Why can my desk fan can turn off my USB mouse?

Discussion in 'General Electronics Chat' started by moeburn, Sep 2, 2013.

  1. moeburn

    Thread Starter Member

    Aug 16, 2013
    So I've got one of these situations going on here:


    I have a 25W desk fan, sitting next to my computer. The desk fan is kinda far away, and I turn it on and off all the time depending on how hot I am, so I plugged it into one of those christmas light switches; its basically a really short extension cord with a really long 125v/15a switch coming out of it.

    Every time I toggle the switch, I hear the "device disconnected" sound play, and my usb mouse and keyboard stop working for a couple seconds. They are plugged into a self powered USB hub, and it only happens when they are plugged into the hub, not the PC itself.

    The hub is self-modded; I turned a non-powered USB hub into a powered-USB hub, by soldering a 5v 1a wall wart to the v+ and ground wires of the hub, and disconnecting the v+ wire from the PC. HOWEVER, when I was soldering the hub, I noticed that the shielding of the USB cable was connected to a small surface mount inductor (i assume inductor, because it was labelled L5), and the inductor was connected to ground. I accidentally tore the cable off, but I thought "pfft, how important could it be to wire the cable shielding to the power supply ground?", so I never bothered to solder it back on.

    So I'm asking, whats the science behind all this? Why is my extension cord switch able to affect other devices at all? Why does soldering the USB shielding to the PSU ground fix it? And why put an inductor in between the shielding and the ground? What is the inductor doing?

    Sure, I could fix the USB hub so the EMI caused by the switch didn't affect it. But how could I stop the switch from sending off so much EMI? Just in case there are other, future devices that are also affected by it, that I can't fix.
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
    A common switch in general causes sparks between it's contacts. I'm sure you could measure them on the 5V power supply and/or the USB data line (with a scope). It either confuses the USB controllers or even resets them, you can't say for sure unless you actually open the PC and measure it. Something you probably won't do because you don't have a schematic of your mainboard.

    If you want to investigate further you'll need to draw a diagram of what you connected where, including the shield etc.