At 08:25 PM 1/26/2003 -0500, you wrote:
>Well, ok, supposing I gnd it to 'earth'.

>First of all, what do you mean
>grounding to earth? I mean if it's not grounding it to the mains ground..
>would an example of 'earth' be grounding it to the psu case? And then
>grounding the circuit board to the case consequently?

Earth is what ends up connected to the water pipe in the ground.

Ground is a bit ambiguous, it could just mean a common connection in the
circuit called GND, one net in your circuit, or it could be connected to
earth. I hope that's clear enough. Your circuit "ground" is not necessarily
connected to earth. On a PC, for example, it is. On many products there's
no ground wire (in the line cord) at all, so obviously it can't be.

And this brings me to my
>next question: What's the point of mains ground then?

Shielding and safety. If your transformer fails you'd like the leakage or
short current to be conducted safely to ground through the ground lead
rather than finding it's way through your left arm and right foot,
thereby permanently "unsubscribing" you.

>What if I am working
>with my parallel port, which is grounded by the computer, which is grounding
>by the mains... and I connect my parport to a circuit powered by my psu, also
>grounded to my psu. They will have different grounds, won't they?

Yes, you can get current flowing through the parallel port wire due to this
"ground loop". Something to be avoided. Under extreme conditions you can
get damaging current. I have not checked to see if printers have the
ground pin connected to earth. The connector shell probably is.

>Somewhere in this whole concept of GND I'm lost!

Hope this helps.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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