RE: [OT] USB voltages and grounds

["Keith Doxey" <ukha@xxxxxxxxxxx>]

Hi Paul,

Lots of kit with switch mode power supplies has a small leakage current
which will result in the entire circuit having a voltage of around half
mains potential on it. This is at EXTREMELY LOW CURRENT but can give you a
slight tingle, particularly if you touch it with the back of your arm etc.

Once equipment is connected together any grounded item will drag everything
down to ground. The problem comes when you "hot plug" equipment
as without
the grounds being commoned the input to the equipment sees the higher
voltage when it is only expecting 5V or so. I believe this is how some
people have damaged their plasma screens with SKY HD.

Unfortunately there is no way of determining which bits of kit are floating
at 90+ volts, it could be the one you want to connect, or it could be the
rest of the system if there isnt a grounded item already connected. The
only
safe way of connecting them is to power everything down first or
deliberately make a connection between the grounds of the two items prior
to
plugging in.

USB connectors *should* make contact on the screens before any of the
actual
connector pins make contact so other than the disconcerting sparking there
shouldnt be any damage.

Hope that helps

Keith

> -----Original Message-----
> From: Paul Gale [mailto:groups@xxxxxxx]
> Sent: 19 September 2006 16:51
> To: ukha_d@xxxxxxx
> Subject: [ukha_d] [OT] USB voltages and grounds
>
>
> Following on from my previous post re USB cables sparking when
> plugging in - I?ve just checked a couple of USB cables for
> different kit by putting a voltmeter between the PC case and the
> USB cable shield. I get around 90V and this is similar for
> several bits of kit.
>
> What would you expect to see?
>
> Paul.
>
>
> [Non-text portions of this message have been removed]
>
>
>
>
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