On 02/05/13 05:46, Major Scott wrote:
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.

Could be time to try another PSU ??