Archive for January 28th, 2009
After grounding the obvious metal bits around the desk as shown there and taking some pains to zap the light switch on the wall (rather than the grounded objects) before sitting down and routing the USB cable away from everything else, the mysterious USB disconnects seem to have Gone Away.
The USB hubs were reporting exactly what happened:
hub 3-0:1.0: port 1 disabled by hub (EMI?), re-enabling...
Which wasn’t much help in the beginning, because I couldn’t correlate a static zap with the disconnect. Quite often, it’d be something innocent like plugging a camera into its USB/charging cradle with no obvious discharge.
The onset of 0 °F weather and the ensuing 0% relative humidity, plus my donning a synthetic fleece jacket while venturing into the rather too-chilly basement laboratory, brought the problem to the fore. An inch-long arc to a light switch gets your attention pretty quick!
Hint: when you know you’re charged, pull a pen from your pocket, get a good grip on the metal pocket clip, and use that to draw the spark from the light switch. The larger surface area contacting your fingers reduces the current density to the mild tingle level, rather than leaving a charred pit on the end of your finger.
In round numbers, the dielectric breakdown voltage of air is 1 kV / mm. That inch-long arc required upwards of 20 kV: not bad for an acrylic jacket!
When we go riding in the winter, we dress in layers of acrylic this and synthetic that, to the extent that simply moving generates a nasty charge. Hence the punchline: nobody moves, nobody gets hurt.