I keep meaning to make one of those power strips that tun off when your USB port goes into sleep mode. I have a friend who could use it as he sleeps his mac mini most of the time, but the ready made ones are out of his price range. Make a nice gift for him.

I thought about that, but the ATX supply is turned on by shorting 'power on' with a ground so there is no real signaling there. I'm not sure how computers are built now but I'm sure an extra 90ma from 3 relays grounding through the atx on circuit will probably cause too much current to be flowing.

Toss a potentiometer on there and see what level it comes on at.

Usually the motherboard has the power on off signal from the front panel and it feeds through the motherboard to the power connector controlling the power in the supply from a connection pin 14 which you should be able to wire into the seond power supply and have them both turn on and off together. I don't see a need to have that many amps from the 12 volt supply as the hard drive takes less than an amp and the extra power to the video card is usually handled by the extra 4 pin connector from the power supply that would plug into the motherboard near the video card slot to provide extra current to it. Usually the processor takes 25-30 amps at 3.3 volts and regulates it down to the 1.6 volt or whatever a Intel chip needs with the switch mode regulators on the motherboard. The other thing you might be able to do is to take a bigger supply apart and stuff it into the case without an enclosure since the supply case is usually full of air anyway. Make sure you have a supply with the 24 pin power cable if the motherboard requires it so that the power connection pins are not overloaded.

I wish it was as easy as wiring into the motherboard connector but it really isn't. The green wire (pin14) needs grounded for the power supply to go active. I don't know what kind of current grounding that pin takes so I don't know if 2-3x relays with 30ma each will overload the switching circuit or not. I can only assume it would because we are talking about small SMD transistors.

As for the processor power. I see that the 3.3v rails have a lot of power through them and it makes 100% sense that the 3.3v would be regulated down but was wondering if you knew why the 'P4' connector is used because of higher processor power demand now and it is 12 volts instead of 3.3v?

https://www.playtool.com/pages/psucon...onnectors.html
https://www.formfactors.org/developer...12V_PS_1_1.pdf

This is where I'm getting my info from.

Well, having looked that up, I needed a standalone power supply this morning for a hard drive hooked up via an IDE->USB adapter whose power brick had gone missing. I looked for the nearest wire-like item and found some solder wick, stuck it in the green pin, and sat the power supply body down on it. Success! :)

I did learn something. I wasn't sure if the wire takes a pulse and the PS latches that, or if the wire has to have a constant ground. It does need to stay grounded, when you unground it, it powers off again.

Anyone know how to de-pin the ATX connector so I don't have to cut the wire to make a semi-permanent connection?

If you look into the connector each pin has two tabs on it that oppose each other. They are easily bent inwards with a tiny screwdriver or paperclip and it pulls right out.

I have read that some ATX supplies don't like to run without a motherboard connected to them because the PWR_OK line (Grey, pin 8) stays silent and the supply starts to swing above and below normal on the rails trying to make power that will give it the OK.

Yeah you should have loads on each of the major outputs since they rely on the 5 volt load to keep the 12 volt output from climbing too high since they come from the same switching transformer on separate windings they tend to interact a little. If you get into the actual motherboard specs you should be able to find out how much power is going into the various supply voltages. The newest power supplies provide a 4 and 6 pin connector for powering up the video card slot depends upon what the motherboard needs plus there are some extra power connectors for the processor - intel needs some extra power usually and not AMD.
The Cooler Master 460 watt supply I have here has 3.3@22a 5@25a 12@18a 12@18a YEAH two 12's -12@0.5a 5Vsb@2.5a
This supply is rated an Intel Form Factor ATX12V V2.3 and over 70% efficient at typical load.

what you could do about having to use 2 switches just use a STDP switch

Single Throw (one lever)

Double Pole (2 separate switch contacts that turn on and off at the same time)