730w atx power supply not suited/too weak ???

  • I'm trying to use a 730w bequiet L8 pure power atx power supply for the bed and hotend so I wired the 8 cpu cables to the board (4 x12v and 4 x GN) wich should give it 360w (it has two 12v rails rated at 30A each). When I heat even only the bed just to 60°c it draws so much current that the fan of the power supply stops and the cables get warm (would say ~40°c).

    I tried putting a 5v load by putting a 5.5ohm 20w resistor on a molex connector and the fan does doesn't stop spining anymore but I got a low voltage warning (8.4v) and the drivers shut down because of overheating. The power supply is haswell compatible so I shouldn't even need it.

    The only thing that worked was limiting the PWM of the hotend and bed to 60% but it's awfully slow and I got a heater fault once (hotend shut itself down mid print).

    Thanks for helping.

  • I think for any type of answer, specifics about your printer, wattage of the heated bed, how many fans, stepper motor ratings, etc is all needed.

  • An atx psu is designed to supply multipe different voltages to the mainboard and other components Since you only use the +12V output is it possible that all the other voltages are running away caused by the unbalanced load making the psu degrade the performance?

  • @jamesm

    • 120W heated bed
    • 3x stepper motors rated at 2.5 A
    • geared stepper motor rated at 1.3A
    • 30W hotend heater
    • 12V 0.6W fan

  • you can user the 5v line to power the duet. connect 1 5v to the external 5v in and set the jumper to external power.

    which wires get hot? all of them, or just the ones connected to the bed?

    when i was using an atx power supply i kept the rails separate and used a mosfet to power the bed from one rail, and the board from the other rail.

  • As others have mentioned above, I've frequently read that ATX supplies are designed to have some load on each rail/voltage supply to work properly. Without that loading, any given voltage may not not perform anywhere near spec.

    Personally, I was happy to leave computer supplies behind a few years ago and go with proper power supplies. At least in the States, MeanWell supplies work very well and are <$30 for the sizes used in printers, and are smaller as well.

  • administrators

    It's a bad idea to parallel the two 12V outputs. I suggest you use one for the bed heater and the other for everything else.

    You will almost certainly need a significant load on +5V to get the proper 12V form the 12V outputs. You may need as much as 1A load on +5V.

  • @dc42 said in 730w atx power supply not suited/too weak ???:

    You may need as much as 1A load on +5V.

    that could be achieved with 5v fans on the duet.

  • administrators

    @nophead supplied two resistors for the ATX supply on the Mendel90.

    THS15 Aluminium clad resistor 10R
    THS15 Aluminium clad resistor 4R7

    Obviously those values may need to change depending on the PSU you use.

    See the M90 assembly manual for how they were fitted

  • @dc42
    Currently im only using one of the two 12v rails, I guess I could take a pci-e cable and connect it like the wiki says to do when using 2 power supplies to put a load on the 2nd one. My only concern was the wiki saying to " not connect the bed heater PSU +ve output" and in a way it would be, but I guess thats just a technicality and it should be fine.

    I will try using the 5V rail for external 5v and if its still not enough I will buy 25-50W halogen lightbulbs to draw more current.

  • @veti using the 5v rail as external 5v the PSU fan doesn't stop spinning anymore and voltage stays around 11.6V whereas previoulsy it was around 11.2V . So I don't know how much the board draws but it seems that its more then the 0.9 A from my resistor. Maybe I can get stable 12v with an even bigger load, will try a halogen lightbulb.

    The connector from the cpu extension cable I'm using is still getting warm though, even when only heating the bed to 60°c, so I still should put the bed on the 2nd 12v rail by connecting it to the board like its a 2nd psu as you did. Why did you need mosfets ?

  • @wylley do you have 5v fans?

  • @wylley said in 730w atx power supply not suited/too weak ???:

    Why did you need mosfets ?

    just to make the wiring easier.

  • @veti yes I have two 5V fans but even when I put them at 100%, it doesn't change anything, they only draw 0.05A each.

  • I run a raspi 3B+ and a 2004 LCD on the +5VFSB and have LED lights on the 5V leg of my ATX when it power up as printer lighting. Ikea Jansjo

    i think they are 0.3W each. i like that they are semi-rigid flexi and i can 'aim' them at thigns.

    I will admit that occasionally I get an odd fluctuation in temp readings, not sure if its a wiring issue or PSU issue.

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