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

**shchua** · 04-09-2004, 08:28 AM

Originally Posted by Laidback

What's the diode for?

It's to clamp the back EMF from the relay. The back EMF is caused by a rapid collapse of the magnetic field in the relay coil when power is removed and can easily reach 60 to 100V. This can damage the component driving the relay coil, so a diode across the relay coil is used to clamp this voltage and dissipate the stored energy safely. The polarity is important - the diode must be reverse-biased when the relay coil is energized.

**dominic** · 04-09-2004, 10:54 AM

I get this same problem with my EPIA M 1000 board, i.e. the gps mouse LED stays lit after the board has shut down.

Has anyone found a way round this on these mobo's or will i have to use this circuit too?

thanks

**Laidback** · 04-09-2004, 02:38 PM

Originally Posted by shchua

It's to clamp the back EMF from the relay. The back EMF is caused by a rapid collapse of the magnetic field in the relay coil when power is removed and can easily reach 60 to 100V. This can damage the component driving the relay coil, so a diode across the relay coil is used to clamp this voltage and dissipate the stored energy safely. The polarity is important - the diode must be reverse-biased when the relay coil is energized.

Ah..Ok, so in English that means what?

That you need to stick a diode between the two connections that activate the relay coil? 12v at one connector and ground at the other, which way does the diode go? And what type of diode?, if that isn't a stoopid question!

**CdRsKuLL** · 04-09-2004, 02:54 PM

To break the power on your usb, I would opt for a relay. I think the best way would be to power the relay from a harddriver splitter cable, you could either get a 5v or 12v for this. then you would need to break into your usb cable and cut either the black or red wire, and put the relay inbetween. Another but i'm less sure about this one, is if you have a ps2 port on the back, you could poss take the 5v from this and just wire the red/black to this port for the usb cable, this way you dont have any relays, and you dont need to open you pc. You may even have a ps2-->usb connector, they come with mice. Then all you need to do is take the red/black from your usb cable, and wire it upto your ps2 port.

**Zip-Lock** · 04-09-2004, 05:51 PM

Originally Posted by dominic

I get this same problem with my EPIA M 1000 board, i.e. the gps mouse LED stays lit after the board has shut down.

Has anyone found a way round this on these mobo's or will i have to use this circuit too?

thanks

If you’re using an ITX board then is it safe to assume that you're using the 90 watt OPUS? If so there's a jumper that you can take off which will disable the standby power. If you're using the 150 watt model it's an older design without this feature.

For the record it should be made clear that the 90 Watt OPUS offers a user selectable (jumper) standby power status. Factory default is that of the ATX spec which calls for standby power to be constantly on. Considering the automotive environment and user requests the newer 90 watt OPUS offers the option to disable standby power.

The older 150 watt OPUS doesn’t offer an option and standby power is constantly on (per the ATX spec).

**Ricky327** · 04-09-2004, 07:38 PM

CdRsKuLL,

then you would need to break into your usb cable and cut either the black or red wire, and put the relay inbetween.

It got to be the +5V that need to be broken. Cutting the black wire may work but the USB device is most likely grounded somewhere else too. What ever colour the +5V is (most likely red) thats the one ill break.

Another but i'm less sure about this one, is if you have a ps2 port on the back, you could poss take the 5v from this and just wire the red/black to this port for the usb cable, this way you dont have any relays, and you dont need to open you pc. You may even have a ps2-->usb connector, they come with mice. Then all you need to do is take the red/black from your usb cable, and wire it upto your ps2 port.

The problem with this is if the USB devices overload the +5V of the PS2 port, you will blow it. The USB port do have some sort of overload sense and protections...so using the original +5V from the USB is much safer to use.

The idea of using PS2-USB converter is infact very good as long as you only use one device and dont overload the PS2 +5V. In this case you are only cutting the supply of one USB device, the rest still gonna drain as normal. I have a cable that do this, it steal the power from PS2 in a form of piggy back PS2 plug/socket joint with a normal USB cable...I got one of this cable on my external USB 2.5" HDD drive.

S4Per idea was to cut the power of the USB ports from the inside of the PC and not cut the power from outside of each device.

I dont know what his exact requirements are but either way it can work.