Need to activate switch with power

[sound2man]

I'm building a project that requires a computer to be turned on 24/7. In the case of a power outage that lasts longer than the battery capacity of the UPS, I need the computer to turn back on. Is there a normally open relay that could close the circuit for a second or so upon initial power?

The entire circuit will be running on 12VDC if it makes any difference.

 

[Cabwood]

In the PC's CMOS setup there may be a setting called "Power on when AC returns" or something like it. Try that first.

 

[Reloadron]

First, any system that is that mission critical should have a very, very good UPS backup system.

That said, try looking in the system BIOS as most newer system have options on what the system should do following a power failure. The default is remain off but there is generally an option to resume last opower state. Therefore if the system was up and running it should automatically turn on when power is restored and boot into windows. I would start in the BIOS.

<EDIT> Cabwood beat me as I typed! </EDIT>

Ron

 

[sound2man]

I checked the BIOS, and this particular board does not support auto on after power outage.

This is going to be a solar powered project, and while the computer needs to be on as much as possible, nobody is going to die if it doesn't. The main issue is that it is going to be located a couple of hours from where I live, so hitting the power button is not practical.

Also, this board has an integrated power supply that is powered by a wall wort, so I can't ground pin 14 of the ATX supply for always on.

 

[Reloadron]

I checked the BIOS, and this particular board does not support auto on after power outage.

This is going to be a solar powered project, and while the computer needs to be on as much as possible, nobody is going to die if it doesn't. The main issue is that it is going to be located a couple of hours from where I live, so hitting the power button is not practical.

Also, this board has an integrated power supply that is powered by a wall wort, so I can't ground pin 14 of the ATX supply for always on.

OK, bummer on the BIOS.

I can suggest how I would approach it. I once had a Dell system that used an external power supply so I know what you mean on that note. Also, grounding the PWR_ON pin is not real bright as it forces the PSU in an ATX form factor system to never shut down for a system problem and can lead to a small disaster.

Somewhere in the system is 5 volt standby power and that power is always present anytime the power supply is connected to mains power. That power could be used to generate a pulse so if power is lost on return when the 5 volt standby was present it would generate a pulse of a short maybe 1 second duration. That pulse could be used to close some small relay contacts across the front panel power button. There is also another option. A 555 or 74123 IC could be configured to manage that.

The front panel power button is a momentary normally open switch. When you press it it places a logic low on one of the pins it is connected to. One pin is signal and the other pin is common. You could likely skip using a relay and have a logic low pulse applied to the high pin and get a start.

Most of this depends on the motherboard and exactly how things are configured. Therefore the suggestions are winging it. You could also use an external PSU to power a one shot rather than system 5 volts and the small relay method. Really depends on how failsafe and elaborate you want to get.

Ron