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I'm building a system without a case, so I don't have an on/off switch to start the machine up. Is it ok to permanently short the on/off pins on the motherboard and simply use the on/off switch on the power supply unit?

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om/off pins are made for a momentary short, not continuous. Some BIOS have an always on for power failure option. –  cybernard Jan 2 at 3:45
    
It would not work, it would cause power OFF-power ON cycle, better is connect a switch similar to a case switch (momentary short).You can use one from an old case, or buy from the store. –  miggy Jan 2 at 4:02
    
Just use the bios settings for resume after power failure to always turn on. –  HTDutchy Jan 2 at 8:22
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5 Answers

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It most likely won’t work.

It would definitely work with an AT motherboard and PSU because that uses a permanent switch which stays in the new position after you toggle it like a light switch, so you could toggle the power button once and turn it on and off by flipping the switch on the PSU.

An ATX motherboard and PSU on the other hand use a momentary switch which returns to its previous state after being pushed. It is more like a button on a TV and is designed to be controllable through software (it logically flips the switch, not physically).

There is an extra complication with an ATX system because unlike with an AT system where the power-switch is actually connected directly to the PSU, the switch on an ATX system connects to the motherboard which then relays the signal to the PSU. The ATX specification says that to turn the PSU on, you need to short pin 16 to ground. I have tried permanently shorting that pin so that I could turn the PSU on and off via its power-cord and it did work, but I was using the PSU for other, non-computer purposes, so it didn’t have a motherboard.

In your case, it wouldn’t work because permanently shorting the switch on the motherboard would be like holding the power-button indefinitely, and ATX motherboards are designed to treat a push of more than 2-4 seconds as a hard-off.

If the problem is that you simply don’t have a switch available, then there are plenty of solutions because all you really need is a way to short the pins for a moment. Check your local electronics store or even better, a computer store, especially a mom-and-pop shop; they may even have something lying around that could work, and might even give it to you for free. If the problem is that you specifically want to control the power through the PSU, then that just won’t work.

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Early AT motherboard based computers used externally switched power PSUs. On the adoption of Plug-n-Play, Energy Star and ACPI power management, the switch function moved to the motherboard which then sent the power state to the PSU. From that point forward, the AT motherboard required a momentary NO power switch just like the ATX motherboard. I was building computer systems during the switchover and had to assure customers that if they really wanted to make sure the computer was depowered, that they needed to use the PSU switch, but after turning it on, would still have to hit the front swtch –  Fiasco Labs Jan 2 at 4:25
    
Yes, I remember all the confusion with some Windows 95 systems staying on after being shut down. I still have a couple of the older style AT PSUs that I use for non-computer tasks that need 5 or 12 volts. –  Synetech Jan 2 at 4:43
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If you short them together, you will in effect be holding in the power button. If you do that on any soft power systemboard, it will immediately turn off after 5 seconds. So no, you cannot do it that way.

You could momentarily place the jumper and then remove it.

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Or salvage an old led with header connector, solder in place of the LED, a normally open button switch and click it when you need to turn it on. –  Fiasco Labs Jan 2 at 4:29
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You cant do that. As K.A stated your system board will get turn off, if we hold the power button. Instead go for digital switch button. This will serve your need.

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You could use your on/off switch and put a 10-100µF cap (16V or more) in series, that should do the trick.

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I would grab a switch / LED or something with the same plugs from another computer / case / spare stash and use that / rip the LED off and short the ends together.

That would be much easier than trying to put a jumper on and taking it off to turn a computer on and off. Otherwise you could use cybernards idea and just set the computer to power on when it detects power.

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1) Not what the OP asked. Quoting: "Is it ok to permanently short the on/off pins on the motherboard and simply use the on/off switch on the power supply unit?" 2) It will not work. The PC would power down after a few seconds because it gets the impression that someone is trying to hard power off the PC by holding the power button. –  Hennes Jan 2 at 12:17
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