Technically possible, though not in the way you're probably thinking of. To start an ATX PSU you need to momentarily short two pins, while an AT style switch closes the connection permanently. If you're okay with flipping the switch to on and then immediately back to off again to turn the machine on, then you can hook it up there. If you forget, the machine will turn back off after a couple seconds, just like if you'd held down the power button on an ATX machine - annoying, but no real harm done. If, however, you'd like the switch to be able to remain in the "on" position, and maybe even use it to properly shut down the machine this way... then it gets a lot more involved.

I'd keep the wires to the ATX connector connected the way they should. The at power button is on/off toggle and the ATX motherboard expects a momentarily button.
If you connect the ATX power connector on the motherboard to the AT power button it should be fine if you follow this sequence:

Push on and instantly off, the system should turn on like you're used to.
Push twice again and it'll turn off/standby/however it is configured.
Keep button on for 8 seconds to simulate the forced turning off and push button to off again.

Done this before with a Pentium 2 baby-AT style board, works perfectly. In the end I got tired of this sequence and found a momentarily button that fit the at style mounting on the case.

And fifteen years ago it ended up at the recycling centre........

But all the videos i see of jump starting a PSU they keep the paperclip or cable in place and the PSU keeps running. Wouldnt a AT power switch have the same effect?

I might've made a boo-boo earlier. The ATX push buttons are momentary, but get connected to a pin header on the mainboard, not the PSU. The power signal pins on the ATX connector (which are probably what you saw getting paperclipped) stay shortened until the mobo decides to release them. So if you hook into the latter then yes that switch would work... However, depending on the OS you're using you likely want the machine properly to be able to shut down in a clean way, and to allow for that, you should to go for the mainboard header.

So the power_good green wire then. Yah that should work on boards with dual AT/ATX connector, but might have weird or even bad effects on a pure ATX board. Should use an ATX supply with it's own switch on the back, because slots may still have standby power when PG switch off, so you need to switch PSU off or unplug to change or install a card.

However, just using this one switch, you might have problems if software puts system into suspend or sleep power states, where normally a tap on the ATX power button would wake it up. On some boards the BIOS lets you set keyboard power on, so tapping spacebar would wake it up. This may also be necessary on boards that are more ATX than AT to get them to start.

Alternatively, there are some types of power switch of the push and latch variety, that have their latch springs on the outside of the switch, so you can pull them out and it becomes a momentary switch with no latch. So that type you could mod to no-latch and hook up to the power switch pins.

Some problem that occurs after a period of time with higher AC or high DC rated switches used at low volts, low amps is that the contacts oxidise, where at higher voltages and currents they would self clean from the slight arcing. This varies by switch design.

Thanks for all the info, it's a board with an at and ATX power connector. I could also use an ATX to AT adapter but I rather use the ATX plug directly.

One question though. Removing the green wire from the ATX plug and attaching it to the AT switch I understand. But where should the ground cable from the AT switch lead to or come from?

Thanks!!

Why not just replace the latching power switch on the AT case with a momentary one? They are available in the exact same form factor from ebay/aliexpress

maxtherabbit wrote on 2024-12-24, 18:22:

Why not just replace the latching power switch on the AT case with a momentary one? They are available in the exact same form factor from ebay/aliexpress

That would be amazing! I searched quite a bit but couldn't find a similar switch to the one i have. Do you maybe know where to find one?

Oh I'm sorry, you have the toggle type. I assumed it was the push button variety

You could remove the switch and measure the cutout. Most likely it's the Taiwanese "standard" 25x25mm in which case you're out of luck. If however it was the smaller 22x19mm like the classic macs used, then this switch would work as a replacement: https://www.digikey.com/en/products/detail/e- … 5HC1921/8540804

Here's an idea: Leave the power switch disconnected and use Reset for power instead. One of them would've ended up unused anyways - might as well use the one that gives you the least amount of trouble. Just hook it up to the mainboard where the ATX momentary power button would've gone, and that's that.

elszgensa wrote on 2024-12-24, 23:31:

Here's an idea: Leave the power switch disconnected and use Reset for power instead. One of them would've ended up unused anyways - might as well use the one that gives you the least amount of trouble. Just hook it up to the mainboard where the ATX momentary power button would've gone, and that's that.

I've used several AT/ATX motherboards in AT cases with an ATX PSU for years this way. If you use Win98 it will almost certainly have an issue once in a while where it would be nice to still have a reset switch, but worst case scenario you simply hold the button down until it shuts the PSU off.

Usually those dual power boards are past the functional turbo header years, so unless you wanted that to switch bus or multiplier jumpers instead, you could rig turbo to reset and reset to power. Less of a PITA to have to double click turbo for the occasional reset than not have soft power switch.

Although functionality wise it would work, part of the cool experience for me is the on off toggle button. I'm going to try my original plan, although I still don't know where to or which ground cable I should connect to the AT on off switch.

If that doesn't work I'll go for the ATX to AT converter and if that doesn't work I'll go for the turbo button instead

Another option is creating a small circuit that provides an impulse to the motherboard, every time the switch toggles from off to on.

I remember seeing something similar on the Internet for this purpose, maybe a simple NE555 with some resistors would do the trick?

Green wire, and Black wire, bridged by switch.

See this instructable.
https://www.instructables.com/How-to-Turn-on- … ly-Without-a-C/

Full rundown:

This is a 5v data level signal, that when bridged to ground, tells the ATX supply to turn on.
https://en.wikipedia.org/wiki/PS-ON_Signal

A normal AT style switch is MORE than adequate to create this signal. It is usually broken out on ATX-->AT motherboard power converters for this explicit purpose.

Since this is *JUST* a 5v data level signal, and is *NOT* related to the PowerGood signal that an ATX motherboard sends, breaking it out of the connector for use with an AT switch should be just peachy.

If you look closely at this pre-made cable, you will see that this green signal is NOT passed to the motherboard *AT ALL*, and is ONLY carried by the breakout.
https://www.rackmountnet.com/product/atx-to-a … on-cable/?gQT=1

The specific ground used should not matter; on most pinouts it lists them as COM or, common ground. It means they are all connected to the same plane internally anyway.

Unfortunately it didn't work, i removed the 2 pins from the ATX plug, connected them to the AT switch. Connected the ATX power connector to the mainboard. Switching the AT power button on turns the fans etc on, the mobo gets power, just nothing happens. No beep, no post, no nothing. It seems like it gets power but doesn't 'know' that it's turned on?

That's not surprising. The motherboard expects to be controlling the soft power of the PSU when being fed through the ATX connector. I think you should just use the ATX to AT adapter (or better yet source a real "AT" (LPX) PSU)

I ordered one, i was just hoping the adapter would not be necessary