[SOLVED] Turn on after AC Power Loss - what that option means in reality?

[litwicki23]

Hello. My mobo Aorus Z490 Pro Ax. I have option
Turn on after AC Power Loss in UEFI.


So if i have brownout for a split of second , pc should shutdown and then automatically reboot itself right?
Its like pulling off/on fast power cord,then pc shuts off and reboot again itself. Thanks for explanation. Any differences?

 

[drea.drechsler]

I think it's an option useful for kiosk PC's. When enabled if it loses AC power it will turn back on when power comes back on. When disabled it just leaves the pc off, which is what I prefer for my desktop.

 

[litwicki23]

Thanks but as I said in my post, I have already read the motherboard manual and changed the setting for "AC BACK", the thing is that it is not working when there is a real power failure. for some reason, it works when I pull the plug from the PS or from the wall but not when I turn off the power in my house.

So if i have brownout for a split of second , pc should shutdown and then automatically reboot itself right?
Its like pulling off/on fast power cord ?

 

[BFG-9000]

It's useful for any PC that is supposed to stay on, like a server or one being used as a router or NAS. Just put the applications that are supposed to launch into the startup folder.

I think how well it works depends on your PSU. I have PSUs that don't light up any motherboard LEDs when I plug them in until I press the power button, so they wouldn't work with such a feature (likely they don't energize the +5vsb until it's been turned on once, then it stays on when you select shut down in software). Works great with any PSU that lights up those standby LEDs immediately when you plug it in

 

[litwicki23]

So brief brownout will affect pc the same like pulling the plug from the PS or from the wall ?

 

[BFG-9000]

Well, a brownout to below the PSU's input specifications (typically something like 90-135/180-265vAC ) may leave the PSU running but unstable. So your PC may instead crash and stay on, but crashed--staying on means that BIOS option never comes into play. So a very brief outage may be more problematic than a long one.

The ATX specification calls for at least 100-127 /200-240vAC range and 17ms hold-up time at full load. Operating out of spec is just untested, so it may still work if you are running a light enough load, or it may not. There's no real incentive for anyone to improve on these kinds of specs (and in fact many name-brand PSUs fail the minimum hold-up time test!) when everyone who needs the uptime will just buy a UPS anyway.

However a UPS is more for people who don't want to lose work, as in mining. For something like a router or NAS where there really isn't any unsaved work, such a BIOS option is an attractive alternative. Unfortunately this use case probably isn't a design consideration at all, considering those PSUs I mentioned that simply could not possibly work with it

 

[drea.drechsler]

So brief brownout will affect pc the same like pulling the plug from the PS or from the wall ?

Yes but just what happens depends on the PSU's hold-up time. They should designed to hold power for a brief period when power momentarily drops, a property called hold-up time.

It is really quite brief, I don't know if you can get the plug back in before it expires but maybe. I've had power completely drop out during a thunderstorm bad enough to briefly lose lights in the room then come back on and yet the computer stayed operating. A brief brownout would probably have the same effect.

 

[litwicki23]

My pc:

Seasonic PRIME TX-850 80Plus Titanium 850W

Gigabyte Rtx 3090 Gaming ( 2x8 pin , 2 separate cables )

Aorus Z490 Pro Gaming

Hi. I have an question.

Week ago i launched Metro Exodus and after cinematic advertisement part pc just shutdown. I pressed only power button, rebooted again and its fine again. Happened once and i cant reproduce.

Happened after launching game on intro advertisements part.

Is this possible that i had voltage power fluctuation in house and thats why it shutdown?
But during launching Metro Exodus,coincedence?
Also laptop and router was not affected, router and laptop doesnt turn off. Only pc.

Power after AC loss have ON in UEFI. So if it was just losing power for a split second, it should be rebooting?
So why he stay off.?