USB help needed please

Hi,

I'm having a problem, my MADCATZ STRIKE 7 keyboard (built in 2 port usb hub) this has a mains transformer power the system, (power joins the pc-usb wire by Y junction)

The problem I have is when pc is shutdown or sleeping, keyboard feeds power back into motherboard, my case fans all spin & I'm concerned it'll fry my Asus Z9PE-D16 motherboard.

Is there anything that can be put inline to stop this back-feeding power on shutdown?

That sounds like bad design.

You could try simply disconnecting the +5V feed wire between the keyboard and PC. It means devices in the hub sockets will be invisible to the PC if the keyboard power unit is turned off but it should stop the back feed. Another solution is to add a Schottky diode (1N5817, 1N5818, 1N5819 or similar) in series with the 5V line, wired with the cathode (banded end) towards the keyboard. That will allow the PC to power it or the external transformer or both but if the transformer is turned off, the voltage at the keyboard and USB sockets will drop slightly (about 0.4V). In most cases that shouldn't cause any problems.

Brian.

Hi Brian,

Thanks sounds like a plan, is this something I'd have to build, there is no ready made solution that can be plugged into the usb port of pc then the keyboard wire plugs into it?

Jamie

I'm not aware of any ready made solution but it's only bad design that allows it to happen anyway. A PC USB port should not allow power to flow back in to it.

If you use the diode trick, it is very simple to install, just open the keyboard, find the wire carrying 5V at the end of the cable, cut it and fit the diode in-line across the break. There are only 4 wires in a USB cable and one will be connected directly to the PC case. If you have a testmeter, measure the resistance from each wire to the case and the one with the lowest reading (should be less than about 3 Ohms) is the 0V wire. Then measure the voltage from the 0V wire to the other 3 wires, the one with 5V on it is the one to cut. The diode costs pennies but you must use a Schottky type, a normal silicon diode will drop more voltage, typically about 0.6V which might leave the remaining voltage a bit too low for some USB devices to work.

Brian.

Hi,

usually any USB device should not push back power to the host.
Thus
* either the device is not according USB specifications
* or the device is faulty

Klaus