On 4/28/15 4:14 PM, Scott Lurndal wrote:
Tony Hwang writes:
J Burns wrote:

I believe dust creates heat build up causing possible over heating.
Excess heat makes things expand. No. 1 enemy for electronics comp[onet
is too much heat.

I don't suppose there's much mass on a USB board. I figured if it was
heat, it would have resumed working when he shut the power off long
enough to unplug and replace the board a few times.

USB chips don't dissipate significant heat, even under load. Not
like a CPU, which can dissipate upwards of 160 watts or more depending
on the clockspeed and SKU.


Stray capacitance wasn't evident with a VOM. It was with a VTVM, an
oscilloscope, or a DMM. The lead of a DMM may be nowhere near a
conductor with 60Hz, but it can pick up several volts. The unshielded
part of a scope lead is much shorter, but it, too, will pick up 60Hz
from stray capacitance.

Impedances in modern circuits can be much higher than the input of a DMM
or a scope. Distances can be very close and frequencies a lot higher
than 60Hz. In putting together my own circuits, I've had to add
capacitors because of the unforeseen effects of stray capacitance.

If I got moisture in a digital watch, I wouldn't expect it to work. The
conductivity of distilled water is 5 microsiemens/meter. Seawater is a
million times higher. If distilled water stopped a watch, I'd suspect
capacitive coupling. The dielectric constant of water is 80 times
higher than air.

I don't know the dielectric constant of household dust, but I imagine it
could be a lot higher than air if it absorbed humidity. Maybe he OP had
trouble on that day because the humidity in the room was a bit higher
than it had been.