> I just bought a second hand Epson USB scanner. > > When plugging the usb cable in I bumped the end of the cable against the > metal plate on the back of my computer and noticed a small spark. > > Tapping the end of the cable against the metal produces more small > sparks when it connects.
The computer case is earthed, so you should be seeing the same sparking when touching the USB cable shield to the water tap, or the mains earth wire. Dito when you charge yourself up walking over the carpet in dry conditions and touch anything else like taps, door handles, ... Of the 4 USB pins, 1 is ground, 1 is 5V, the other 2 are signal lines. You can produce sparking on 5V only if there is a noticable amount of current involved. USB has a limit of 500mA per (computer-)connector, anyway that's supplied by the computer, not the peripheral. The fault is with the scanner - perhaps the scanner's power supply is faulty and allows a chargeup of the whole device. This is the case if there is a capacitive component to the sparking, i.e. after a spark it needs a recovery time to get back up to the biggest spark. Put a voltmeter on the plug casing and shorten it to earth, then see how fast it comes up again. If you can constantly drain a significant amount of current (say > 1mA) don't put it anywhere near your computer and get it fixed somehow, but I can't help with that over the distance. Volker -- Volker Kuhlmann is list0570 with the domain in header http://volker.dnsalias.net/ Please do not CC list postings to me.
