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I'm currently designing a PCB to adapt a proprietary Dell redundant power supply to have ATX pinouts. It's a redundant PSU from a PowerEdge Tower, and while it has an ATX 24-pin connector to the motherboard with most of the standard pins, it's in a proprietary pinout and has different pin counts. The missing pins are +5VSB and -12V. There is a +12VSB which I plan on regulating down to 5v, and I will use an Inverting Op Amp to produce a -12V connection. I am confident in these being safe as I have an ATX 2.0 PSU to reference for the maximum power draw on those rails can be and I am choosing components for an additonal factor of safety on top of that.

My main concern however is I don't know what other power draw through the +3.3V, +5V, and +12V Pins is. I know it has to be at least 80W combined at max since the motherboard (ASRock n100m) I intend to power with has only the 24pin connector for power and the CPU draws around that amount at peak. Where can I find a figure for the rated power draw through a 24-Pin connector? I have seen a figure of 8 amps per pin as part of the Molex connector spec, and while I definitely want to remain safe that does seem to be overly excessive.

I am not an electrical engineer (I am a mechanical engineering student), but I do have experience working with electronics and PCB design. Thank you very much for the help in advance!

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    \$\begingroup\$ There's usually a sticker on the side of the power supply that lists the maximum for each rail. Regardless of what the spec says I would make sure you can handle at least what the sticker says. \$\endgroup\$
    – user1850479
    Commented Jul 4, 2023 at 17:23
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    \$\begingroup\$ You can't use an inverting op amp to make -12V unless you already have a negative voltage of at least -12V to power the op amp. You will need a DC/DC converter to get the -12V from +12V. \$\endgroup\$
    – Elliot Alderson
    Commented Jul 4, 2023 at 17:28
  • \$\begingroup\$ That was my initial idea, but the PSU is a 750-watt model, and it while that is certainly the safest option, it also doesn't really make that much sense to me as even a 13900k only really can push 400w and that mostly comes through the CPU 8 Pin connectors. It also poses a challenge for the PCB design, the space needed for the traces (using 2oz copper and a trace width calculator) is wider than the pin spacing on the through-hole 24-pin connector. A standard 24-pin only has 2 12V Pins, and assuming that they need to be able to carry a combined 750W seems unrealistic. \$\endgroup\$
    – Anton Yukovitch
    Commented Jul 4, 2023 at 17:32
  • \$\begingroup\$ Hi Elliot, thanks for letting me know that I can't do that with an inverting op-amp! I will look into using a DC/DC converter. \$\endgroup\$
    – Anton Yukovitch
    Commented Jul 4, 2023 at 17:34
  • \$\begingroup\$ The sticker should list the maximum current for each rail, not just the total power for the power supply. You should make your adapter capable of surviving the maximum current for each rail it connects. If it doesn't connect the rails going to the 8-pin connectors, then you don't need to worry about that current. \$\endgroup\$
    – user1850479
    Commented Jul 4, 2023 at 17:54

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