Re: Oscilliscope and Ground
From: mike (spamme0_at_netscape.net)
Date: 03/11/05
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Date: Fri, 11 Mar 2005 15:02:16 -0800
Kitchen Man wrote:
> On Fri, 11 Mar 2005 01:35:01 -0800 in sci.electronics.basics, mike
> <spamme0@netscape.net> wrote msg <42316645.6060401@netscape.net>:
>
>
>>>Excellent experiment. Even without a ground prong on your oscope power
>>>cord (how old is that thing?) the scope will sense your building's earth
>>>ground through the neutral lead. Your transformer primary sees the same
>>>ground through its neutral. Thus, they are common grounded, and you can
>>>measure a good ac signal. The DC side is isolated from that common
>>>ground via the transformer, so you need that second reference to make a
>>>measurement. You saw an AC signal on the output with the ungrounded
>>>probe, because that's the voltage the scope could see with reference to
>>>the building earth ground. Hope that makes sense.
>>>
>>
>>Ground is a DEFINITION, not a PLACE. People often (mis)use (IMHO) the
>>term ground to mean earth. But that only makes sense if you're sitting
>>on the dirt.
>
>
> It appears we are in violent agreement. Terms, nevertheless, persist,
> and a building's earth ground might just be called that because it is
> sitting in the dirt. Something to consider.
>
>
>>Where there's distance, there's impedance. Your measurement problem
>>gets much more difficult as the frequency increases. And there's also
>>mutual coupling of all kinds...but I digress.
>
>
> Don't you think you're adding a bit more complexity to the issue than it
> deserves? The man's question was pretty simple. If you recall,
Communication is always difficult in a half-duplex medium like the
internet. My experience has shown that people often ask simple
questions about complex issues because they don't know enough to ask the
question that gets them the answer they really need. Then they get
many fragments of information that range from correct to wrong to
downright dangerous.
I tend to overkill the answer so that there's a consistent message in
one place.
Since I'm always right ;-) this is best for all ;-) ;-)
Scope probing is a very complex issue and deserves treatment in a basics
newsgroup.
mike
>
> "I figured I would need to use the ground clip when measuring my DC
> voltage output, but why didn't I need it for measuring the AC voltage?"
>
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