so you want to change your heatbed power supply..

this what i have done to use an external power supply and still use the standard heatbed command

i cut in half the 12v wire (2 reds wire) that goes to the heatbed.... after testing the wire that come from the electronic board to see where is the +12v, i connect it to this step down controller...( 12v to 5v :  http://dx.com/p/dc-4-40v-to-dc-1-5-35v- … mer-126108  3.5$ )

now when the host software want to switch on the heatbed, i receive a clean 5v on or off (you can adjust the output power with the adjustable trimmer resistor for a clean 5.0v)

then i connect the step down to this relay card... ( http://dx.com/p/1-channel-5v-30a-high-p … arm-153000  6.2$ )

the -Vout to the ground (0v)  and the +Vout (5v) to the Ctrl and +5V connector..

now i have a high power relay that can control a up to DC 30V, AC 250V, 30A power supply...

you just need to connect your heatbed (silicon pad for example) to your external power supply with the +12v wire thru the relay connector...

10$ (include international shipping) and 15minutes of work...

this was made because I DONT want to play with electronic component ... i KNOW you can do it cleaner and perhaps for less $ but this is a dirty trick that anybody can do at home if you have not two left hands... (and i have some relay and step-down card in my arduino-box-of-thing that wait...) and the result is a (.. overkill) proper protected electronic