Please be warned:
A computer PSU (power supply unit) can become very dangerous as soon as you open it up. It may have exposed wires that will be live if the unit is connected to power (if it has recently been connected to power, there may even be built up electricity in large capacitors inside the unit which may hold high voltages.)
PLEASE make sure you have REMOVED the power cord BEFORE you start modifying a PSU. Electricity can’t be seen the way we normally can see things, and it can be deadly. If not sure seek advice or DON’T do it. It is not worth the risk.
Tip: Wear safety glasses when working with wires and PSU’s. If you should short something or a wire flips in your face, the glasses will protect your eyes. It is the cheapest insurance on this planet! 🙂
I use a standard cheap PC power supply (PSU) to drive my printer, HBP and Hot End. It has a rating of 18A on the 12V rail and works fine.
It was salvaged from an old computer. I have not tried a laptop PSU yet as I don’t have one and they are not cheap. But the advantage with a loptop PSU is that it delivers 19V instead of 12V. This will most likely make it easier to reach the high temperature in the hot end. 2amps at 12 volt gives you 24watt while 2amps at 19volt gives you 38watt. Quite a big difference. For the heated bed, I think a normal PSU is the way to go, but look around on the reprap forums. It has been discussed there as well.
Look here to find info on how to modify a PSU.
You can skip this step and just get some connectors that will fit onto your PSU plugs, but you may have to use more then one to be able to deliver safely about 10 amps to the heated bed.
I have tried to use 2 power supplies at the same time, one for printer and hotend and another one just for the heat bed (HBP). It made no difference worth the hazzle of having two PSU’s.
As you can see in the picture I added an LED to my PSU, this way I can see that it is on from a distance. I also added some screw terminals at the top. This way I can easily add a fan if I have to. I have +3v, +5v +12v and GND. This lets me chose between 2v-3v-5v-7v and 12v. (If you connect between +12 and +5 you get 7.) But remember to look at the label to see what rating your PSU have got on the various rails (3, 5 and 12) before plugging in any heavy loads.
Rember that a computer PSU have got a “starting wire”. To turn it on you need to short this to ground and keep it shorted. It is normally GREEN in colour. Short this permanently. You also need a “load” to make it stable. But all this is in the link I gave you before.