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I have a computer I'm using as a server, I would want it to turn on on its own when power is cut off, and I figured the easiest way is to connect a micro, maybe an attiny85, that will short the power button pins with a transistor when it is reset (I've checked and the BIOS does not support automatic boot on power on).

However, for this I would need to access the +5VSB rail, is there a standard way to get it from an ATX motherboard? I was thinking about printing a PCB that plugs in one of the PCI-e x1 slots, but it looks like it doesn't have +5VSB, not even any +5V

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  • \$\begingroup\$ Tap splice into the main ATX power connector? \$\endgroup\$
    – vir
    Nov 24, 2021 at 19:09
  • \$\begingroup\$ Why do you need 5V specifically? PCIe has 3.3V standby. It might be possible to configure one or several USB ports to run from 5V standby supply. How about a relay to push the button until 5V or 12V supply turns on? \$\endgroup\$
    – Justme
    Nov 24, 2021 at 19:28
  • \$\begingroup\$ @Justme I've never heard of 3v3 standby, is it step down from +5vsb? If not it will not be available when the power is off \$\endgroup\$
    – Mauro F.
    Nov 24, 2021 at 19:35
  • \$\begingroup\$ @MauroF. It's optional, but if it's implemented, where else would it come from except from +5VSB? \$\endgroup\$
    – Justme
    Nov 24, 2021 at 20:16

4 Answers 4

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Apparently +5VSB is available from PS/2 mouse and keybaord ports. This is not true for every computer I've tested, but for some it is, for the one I'm working on luckily it is, so I can get it from this port.

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+5VSB is pin #9 on the ATX connector. You can run a jumper from the connector (or splice a connector into the wiring harness of the power supply). The purple wire.

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USB headers are the standard ways, and USB supplies are typically fairly fault tolerant. You don't want to ruin your motherboard with a short, do you.

I would want it to turn on on its own when power is cut off,

Solution: don't do anything special. The ATX power supply standard is specified exactly for that, and supplies a 5V standby voltage to keep an onboard controller happy, which typically can turn on a computer on a specific time, or when a specific network packet is encountered. Or, after a power failure! There's even a specific setting for that in all UEFI setup UIs I've seen in a while.

You don't need any additional hardware for this. This is standard functionality.

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    \$\begingroup\$ Please at least read the question before replying. I specifically said that my bios does not support this option (not every motherboard does). Also can you source that USB uses +5VSB? My computer's usb ports are NOT powered when the computer is off but still connected to the power \$\endgroup\$
    – Mauro F.
    Nov 24, 2021 at 19:25
  • \$\begingroup\$ hm, I have one of the on-board USB pin headers being "off-enabled" (marketing-wise, to allow for charging of phones etc) \$\endgroup\$ Nov 24, 2021 at 19:26
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    \$\begingroup\$ Re: your bios doesn't have it: chances are, your bios setup frontend doesn't show it, but it's there! (would be a strange omission from firmware) \$\endgroup\$ Nov 24, 2021 at 19:37
  • \$\begingroup\$ those USB charging port are unfortunately not available on my botherboard (which is almost 8 years old, and they were no as common), also I've seen it more often on laptops than desktops. And regarding the UEFI settings, well it's not I can really do anything about it if they don't allow me to set this option, I've looked in every advanced option and I couldn't find anything. However, I've found a solution for this paritcular problem \$\endgroup\$
    – Mauro F.
    Nov 24, 2021 at 19:57
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The easiest thing way to get +5VSB is to splice it out of the ATX power connector. While it might be available on the motherboard in a few places, it would be easier to cut or use a splice connector like this one

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