[LAU] OT interference / grounding problems

James Cameron quozl at us.netrek.org
Sat Feb 21 00:57:25 EST 2009

On Sat, Feb 21, 2009 at 03:49:39AM +0000, pete shorthose wrote:
> It may not be ideal to have the path to ground traveling though
> several devices but in this instance it works. 

Well done.

> nb: Someone measured the voltage coming off the case of a later model
> dell at around 100v. Nice huh. Good job the current is low.

Yes, roughly 100V is what I've measured, and we have 240V domestic power
in Australia.  The insulation ensures the current is low, but it also
acts as a capacitor and so the AC can cross it.  The low voltage side of
the power supply ends up floating at about half the total input voltage,
and the low current means that even measuring it brings it down.

There's another potential cause of a significant voltage on the low
voltage side, and that is a domestic neutral that is not well terminated
to the building ground ... but it depends on your local electrical
safety regulations, not every regulator does this.  If you have that
problem you're in for even more interesting effects.

> Apples have had problems too or so I've read.

I've a MacBook here, hang on ... yes, you're right, between electrical
outlet earth and the USB connector shell is 74V AC, which drops to 20V
AC if I touch both ground and the shell, and drops to zero if I turn off
the adaptor.

James Cameron    mailto:quozl at us.netrek.org     http://quozl.netrek.org/

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