Thread: Looking for a creative solution: USB bus still powered after CPU is off

I was actually just about to post...why not tap into the PC power connector. But then oh yeah the FDD/HDD power connector will work and the cable are so easy to find too

I think its bed time for me.

Heres a circuit that may work with your problems :


This circuit assume the OPUS will work fine if the +12V and the IGN wire come on at the same time. Otherwise a delay may be needed before the IGN go to +12V.


Heres a run down of how it work. The IGN line from the car put a supply to the OPUS with a +12V, this 12V also triger the OPUS to turn the PC on.

Once the PC is on, the relay powered by the 12V supply of the HDD connector closes the relay switch, supplying the permanent +12V into the OPUS +12V (nothing happen here)


When the IGN is lost (theres still a +12V supply to the OPUS from the relay switch), the OPUS then send a signal to the PC to turn off. The PC then switch off and turning the relay off. The power into the OPUS is now completely cut off.

Hope its clear...as I said I dont have an OPUS and cant test it. Someone who know enough may wanna try...please do so, just dont blame me when something smoke up :P

It may not work as I mentioned earlier as some assumption are made.

Im even assuming thats what the OPUS connection look like...

Those with OPUS may wanna comment

S4Per,

The front USB ports are NOT integrated into the mobo, therfore they have an actual connector that plugs into the mobo. What if a relay of sorts was wired into the particular power line here on the connector, which would be triggered to turn the power off based on the PC being shut down?

Yes you can do this...

On my post #21. Buy a HDD cable splitter, connect the relay coil and a diode on the HDD cable 0V and +12V. The relay switch can now be used to cut off the USB supply between the motherboard and the USB port on the front.

I havent tried it as I have no need but it should work.

Hope that helps

Thanks everyone. You guys rock! I think I'm going to try this last one first, and if it doesn't work, I have a lot of other things to try.

Thanks a ton!

Good luck...

No matter what you do dont forget the diode in parallel with the relay coil. 1N4001, 1N4002 etc should work ok.

It seems that most motherboards power the USB ports off the +5VSB. How about replacing the +5VSB pin on the ATX power connector with +5V ? This will ensure that power to USB ports is cut when the PC is off. I'm not sure if the motherboard will power on without +5VSB but its worth a try.