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Powering ATX-standard mobo with separate supplies

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Ash View Drop Down
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    Posted: 24 Aug 2021 at 4:17pm
I really want to try powering a mini-ITX motherboard with separate regulated linear power supplies but I need to first understand the logic behind how the mobo and ATX supply normally signal to each other to switch on or off.

The easy bit:

Yellow wires are +12V
Red wires are +5V
Orange wires are +3.3V
Black wires are ground / 0V

Simply obtain the power supplies and connect the terminals to the correct colours. Simple.



The tricky bit:

Purple wire is +5VSB
Blue wire is -12V
Green wire is PWR_ON
Grey wire is PWR_OK

Purple wire normally sits at +5V (at standby) once wall power is turned on, regardless of whether the computer itself is on or off. I don't especially need to keep things like USB ports powered up when the PC is off so I could either leave it unconnected (floating) or I could tie it to +5V, which is probably the better option.

Blue wire would best be handled by the -12V from my switching pico-PSU, where I think I have to short the green wire to ground in order to switch the -12V on. I power the pico-PSU with the same +12V linear supply that will supply the motherboard directly.

Green wire is basically used by the motherboard to tell the supply to turn on and provide all the supply voltages. Taking it low by shorting it to ground seems to be how this is done.

Grey wire is how the supply tells the motherboard that the voltages are all within their specified ranges. I think that if this wire is tied high, up to +5V, the motherboard will interpret that the power supply is working correctly.


I would like to confirm these thoughts as I would prefer not to destroy my motherboard, CPU and peripherals.
Pink Faun S/PDIF PCIe card>Lautus coaxial>Majestic DAC>Lautus-R3>2xProprius>Alpair 11MS
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Ash View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ash Quote  Post ReplyReply Direct Link To This Post Posted: 24 Aug 2021 at 4:24pm
My main concerns are providing supply voltages at pins before they are ready to accept them, power supply surges if I bypass the power analytics logic and switching the computer off at shutdown because it will immediately restart if the green PWR_ON pin is held low but the computer won't shut down properly if I disconnect the pin when the operating system is running. I would have to assume that shutdown effectively becomes restart and I would have to access the BIOS menu before disconnecting the green wire.
Pink Faun S/PDIF PCIe card>Lautus coaxial>Majestic DAC>Lautus-R3>2xProprius>Alpair 11MS
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ash Quote  Post ReplyReply Direct Link To This Post Posted: 24 Aug 2021 at 4:59pm
Do I generate -12V by connecting the positive terminal to the 0V of the other supplies and connecting its own 0V to the blue wire??

(Voltage is a vector, has both magnitude and direction. The magnitude is the same, I just have to reverse the direction)
Pink Faun S/PDIF PCIe card>Lautus coaxial>Majestic DAC>Lautus-R3>2xProprius>Alpair 11MS
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ash Quote  Post ReplyReply Direct Link To This Post Posted: 24 Aug 2021 at 8:24pm
Actually, forget it. I am going to continue my research into single board computers and industrial boards. Some of the more custom boards, whilst they can be pricey, offer plenty of PCIe connectivity options, x86 architecture, RPi-compatible 40-pin GPIO connector and 5V/12V power input. I already have DDR4 SO-DIMM RAM, M.2 to PCIe x4 adapter and PCIe x1 riser ready for small computers. Waiting for a mPCIe to PCIe x1 adapter then I should be able to connect my Pink Faun soundcard to most of the boards out there. Will hopefully be able to move away from gaming motherboards.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote CageyH Quote  Post ReplyReply Direct Link To This Post Posted: 25 Aug 2021 at 7:40pm
http://https://www.diyaudio.com/forums/digital-source/241533-linear-atx-power-supply.html

This may help, but as it is a power supply, proceed at your own risk etc.
Kevin
European loan coordinator, based near Toulouse, France.
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Ash View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ash Quote  Post ReplyReply Direct Link To This Post Posted: 29 Aug 2021 at 4:53pm
Thanks. Been looking at the Up Squared and Up Xtreme i11 boards as candidates for my main (music) computer. I could use Graham's DAK 5V 3A supply with the Up Squared but the board doesn't have absolutely all of the connectivity that I want. Therefore I might shell out for the Xtreme i11 to future-proof my setup. Runs on 12V so would have to buy something like an SBooster PSU. Still researching.
Pink Faun S/PDIF PCIe card>Lautus coaxial>Majestic DAC>Lautus-R3>2xProprius>Alpair 11MS
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