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The logisys ATX supply I received with my Rostock Max Kit has a -12v blue wire on the main strip that is capable of handling .8 amps. The other +12v yellow wires can handle 16 amps max altogether. Could I not use the +12v yellow and blue -12v wires for the onyx heat bed by themselves to achieve 24v and then run the rest of the Rambo card off the standard +12 volt wires? I am not sure the -12v lead will necessarily handle the load per se, but the extra voltage will very likely have a positive effect on the heat bed performance if it works. I also have an old 135w AT power supply laying around that I can re-task to power something if needed.

far as I know most pc power supplies only have one -12v lead, if they even have one at all. use of this wire in computer systems are seldom used but when they are used they output only a small amount of current. probably not enough to heat up the onyx. you may have the voltage, but not enough current in this situation. Try it let us know how it worked out for you.

here's how much current the onyx draws at various temperatures.
http://forum.seemecnc.com/viewtopic.php ... =170#p7960

If the current is limited to .8 amps, that would really not work all that well eh? Not sure how the PSU would work like that to be honest. I think I'll set things up the regular way and if the heat up times take too long or never get there, I'll invest in a larger and more powerful power supply.