- Messages sorted by: [ date ][ thread ][ subject ][ author ]
- Next message: LLOYD GARVER: "deaf correction box--still"
- Previous message: Andrea Fuller: "Re: Yet more re: hum and power supplies"
- In reply to: Rose: "Re: efficient e-mail with Dragon products?"
- Next in thread: Patrick Henebry: "Re: efficient e-mail with Dragon products?"
>
> I wondered about that. I now recall seeing that it has a two-prong power
> cord. Of course, one side of the cord connects to the "neutral" side of the
> power line which is nominally at "ground" potential (zero volts). Some
> electronic devices use that fact and connect that side to their metal case
> portions internally.
*** Um,
The neutral side of the power line is a really REALLY poor ground.
Here's why: AC current flows through it, supplying all the loads that are
plugged in. The neutral wire itself has a finite resistance ( after all,
it's a real wire ). All the current flowing through it causes varying
voltage drops across it's length.
By Ohm's law - current flowing through the wire, times the wire's
resistance, equals the voltage across the wire.
The third wire, on the other hand, has NO current flowing through it,
in normal use. Or at least, it doesn't if all the plugged in three-prong
items are properly designed and working well. So you can be reasonably
sure of finding a reasonably clean AC ground at that third prong. Whereas
at the neutral prong you will typically find a couple volts of AC.
Wall-wart power supplies get away with two prongs for exactly the same
reason that double-insulated tools do: they have LOTS ( double) of
insulation between you and all the metal bits in the supply. Also, they
don't depend on the neutral for any sort of signal ground. They normally
have very little circuitry on the "AC" side, just a transformer primary
winding with isolated low voltage on the secondary side.
- Jerry Kaidor ( jerry@@tr2.com )
- Next message: LLOYD GARVER: "deaf correction box--still"
- Previous message: Andrea Fuller: "Re: Yet more re: hum and power supplies"
- In reply to: Rose: "Re: efficient e-mail with Dragon products?"
- Next in thread: Patrick Henebry: "Re: efficient e-mail with Dragon products?"
 |  |