[ILUG] OT Possible Earth loop on modem connection
conor.daly at oceanfree.net
Tue Sep 3 21:39:17 IST 2002
On Tue, Sep 03, 2002 at 05:10:43PM +0100 or so it is rumoured hereabouts,
Anthony Staines thought:
> My favourite laptop = a Compaq EVO (runnning Red Hat 7.1 of course) =
> has a problem. When I connect it to the telephone line (through the
> built-in modem) and the mains (through the power supply) at the same
> time, there is an unmerciful hum (a 50Hz hum) on the modem speakers, and
> the line drops.
> I have two power supplies for this beast, and the problem occurs with
> both of them. Therefore I assume that this is an earth loop.
> Any suggestions on ways of killing this with no access to electrical
> testing equipment (but without killing me, if you follow)?
Isolating transformer? Though, mains power supplies usually have a trafo
within and are normally isolated already.
if you have a battery and a torch bulb handy, you could series them with
the earth pin on the power supply and the dc end (unplugged of course!)
and see if the bulb lights. Before that, you could open the plug and see
if there is an earth wire at all. Many of these are "double insulated"
and so don't require an earth (the label should have "concentric" squares
symbol to indicate double insulated ______ )
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Conor Daly <conor.daly at oceanfree.net>
Domestic Sysadmin :-)
7:59pm up 4 days, 27 min, 0 users, load average: 0.00, 0.00, 0.00
8:01pm up 4 days, 10 min, 1 user, load average: 0.10, 0.11, 0.08
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