RE: [OT] USB voltages and grounds

["Paul Gale" <groups@xxxxxxxxxxxxxxxx>]

Thanks Keith,

I've caused Windows to hang a number of times when plugging in USB cables
and had the spark :( I'm pretty sure it also damaged one of my mainboards
which I had to RMA back to EBuyer (Actually a bit of luck as I wanted to
upgrade this board - it's now obsolete with them and they had to credit me
back ... 9 months later - bonus !).

Is there nothing else that can be done? Seems pretty poor design?

Some of my kit is powered from sockets on opposite sides of the room (some
through UPS's, filtered power strips etc - quite a mess of 50+ plugs and
transformers!!!). Will this contribute to the problem?

Thanks,

Paul.

> -----Original Message-----
> From: ukha_d@xxxxxxx [mailto:ukha_d@xxxxxxx] On Behalf Of
> Keith Doxey
> Sent: 19 September 2006 18:34
> To: ukha_d@xxxxxxx
> Subject: RE: [ukha_d] [OT] USB voltages and grounds
>
> 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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