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jason.allen.750331 3 Newbie Poster

I saw lots of old threads on this problem but could not find a solution so I thought I would share my experience and solution.

Hi,
I just had a problem that my cpu and case fans would not switch off after shutdown.

It turned out to be a dvi-d connector for my graphics card from a vga monitor. It had an option for a usb power boost I.e I could plug the micro usb end into the adaptor and the other into a normal usb to provide power to initiate the diplay.

It seems I did not need to use this extra power usb cord with the 'active' dvi-d adaptor and it was causing a small wattage power loop to go back into the motherboard which was just enough to activate the cpu and case fans.

I assume this could happen with other devices which plug into the usb and use an external power scource like old external hard drives. I already replaced my psu twice because of this problem needlessly. In my case the monitor was giving adequate power to the vga to dvi-d adaptor and the aditional usb power cable was overkill!

Unplug everything except mouse, keyboard and har drive when trying to diagnose this problem.

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hithirdwavedust commented: Right on! It's always nice when people share their tech triumphs! +0
thepastimer commented: I didn't even know that was possible. Thanks for the post. +3

Dani AI

Generated

As reported, an externally powered display/USB accessory can keep a system partially live after an OS shutdown. The underlying cause is a small reverse current (backfeed) into the motherboard's standby circuitry. Motherboards keep a standby rail active so wake-on events work; if an attached device sources a few hundred milliamps into that circuitry it can be enough to power fan headers or logic and make fans keep spinning even though the OS has shut down.

A compact technical note: the standby rail (commonly called 5VSB) remains present in S5. Backfeed from a powered adapter, monitor USB hub, or externally powered USB device can drive parts of the board from the outside, so fan headers that are normally driven when the board is active can still see enough voltage to move the fans. That is why swapping the PSU often does not fix the symptom.

Recommended diagnostic workflow:

  • Shutdown the system and observe whether fans continue to spin.
  • Turn the PSU mains switch off (or unplug mains) to confirm whether the fans stop; if they do, the issue is almost certainly external backfeeding.
  • Systematically disconnect powered peripherals (powered USB hubs, active display adapters, external drives with separate supplies) and repeat shutdown tests.
  • In BIOS/UEFI, enable ErP/EuP (kills most standby power in S5) and disable wake-on-USB, wake-on-LAN, and similar "power on" options.
  • If comfortable with measurement, check the standby voltage on the ATX connector with a multimeter to confirm present voltage after shutdown (observe safety precautions).

Practical fixes and cautions: replace the powered adapter with a passive one or a model that does not require USB VBUS; use a data-only USB lead (VBUS removed) or power the accessory from a source that cannot backfeed the motherboard; avoid cutting or modifying cables unless confident. Enabling ErP and disabling wake settings is the least invasive fix. This is a common, non-fatal issue — diagnosing for backfeed first can prevent unnecessary PSU replacements.

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