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Mobo SB_PWR LED stays on when computer unplugged

L

Loosley

Member
Hi,

The green SB_PWR LED on my motherboard stays on even when I shut off the power supply switch and unplug my PC completely from the wall. The light stayed on for over an hour before finally turning off. Is this normal? What kind of phantom power could be causing this?

I even tried shutting off the PC, unplugging it from the wall, and unplugging the 24pin power connector to the motherboard, and the light stayed on.

The computer seems to boot normally otherwise. Does this happen to anyone else? Should I RMA the motherboard?

mobo: Asus P7H57D-V EVO
PSU: Corsair AX850 (850 W)
vid: eVGA GTX460 768 MB
RAM: Corsair 4 GB DDR3
HD: WD Caviar Black 640 GB
 
Loosley said:
But would stored capacitance cause the motherboard LED to stay on for over an hour? Is it safe to unplug/plug components into the mobo while the LED is on (and the PSU is off), or should I wait until it goes out before doing any work on the PC?
Click to expand...

Yes, capacitors can keep an LED lit for any length of time if they're big enough. You can try draining it faster by turning on all the Power on by PCI, PCIE, and PS/2 options in the BIOS. Connect a PS/2 keyboard if you have one as well. Connecting a mouse or a keyboard to the USB ports under the PS/2 port might also help. Make sure you've got the power supply turned off at the back or the light will never go out, as it'll continue to provide standby power to the motherboard.

Failing that you can try disconnecting the power supply from the motherboard. The capacitance keeping the LED lit is likely all in the power supply. However, there's very little risk in replacing components with just standby power supplied to the board. Removing and reconnecting the power supply each time is more likely to damage something.
 
Loosley said:
But would stored capacitance cause the motherboard LED to stay on for over an hour? Is it safe to unplug/plug components into the mobo while the LED is on (and the PSU is off), or should I wait until it goes out before doing any work on the PC?
Click to expand...

You should always turn off or unplug the PS and then press the power button on, and watch the LEDs go out as the power drains out (only takes a second). You could kill your RAM installing it without draining the power from the board.
 
schmunk said:
You should always turn off or unplug the PS and then press the power button on, and watch the LEDs go out as the power drains out (only takes a second). You could kill your RAM installing it without draining the power from the board.
Click to expand...


This is the correct way.
 
schmunk said:
You should always turn off or unplug the PS and then press the power button on, and watch the LEDs go out as the power drains out (only takes a second). You could kill your RAM installing it without draining the power from the board.
Click to expand...

Cool trick. RAM doesn't get standby power though, so it's safe to replace without doing this.
 
Ross Ridge said:
Cool trick. RAM doesn't get standby power though, so it's safe to replace without doing this.
Click to expand...

Thanks for all the replies. I finally figured out what was causing the light. It turns out I had a powered USB hub plugged into one of the USB ports, and that was generating enough power to light the SB_PWR LED. I had no idea a USB hub generated that much voltage.

Hopefully I didn't fry anything by leaving this USB hub plugged in while replacing RAM. I guess a run through MemTest should verify things?
 
Ross Ridge said:
Cool trick. RAM doesn't get standby power though, so it's safe to replace without doing this.
Click to expand...

The DFI NF4 LanParty boards supplied voltage to the RAM when power was off, this was the famous "Cold Boot" issue, the RAM had this bias voltage when off which would prevent it from booting on power up. Oskar lead engineer at easystreet forums for DFI confirmed this.

I actually RMAd one of these boards once, it would only start after PSU was unplugged and then plugged back in.

Other boards could do this also, so why not play it safe?
 
I had this problem with my board (Asus M4A785-M) and it turns out my USB hubs ac adapter was backfeeding power through the USB port and thus made the board SB LED stay lit even though the PSU was unplugged.
 
ComputerWizKid said:
I had this problem with my board (Asus M4A785-M) and it turns out my USB hubs ac adapter was backfeeding power through the USB port and thus made the board SB LED stay lit even though the PSU was unplugged.
Click to expand...

This worries me... The upstream USB port should only be able to sink (draw) current, and not supply it, at least according to the USB specifications. I wonder if it is not a good idea to use the USB hub that I'm using now if it is actually supplying power to the upstream port. It seems to be working fine though...
 
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