Take the 2-minute tour ×
Super User is a question and answer site for computer enthusiasts and power users. It's 100% free, no registration required.

I have a desktop PC which was recently moved. Since moving I've noticed that even while the PC is turned off (but wall socket turned on) the NumLock LED on the keyboard is on.

It may have been doing this before the move, but I've only noticed it now.

Maybe it matters that the keyboard is connected via a shared mouse/keyboard PS/2 port and I'm running Windows 7.

I've not checked whether the LEDs are just keeping whatever state they were in when I turned the PC off but will do this if that information could be relevant.

Why does the led stay on when the PC is turned off and how can I prevent this?

share|improve this question
    
Have you removed all the other cables connected to the PC? Sometimes the power comes in from an external device such as a self-powered USB-Hub. –  Robert Sep 17 '12 at 11:04
    
Nothing connected to the PC has its own power supply apart from the monitor. The socket on the wall and on the PSU are both on though. –  George Duckett Sep 17 '12 at 11:07

3 Answers 3

up vote 14 down vote accepted

The behavior is of the keyboard LEDs being ON is fine, and it stays on because:

  • After a normal / successful shutdown, the motherboard LED is on, and that means the motherboard is still receiving power.

  • As long as the motherboard receives power, it will give +5V AUX supply. The +5V AUX voltage is what is used to power up the system.

  • It can also power the USB ports when the PC is off if your motherboard supports that function.

  • It also maintains the state of the LEDs on the keyboard as far as my knowledge goes, for example if your NumLock was ON when your PC was ON, then it will remain ON

The only way you can prevent it is to shut off the power from where it is plugged in, or just pull out the power cord connecting your SMPS / PSU from the back of your CPU.

For more reference and details read this on SevenForums. He does mention that even his NumLock remains ON along with his motherboard LED.

share|improve this answer

Certain desktop and laptop motherboard manufacturers came up with an idea to supply power to USB devices even if the computer is shutdown. As long as the power supply is plugged in and receiving electricity, devices attached to the computer will receive power.

This is a motherboard feature, and not dependent on the operating system.

To control when this is enabled (if at all), check the documentation for your motherboard or your desktop manufacturer. It will be a BIOS setting that you can change.

share|improve this answer
    
I have noticed this behavior with external keyboards plugged into a laptop but never in a desktop system, so I agree with this answer. You can simply unplug and re-plug the keyboard to turn it off. –  Synetech Sep 17 '12 at 15:32

It could be that the computer is waiting for a 'wake on HID (Human Interface Device, e.g. keyboard)'. This is especially likely if you are hibernating instead of actually turning it off. There may also be a BIOS setting controlling this.

You could try hitting a key and seeing if the computer wakes. It may not necessarily do this, and if you don't need wake on keyboard you may as well disable it anyway.


You could try disabling wake on keyboard in the Windows 7 device settings:

  1. Open the Device Manager

  2. Find your keyboard, right click, Properties

  3. Go to the Power Management tab

  4. Uncheck Allow this device to wake the computer


You could also take a look at your BIOS settings for something related. I can't give more precise instructions on that.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.