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Prusa Mk2 heated bed + ATX power supply. Help!

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Hi guys,

I just got a Prusa MK2 heated bed. I've got an quality ATX PSU I'd like to use from a PC I don't use anymore. It is rated for 350W and 12V/16A. I suppose that should be enough. I wired the thing according to the ultimaker Wiki.

I first uploaded Marlin from marlin-build. I tested the thermistor and it works. So I continued to wire stuff up. Now, I connected one side of the PCB to the 12V (yellow) wire of the PSU, and the other side of the PCB to heated bed pin 2, as instructed. I soldered the PSU ground to the UM ground on the UM power supply jack (on the side pin, towards the power switch).

I turn on the printer, connect the USB and turn on the ATX PSU. Everything works, and the ATX fan spins. I also get a temp readout in printrun. But as soon as I set a temperature for the bed, the ATX PSU switches off.

What to check, and what to do? I know there's electronic experts here (I'm definitely not one of them) so any help would be much appreciated, I'd like to get this thing running as soon as possible.


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A lot of PSU's have an operating range of loading ratios between the rails (3.3, 5V etc) where it will safely operate. When you're using it like for a heated bed you may exceed those limits (because the other rails are completely unloaded) causing the rail voltages to fluctuate and the PSU to shut itself down.

You may find that putting a couple of watts of resistor load onto the 3.3V and 5V rails will allow the PSU to do what you want to do.

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Oh. I guess I should have known that earlier. :D

I found a ceramic 10W resistor I think it was around 50 Ohm and hooked that up to the 5V rail, and another smaller one on the 3.3 Rail. Connected the green wire to the ground to start the PSU, and repeated the procedure from above. The fan twitched for a milisec and that was that. Now whatever I try to do with the PSU (like connect peltiers and fans like I used to do before) the PSU fan just twitches and nothing happens, I can't get any voltage readout on the wires.

I'm presuming I accidentally burned something in the PSU in my previous attempt?

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