Duet WiFi 2, ATX Power supply, and Heated bed replacement

  • Hello!

    I've decided to upgrade to an ATX PSU, since my last PSU was a bit sketchy. I have watched some videos on youtube, showing how some people have wired their ATX's. I've also looked at the "Duet 0.8.5 Wiring ATX Power Supply" thread on the forums, but before I go through with it, I wanted to clear some things up.

    I bought an EVGA 500W ATX PSU. This power supply has two "CPU" plugs. They are each square shaped plugs with a total of 4 wires (two yellow and two black). On this thread https://forum.duet3d.com/topic/1570/duet-0-8-5-wiring-atx-power-supply it is reccomended to put as many of the power wires into the provided ferrule. I was able to fit 2 of the yellow wires in total into the ferrule, which will go into the + power input on the Duet.

    Question one, will this be enough current for everything?

    Question two, I'm not sure what I should wire into the - power input on the Duet. Since I used the two power wires from the square connector, should I use both of the negative wires that were also on that square connector? Or just one? Or a different wire entirely?

    Also, (thank you for your patience) I am replaceing my heated bed to a more powerful one, do I need to change anything in the Gcode? Or do I just need to retune the heater?

    I appriciate your help, Thank you.

  • according to the spec the 4 pin cpu connector is rated for around 75W cpus.

    it really depends on the diameter of the wires.
    What i did when i was using an old atx power supply was to use a wago 222-415 to connect 4 wires in and connect it with a 4mm^2 wire to the duet.

  • The CPU and GPU rails are going to have the most amount of rated current.

    I once used an ATX PSU to power a R/C lithium charger, I went inside, bundled the 12V rail together to one larger cable.

    Just note, those caps can harm or kill you well after it's been powered down so take care.

    Also note ATX PSU's are rated at maximum power, this means all rails together, you need to see what the 12V output is, some older or cheaper PSU's have very weak 12V rails.

    You are also going to need to short the signal wire if you were not aware already- some PSU's will prevent you from drawing too much without it actually connected to a computer, as some will sense the 5V rails and see that nothing is drawing power and shut down, I had one do this to me and it was useless for this reason.

  • @veti Do you have a suggestion on how I should wire this now? If i need a little more than 300w for the heated bed, hot end, etc.. how do people get away with just wiring up half of one of the CPU plugs?

  • I should add that i need a total of about 335W:

    • My new bed heater is rated at 200w

    • The stepper motors require about 90W

    • My extruder is rated at 30W

    • I have 3 fans, and according to the "Choosing the power supply" guide, i'm allowing about 3W per fan.

    • And finally, 2W for the Duet Wifi 2.

    As Veti said above, the two CPU connectors on the PSU are possibly rated at 75W each. If I were to use all 4 power wires from these, wouldn't I still need 185W of power? At that point, that would be a lot of wires going into the Duet, so it confuses / concerces me. Would that be correct?

    If that is the case, how can I go about finding the required power? I'm unsure how much power each wire provides.

  • your psu has a single rail. connecting more wires in parallel just reduced the current that passes through each wire.
    it really depends on the wires used in the psu
    see https://www.engineeringtoolbox.com/wire-gauges-d_419.html and the max current rating for a wire.
    Normaly the wires for the 8/6 pin PCIE are a bit bigger than the rest.

    With a 200W bed you are getting very close to the max 18A for the bed channel.
    You could consider getting a mosfet with a good heatsink (like a MKS MOSFET ) and connecting the bed with separate wires.

  • If you find you need more power, which you are pretty much at the peak (PSU's loose power over time) look into 12V Server PSU's they are simpler, cheaper and more powerful.

    Or, depending on your rig, make the jump to 24V.

    You really don't need too much gauge to wire these things, if you are running a long rung, then yes, go high guage, but my overclocked GPU at 400W with two PCIE plugs and about 2 feet, no problem.

    Simply put, bundle as many 12V wires as you can and solder it to a heavy guage wire, or, go inside and connect the wire directly to the 12V rail, you will see a big blob of solder where many if not all 12v wires connect to.

    Make sure you do a good job at soldering and insulating the joints and use as little length wire as possible.

  • a wago 222-415 is better than soldering.

  • administrators

    You might find something like this useful https://ooznest.co.uk/product/atx-adaptor-board/.

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