Here's a little PCB I made for myself that I wanted to share here in case it can be useful to anyone. It's a latching circuit that will allow you to use an ATX PSU and case (with a momentary power switch) with an AT style motherboard.

https://oshpark.com/shared_projects/h6LgDD3k

I have a baby AT board that I'm using in an ATX case along with an ATX power supply. I have one of those generic ATX-to-AT power adapters that I'm using. Such as this one...

These adapters already break out the PS_ON and a ground wire (green and black) but you will also need the +5VSB wire (purple). My adapter did not have this pin/wire installed, which I'm guessing most wont since it wouldn't normally connect to an AT board. Adding the pin is easy, you just need a crimping tool. If you don't already have one, you can get a multi function crimping tool that does JST, Dupont and Molex style pins. This is very useful to have for making various cables and adapters for vintage computers. Otherwise, you can simply splice into the +5VSB wire from your ATX PSU.

Normally, you would wire the PS_ON and COM (green and black) wires to the switch on an AT style case. But since I'm using an ATX case with a momentary power switch, I can't do that. So at first I just twisted the green and black wires together and used the standby switch on the back of the PSU to turn the computer on and off. This works well enough, but then I switched to a PSU that does not have a standby switch (it's a quality PSU that I decided to use for a number of reasons). So another option would be to wire the PS_ON and COM wires to a latching switch, and mount that switch on the case somewhere. Well, I didn't want to do that. Yet another option would be to get a little IEC C13/C14 cable with an inline power switch such as this one. Again with the PS_ON and COM wires twisted together, a cable with a switch like this would work well enough. And I actually got myself one of those cables just so I wouldn't have to leave the PSU always in standby (or else unplug it). But I wanted to be able to use the actual power switch that's on the ATX case, and since the circuit was simple enough I decided to go this route. I just wanted to list all of the options I thought of in case it can help anyone.

I made this PCB from a circuit that someone posted on Stack Exchange. You can find more info about it on that page: https://electronics.stackexchange.com/questio … utton-as-switch

I have since decided to update my PCB with some LEDs for status indicators. One for the +5VSB signal, and another that should illuminate when the PSU is turned on. I figured this could be useful, especially for older boards that may not have any status lights or fans. I have this updated PCB on order, but have not shared it yet. When I get it in and have a chance to test it, I will post it if everything works as intended.

And just in case it's useful for anyone, here's another latching circuit I found that instead uses a 555 timer: https://www.instructables.com/Latching-Moment … PSU-Conversion/

I went with the dual inverter circuit for now because it's slightly less complicated. The 555 circuit has roughly the same component count, but also requires the Power OK signal from the PSU. No big deal, it would just require one more wire from the main connector. I may build the 555 circuit at some point, just to mess around and see which one I like better.

Of course, everything is use at your own risk.