Watch out when playing with mains voltage.

Now, if you can fit the ATX PSU insides safely in the XT PSU casing, you have a couple of options that depend on how the ATX supply works.

First of all, the ATX PSU has 5V STANDBY supply when it has mains connected even when it is otherwise turned off. It is the purple wire. You don't need to use that but you could connect it to a LED (with a resistor) to indicate you have mains turned on.

Second, the ATX PSU is turned on by connecting the green PS_ON wire to ground. So you could use the old big mains switch to connect green wire to ground so you can use the ATX supply like it is normally used.

There might be another way, if you always connect the PS_ON to ground, then the power supply could work just like AT/XT PSU so it is always turned on when it has mains connected. So this would let you just use the old big mains switch to actually switch mains to ATX PSU just like it did before. I am not 100% sure if you can use ATX supplies this way, but you can easily check your supply by just connecting green wire to black wire with a paper clip or something and see if it turns on.

Now, many power supplies do not start up when they see no load or too light load, so at least have something connected to the PSU like two hard drives if it does not turn on. Also the XT does not use the 3.3V from the ATX PSU so it is possible you need some extra load on 3.3V supplies so the ATX PSU is happy and stays on.

Happy hacking.

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If you want the XT switch to control ATX mains wires:

The old mains switch connects or disconnects both the live and neutral of mains for your safety, plus it carries an extra grounding wire to connect to case along with the switch.
Why both? Where I live you can connect the mains plug either way so if the switch disconnects only one wire, you can't be sure if it is the live or neutral.

I can't see from the pictures but if I interpret the picture right, you have desoldered the mains switch of the ATX supply and it only switches one mains wire, so you need to rig it so that the switch would be on. Another option is that it is mains voltage select switch between 115/240V but this is unlikely. You should know this. Basically, you could solder the original ATX mains switch back to the PCB and leave it turned on, or just replace the switch with a wire.

Of course you could also just use the XT switch in place and switch only one mains wire, but it would be safer to put the XT switch between mains input socket and ATX mains input so it switches both mains wires. Always connect earth grounds directly, so they are always connected. Then you need to rig the green PS_ON pin from the 20-pin connector to black GND to make the ATX PSU start up when it has mains.

Sorry if this sounds like a long description but I like to make sure people understand what they are doing or what they should be doing when mains voltage is involved. I am not even sure if legislation allows me or you to mess with mains inside devices so that me or you can be held responsible if someone gets electrocuted or a house burns down because of your wiring.

If you want the XT switch to control ATX in a normal way:

Just connect mains from XT mains socket to ATX mains PCB (the ATX power switch needs to be there or rigged into ON position). No other messing with mains inside the PSU.

Then connect the green PS_ON wire to black GND wire on the XT PSU switch.

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I'm not totally familiar with the differences between XT and AT. I do know however that you can buy an AT power supply made by Athena Power on Newegg. Not sure if that would do the trick form-factor wise, but it certainly seems to have the requisite outputs, sufficient wattage, and a button.

x

Heheh yeah, Athena Power. Sounds... reputable.