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I have an old Power Mac G5 power supply from 2003. It's an excellent quality high-power unit which I'd hate to have to throw out. The problem is that it's 100% proprietary.

All voltages are the same as an ATX PSU: 12 V, 5 V, 3.3 V, -12 V, 5 Vsb, and ps_on are all present, so I thought I'd have a go at rewiring it.

About halfway through the project I found the first issue with this. There is no pwr_ok in the PSU.

So I got stuck on that issue. There are 3.3 Vs and 5 Vs rails in the Mac PSU; I believe they mean sense, but I'm not sure. Am I supposed to use 5 Vs here? How should I proceed?

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The sense pins are for sensing the voltage at the motherboard side, so the power supply can adjust the output voltage based on what will it be at the motherboard end of the wiring, instead of just outputint some voltage which drops over the wiring before it reaches the motherboard.

They can't be used for powergood PS_ON signal.

You could fake the signal and tie PS_OK to 5V supply with a resistor. Maybe add a capacitor to make it rise slower, so motherboard starts when supples are settled.

The PS_OK is a signal which the supply gives to motherboard that voltages are now within ATX specifications and have been for a given time required, and it will also signal to motherboard when the supplies have gone out of spec or as a forewarning that they soon will go out if spec, if for example the supply detects that AC was lost and there is still energy for a few milliseconds.

The Mac motherboard obviously has the supply monitoring circuitry on the motherboard, not in the supply.

You could build such a circuit, but the power supply specs and tolerances are rather unknown.

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