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• Oct 2, 2006
> Leonard:
>
> The "grounding" issue leaves me totally confused. If some of the
> equiptment operates off house current, some of it operates off 12 v
> battery, etc, what is "ground"? the house current is returned
> eventually to earth ground; the battery current is returned to the
> neg. term. on the battery -- and therefore floats wih respect to earth
> ground... and I may eventually buy a little generator to power
> everything,
> and its ground _must_ be returned to earth ground.... aarrgh! How do
> you untangle all these things? oh, and what is a "ground loop"?
> <snipped>
'Ground', has two distinct meanings. Generically, the 'ground' in a
circuit, is the point which voltages are referenced to. This is often
connected to 'ground' (the local 'mains' safety/reference).
So you can have a 'ground point', in a completely isolated circuit, and
this still makes sense (I tend to tell people to seperate the concept of
'ground', from 'earth', and also to use a distinctive term to make the
segregation obvious - so an isolated part of a circuit, might have it's
'ground', called 'ISGND' - isolated signal ground).
In your case, the 12v battery 0v terminal, is the 'system ground' for this
part of the unit.
The big problem, is when there are multiple different 'ground paths'. If
(for instance), you have a signal cable betwen two parts of a system, and
both the 'parts', have their own 'grounds', and then these are linked by
the cable, what happens?. The answer depends on the resistances involved in
the cables, the resistance between the 'grounds', and the actual current
involved. If (as is commonly the case...), the 'grounds' have slightly
different voltages present ('earth', is not a perfect conductor...), the
current, finds the lowest resistance path through the cable between the
units, and current will be flowing through the cable as a result of having
the two distinct 'ground' points. This voltage can result in unwanted
signals being generated. This is why if you look at the circuits for
systems that are likely to have problems from this (audio/video systems in
particular), you will see local ground reference areas (usually 'planes' on
the PCB for example), with just one ground path back to the main reference
point, usually using a 'star' shaped ground layout (you want to avoid
'tree' type ground layouts, since the current flows then encourage voltage
differences to exist along the tree branches...). You will also see in the
same systems, the cable 'shield', commonly connected at only one end of
each connection.

Best Wishes
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