Re: newbie DC question

From: Terry (tsanford_at_nf.sympatico.ca)
Date: 12/28/04 

Date: Tue, 28 Dec 2004 12:57:37 -0330

"Rob Snyder" <rsnyder@reversedthebrokenglass.com> wrote in message
news:qJ3Ad.38117$ld2.15952316@twister.nyc.rr.com...
>
Rob wrote in part:
>
> What *is* this ground thing? Does current flow from the negative output
> to ground? From ground to the positive? Both? Neither? What they heck am
> I missing?
>
ROB: Just to see if I can add to your confusion by using some examples. :-)

My car has a 12 volt battery. The negative terminal is connected to the
chassis of the vehicle.
So within the vehicle that is a 'common' point for all the negative
connections. I suppose in a way it is the vehicle 'ground' although clearly
it is not connected to the ground since the vehicle rolls on rubber tyres!

Many years ago my father had a car in which the positive end of the battery
was connected to the frame/chassis of the car so the positive potential was
'common' in that car!

In telephone exchanges they used very large 48 volt battery installations
charged by large rectifiers. In almost all cases the positive side of the
battery was connected to the ground of the building. This -48 volt power was
used to operate electromechanical telephone equipment. But there were often
other 130 volt power supplies inside the same building that had their
negative end connected to the same ground; this +130 volt power was mainly
used to operate tube type equipment. So that 'ground' was a common point
reference point for both supplies.

Occasionally you will get circuits where neither positive or negative is, or
needs to be grounded, these often referred to as 'floating'.

So the term 'ground' is best thought of as a common reference point, for a
particular piece of equipment or situation.

Just to confuse some more? Most telephone circuits require a pair of wires!
But there were and are very simple telephone and signalling systems which
use one wire as a connection and the ground as the other connection. Often
called "One wire and ground/earth systems".

Email if you'd like to discuss some more. Terry.

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