Here is a build from Team RepRap:
Many people use the Xbox 360 power supply to run their heated bed but few ever mention it or offer information on it. So I decided to add it here. Here is some basic information on the power supply and how to use it with Ramps setup.
With Ramps electronics it's really easy to use a separate power supply for only the heated bed. For me, a 203W X-Box 360 Power Supply has proven to be a functional, cheap solution for powering the heated bed or the electronics (~17 amps is not enough for both). This guide might help whoever is brave enough to try it:
Materials: 203W X-Box 360 Power Supply ($15 from Craigslist, remember to make sure it's 203W!) 2 Short Lengths of 12AWG Stranded Wire ($0.37 a foot from Home Depot) 2 12-10AWG Butt Splices (~$2 for a package of 15 from Home Depot - Link)
1. Ensure the X-Box 360 Power Supply is unplugged and fully discharged.
2. Cut off the connector which goes into the back of the X-Box 360 (as close to the connector as possible, or just before the magnet).
3. Peel back the cable's shielding to expose the 4 yellow wires (12V), 4 black wires (Ground), 1 red wire (5V), and 1 blue wire (Standby I assume).
4. Strip a small amount of the red and blue wires and connect them to each other (optionally solder). This will prevent the power supply from staying in stand-by mode after you give it power.
5. Strip and twist the ends of the yellow wires together, then strip and twist the ends of the black wires together (possibly solder these and the yellow wires as well).
6. Place the yellow (remember these are 12V) and black (ground) wires into their own butt splices, crimp, then crimp the 12AWG wire into the other side of each of the butt spices. It will help you to remember which one of these is ground by splicing black wire to black.
7. Put some electrical tape around the connections.
8. Power up (you should see a green light on the power brick) and test with a voltmeter. If everything seems to be working, you're ready to attach the leads to your electronics. (below is a picture of the leads attached to RAMPs 1.4 electronics)