Instructables
You can hack some parts you probably have lying around into a cable that will let you power electronic projects from a PC.

I initially did this article on my web site at uC Hobby but figured I would give it a try as my first instructable. Let me know what you think.

First let me say that this is dangerous. Your PC power supply should have protection from shorts but you are sure to loose any open files not if, but when you make a mistake on your breadboard. Also the PC power supply can output a great deal of current so things can get out of hand quickly. In fact, the more I think about this, the more I realize it is a bad idea.

The first two pictures show the finished product. A PC power plug with long extension wires that reach my microcontroller breadboard. The PC provides +12 and +5 VDC.
 
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Step 1: Find parts

I scrounged an old PC fan for this hack. We can use the long red and black wires and the male power connector to make our cable. I dug up a long piece of yellow wire to extend the +12V.

Step 2: PC power connections

PC drive connectors have 4 connections, 1 Red = +5V, 2 Black = GND, 1 Yellow = +12V.

Step 3: Connect extension wires

Notice that the wires are cut at different lengths. This is an old trick; each wire is cut to prevent shorts. The red wire splice will be in a different place then the black or yellow connections. When you tape up the splices you can wrap around all the wires which is easier and ultimately safer then trying to protect each splice when they are all side by side.

Step 4: Solder the connections

I wrapped the wires together then soldered each connection. The two black wires are ground and were wrapped together along with the black extension wire.

I like to strip a long section of the insulation, wrap, solder then trim about half the length. This makes it easy to ignore large solder blobs at the end of the connection. You don't need a 3/4 inch connection after it is soldered.
I did something very similar to this about a week ago, I had just purchased a small solderless breadbord and didnt want to have extra batteries involved with powering it. I grabbed a USB cable, cut one end off zip-tied it to the breadboard and reinforced it with my glue gun. It was a much smaller breadboard, about 2 by 4 inches. For the connections I just tinned the tips of the red and black wires and popped them in the 2 rows of holes running down the outer edges, added a bit of heatshrink and voila! I've been using it with my laptop while surfing Instructables etc to test out circuits etc, it's a great learning aid. I have shorted it by mistake and on purpose (on an old junk PC of course) and no problems so far, one could add a simple switch built on to keep power off till everything is wired.
I have accidentally shorted my USB voltage lines :/ On my laptop too. The worst that happened -- I got a little pop up that said the hardware was disabled due to high current. Once I restarted the box -- everything was back to normal (after a little nervous sweating :P).
USB is (by specification) supposed to have current limiting built into the interface. Sometimes this is just a "polyfuse" - a self resetting thermal fuse, and other times it is something more intelligent. Not so the bare connectors from your power supply (they'll likely have SOME protection, of the sort that will kick in AFTER your external wires are glowing :-)
I did not realize it was a standard -- but that's good :) -- I was responding to geekboxjockey's last sentence about his/her accidental shorting...
At my school all the students have HP 2140 netbooks and we always play the prank of putting gum wrapper foil or another type of metal in the USB port and it instantly turns off the netbook. i haven't studied it much but it appears not to do much damage unless the netbook belongs to someone with a short temper.
In my experience, PC power supplies are "very" sensitive to shorts. I accidentally shorted the output on an older supply and it died immediately. No leeway for error on that one.
chilll20095 years ago
nice. i had trouble doing it 'till i read yo instructable.
russ_hensel7 years ago
It is also useful to know that many supplies have the 0 volt line connected to the AC ground, not floating. This can introduce a ground where you do not want it and means that you cannot put supplies in series to get higher voltages ( you can use parallel if you have some load sharing resistors ). I have opened some supplies and remounted the circuit with nylon nuts and bolts to get a floating supply, I think it is safe.
uCHobby (author) 8 years ago
I have built a current limiter to make it safer to use the direct PC power cable. You can read about it at my blog. uC Hobby
uCHobby (author) 8 years ago
Thanks for all the comments. I do beleive USB would be a safer way to get +5V for project use.
rotor8 years ago
I just used an old power supply all on it's own. If you don't have one laying around, just salvage it out of the next old PC you see laying on the street corner or in a trash can. There's an instructable somewhere that shows which wires to hook up to a switch, and viola, easy 3,5 and 12 volt power supply that's actually not totally crappy.
Myself rotor8 years ago
Be careful with running a PC supply with nothing, or almost nothing, connected to it. Many switching power supply designs have minimum load ratings, since they can't take the switching element pulse width down to zero, and without the minimum load connected, the output voltage can go unregulated. If you're lucky, this triggers an overvoltage detection circuit which shuts down the input and/or crowbars the output. If you're unlucky, you see 200V on the output. That being said, a few fans should be plenty of load to keep that from happening, and who doesn't appreciate a little airflow? Pick up one of those charcoal filter mats and set up a 120mm fan as a solder smoke sucker for your bench.
rotor Myself8 years ago
I don't know what "crowbars the input" means, but I routinely use it to run nothing more than a few LEDs and 555s (I am not good with electronics), and notice no ill effects. The power supply I use isn't anything fancy, just a generic 200W PSU from some beige box that had outlived its usefulness. I can't imagine the PSU has any more logic in it than the spec dictates, but who knows, maybe I got lucky.
thinman8 years ago
Ya, I would also be careful of over working the power supply. Your computer power supply can certainly be overworked if you have too many components drawing current from it. Not, to mention that besides overheating, under powering components is one of the leading causes of death in electronic components. Just a warning, I have overworked one too many power supplies and royally fried them beyond recovery. Granted power supplies aren't usually that expensive, but your probably better off ripping one out of an old box that are readily available (especially at the end of a semester in college dumpsters). Then you will have your own dedicated power supply that won't cripple your computer if you break it. Here are some plans for one here.
westfw8 years ago
Fuses might be a good idea. The average PC power supply can make normal sized wire GLOW...