectors")
~ +12V2 connector (aka "P4 12V" connector)
~ PCI Express (PCI-e) power connector
~ SATA power connector
~ Floppy drive power connector
~ Aux power Connector(s)
I would suggest that you ignore the last three connectors listed, as they are small and fiddly to deal with.
To safely get the maximum power from your PSU, you are likely interested in the 4-pin molex, which will give you more wires coming from the +12V1 and +5V rails, allowing you to spread the load and lower the resistance. Many PSUs have a "daisy chain" of multiple 4-pin molex connectors that are connected in series on the same set of wires (see the last picture). Obviously, you only need to connect to one connector per set of wires.
The +12V2 connector is intended to be connected to some motherboards in order to supply additional power to newer, larger processors. It is labeled "+12V2" because it is usually a completely separate rail from "+12V1". On the PSU's label, it will give separate maximum loads for each +12V rail. You can use them separately, or wire them together to achieve a higher maximum load. Realize that the PSU may have a maximum total load as well, or that there may be maximum loads for groups of rails (e.g. +12V1 is rated to 16A and +12V2 is rated to 14A, but the side panel may say that maximum load for both +12V rails is 20A, so even if you wire them together, you're not going to get the 30A that you might have thought).
The PCI-e power connector is intended for graphics cards with power demands higher than 75W. It will likely only be present on fairly new PSUs that supply >450W. These connectors may have rails of their own ("+12V3" and up), or they may not.
Image Credit: Please note that the two supplied graphical charts are not my work. They are reproduced here for informational purposes only from http://www.smpspowersupply.com/connectors-pinouts.html This site is also linked to in References & Further Reading (step 6).
Step 5: Tips, Suggestions, & Anomalies
As I mentioned in a few of the previous "steps"; there will be maximum rated outputs for each rail individually and probably also for groups of rails. See the first image for an example of how a PSU label shows these limitations.
If you are planning to modify a PSU ("method A"), be sure to test it for functionality before you put all the effort into modifying it. You can do this by plugging the PSU in and then using a small piece of wire to connect the green "PSU on" pin on the ATX connector to any of the black "ground" wires. This will turn it on so that you can check the outputs with a voltmeter. Don't forget to check for +5V on the grey "self-test OK" wire. Alternatively, you can use a power supply tester designed to test ATX PSUs, which you can typically buy for around $20 USD online. I would recommend against testing an unknown or salvaged PSU by installing it in a computer, both because it may damage the computer and because your "testing" may not be comprehensive.
If you plan to use fuses on your project, put your fuses on the outputs, NOT the ground (Yes, you will need quite a few fuse-holders). The PSU already has an internal fuse or other overload protection, so using fuses mostly enables you to protect whatever you're powering from the PSU's full wrath. You could also use small circuit-breakers, which are available at many electronics retailers.
In order to figure out which of a PSU's +12V lines are on different rails, you can (after unplugging it) use a multimeter to check resistance between the +12V pins on the different connectors. Any resistance greater than essentially zero is indicative that the two connectors that you are testing are on different rails.
If you are adding components inside the PSU's enclosure, be careful to leave enough space for airflow so that the PSU can still cool itself. if you need more space, you can always move the fan onto the exterior of the case, using the original screw holes.
If you drill any holes in the case, try to keep the metal filings out of the electronics!
If you would like to add a variable voltage feature to your PSU, there are several ways to do it (see some of the links in step 6). The simplest way is to use a potentiometer (a variable resistor), which will probably limit the amperage of your adjusted voltage to somewhere between 1 and 2 amps (unless you find a really monstrous potentiometer, in which case, please buy me one too). Another option is to use adjustable regulators (e.g. the Texas Instruments LM338). This approach would be more complex, but could allow for a higher maximum amperage on your variable output (see the comments for more discussion on this topic).
Step 6: Sources & Further Reading
Many thanks to everyone who has gone before me and made this type of information available on the Internet. I can't lay claim to very much original research, but I hope that you have found my aggregation and condensation of information to be useful. If you need more information or more detail, I'm happy to try to help you myself, but you many find the following references helpful as well. Also have a look at the comments section on this Instructable--I've answered a few questions there, when the entirety of the answer didn't seem to fit into the rest of the Instructable. I hope that you take your new knowledge and parlay it into some serious DC amperage. Remember, if knowledge is power, then knowledge of power is power squared.
General Information on ATX PSUs:
Wikipedia's page on Computer PSUs
Wikipedia's page on the ATX standard (link is pinned to the section on the PSU connector)
Great info on load-balancing and rails, as well as a nice page on connectors
Pin-outs for all common PSU connectors
Pinouts.ru wants you to know how every connector ever is pinned!
Helpful Project Pages from Outside Instructables:
WikiHow has a page with some helpful photos and good tips
This page has good information, but I wouldn't follow his instruction to only use one wire per binding-post!
From the above page: From 1996-2000, Dell used non standard wire-colors! Gah!
An interesting project to create a variable-output benchtop power supply from an ATX PSU
Other good Instructables on PSU modification:
Excellent diagrams here, and good ideas about fuse-use
A very slick supply, built to allow PSUs to be easily exchanged (<-- this project is my personal favorite)
This hacker has created and sells an adapter to easily use a PSU's power without opening it up
(I would note that the product sold in the last link only uses an ATX connector, so you probably can't safely use the PSU's full power with such a product.)