On Wed, 01 May 2013 22:38:58 +0100, Ian Field wrote:



"Major Scott" wrote in message
news
On Wed, 01 May 2013 15:59:33 +0100, David Moorman
wrote:

In article , "Major Scott"
wrote:

I have a USB hard disk enclosure, and I'm running it without it's case
as I'm
testing another drive with it. When I touched the hard disk with my
hand and
the case of the computer (which is earthed) with my arm, I felt a
tickle. So
I connected a multimeter to it, and the hard disk is at 90 volts AC with
a
short circuit current of 0.136 mA. Is this normal? Is this a good idea
when
working with delicate computer electronics? I suppose the ground of the
USB
lead will get rid of it but it seems a bit odd. The USB disk unit is
powered
by a cheap wall wart that came with it, rated 12V DC 1.5A.

Something is effed up.

It works though.

I've had a similar voltage out of the composite output of a VCR. That's
the trouble with these double insulated things that aren't earthed.

Any double insulated SMPSU has filter caps from each mains pole to the
equipment 'ground' - they form a capacitive voltage divider (Xc + Xc).

That sounds rather strange to me. Especially where delicate electronics are involved. Imagine I connected the USB hard disk, while it was powered up, to the computer, and the USB plug's grounded chassis managed to touch one of the signal contacts.

--
H lp! S m b d st l ll th v w ls fr m m k yb rd!