Ethan said:
Hi,
I got a new pair of headphones for my iPod and touched a metal part of
my home-built PC. I got a small electric shock, then the video card
wasn't working. I tried every monitor I had, but nothing worked. I
then shut down the computer, cleared the CMOS, and it booted up fine.
Any ideas what I should have done differently?
I'm not sure exactly from your description, how the
headphones, iPod, and computer are related. Was the
iPod plugged into the PC, at the time you were
connecting headphones ?
Modern computers come with a three pronged plug (at
least in North America they do). One of the prongs
is safety ground. The safety ground is connected
to the PSU casing and as a result, to the chassis
of the computer as well. If your house has three prong
outlets, there should be a ground path to the chassis
of the computer.
Some older houses, have two pronged wiring, and the
safety ground is missing. Modern computers have
EMI (noise) filtering near the AC input
to the PC. Some components shunt any electrical
noise, into the safety ground. Unfortunately, at
the same time, they also shunt a small amount of
120VAC current into safety ground as well.
(In this picture, leakage is via C2 and C3.)
http://www.pavouk.org/hw/en_atxps.html
(Good quality filter block, with leakage specified. These
can be added in front of a noisy electrical load, to prevent
electrical noise from traveling back into the wall. The
10VN1 has two pairs of capacitors that leak to shield ground.)
http://web.archive.org/web/20031114125934/http://corcom.com/pdf/rfi/N+Series.PDF
If the computer is connected to a three pronged outlet,
the small amount of current harmlessly goes into the
safety ground. The computer is safe and "shock free",
and you can touch the chassis all you want.
If the third prong is not connected, from the
computer to the safety ground in the wall, then
the computer chassis becomes electrified. A small
amount of AC current can flow into someone who
touches the chassis, at the same time they touch
another grounded surface.
Similar issues can arise, if there is a fault in
the safety ground. In my old lab at work, one day
I got a shock when touching the chassis of two computers
that were plugged into different outlets. When measured
with a multimeter, there was 55VAC difference between
the two computer cases. That lab is the only incident
I've seen like that. (The safety was cut on one side
of the room.)
So, I don't know what you were doing, but it may be
that the iPod and headphones had nothing to do with it.
It could be that the computer has always had a safety
ground problem.
Paul
