The amps you get depends on the power supply you use. The power supply will show the amps rating somewhere on the label. The one I did does 10 amps at 12V and 25 amps at 5V.
Not really hard to make.. just need an old AT power supply (it can be done with an ATX supply, but a lot more wiring is necessary since these get power on and power good signals from the motherboard), some 10 ohm 10 watt "Sandbar" resistors, and an automotive 12V bulb. If your PS is less than 200 watts you only need 1 resistor. For larger wattage supplies 2 is sufficient.
First thing you do is unplug the PS and let it sit for a bit as there are some large capacitors in there that will give you quite a whack if they're not discharged completely! (I let mine sit overnight.. an electrical engineer could probably give a more accurate time.)
Next, open the box and remove the circuit board. Disolder all the computer wiring from the board remembering where the red, yellow, and black wires were. The red are the 5V+ wires, the yellow are the 12V+ wires, and the blacks are grounds. There are also 12V- (blue if I remember) and 5V- (green?) that could be used to get 24V or 10V when used with their positive counterparts, although not much current there.
Anyway, I only used the yellow, black, and red circuits on mine. I wired in one piece of 14AWG to the board where the yellows were connected, one 14AWG and 2 18 gauge wires to the grounds, and 2 18 gauge wires to the 5V (reds). I hooked the 2 10 ohm 10 watt resistors in parallel and connected them with the auto bulb to a 5V 18 gauge wire and the other side to one of the 18 gauge ground wires. The remaining 18 gauge ground and 5V wires will connect to the binding posts that will be installed to the case. The 14 AWG wires will connect to the binding posts for 12V out. (As an aside, you could just wire in 2 binding posts for 12V only if you don't need the 5V out.. I found that the 5V works great for running in brushed motors though!)
Mount the binding posts on the case (make sure they don't ground to the case! This will shut down the PS if your lucky and if not it will ruin it!) and connect the wires to them and reassemble the PS.
That's it.. you've now got a power supply capable of running field chargers and (if you wired the 5V in) running in new motors/brushes! If you want to get real fancy you could paint the case as well and label the outputs. I haven't done this yet but plan to. I've used mine for many hours since I made it (almost on constantly since then) and it's still working like a charm!
The only thing I noticed is that if you're running in a motor on the 5V line it's better to shut off the supply, connect the motor, then turn it back on. Sometimes the sudden hit of the motor can temporarily shut down the PS (usually 20 seconds or so and it powers back up). This can be built in less than an hour for less than $5 (USD) and will work as well as if not better than most commercial 12V only supplies.
Here is a little draw of the long text… (click on it for the large version)
And here is a photo of the final product…