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From: jgk@osc.COM (Joe Keane)
Newsgroups: sci.electronics
Subject: Re: polarity
Summary: No DC.
Keywords: common mode
Message-ID: <4761@osc.COM>
Date: 30 Apr 91 18:23:35 GMT
References: <1991Apr23.152315.22825@news.larc.nasa.gov> <1991Apr24.111307.2296@dcs.simpact.com> <2226@gold.gvg.tek.com> <1991Apr29.132059.9403@news.larc.nasa.gov>
Reply-To: jgk@osc.COM (Joe Keane)
Distribution: usa
Organization: Versant Object Technology, Menlo Park, CA
Lines: 34

In article <1991Apr23.152315.22825@news.larc.nasa.gov>
kludge@grissom.larc.nasa.gov ( Scott Dorsey) writes:
>   If this were the case, then you'd be able to get 60 volts (rms) between
>each side of the plug and ground.  But this isn't the case.  Try it with a 
>meter.  You get 120V on one side, and zero on the other (hopefully) if you
>measure from the two prongs to the building ground.

This experimental evidence is correct.

>   You can think of it as having a DC component of 84V and an AC component 
>of 84V.

But this explanation is wrong.

In article <1991Apr29.132059.9403@news.larc.nasa.gov>
kludge@grissom.larc.nasa.gov ( Scott Dorsey) writes:
>Hmm... The point that I was making was that the signal was offset.  The zero
>voltage point was at the bottom of the wave instead of the center.  I think
>that imagining this as being a DC offset is a good way to look at it.

Try another experiment.  Put a scope on the hot line, assuming it's rated for
the voltage.  Even with DC coupling, the zero voltage point will still be in
the center.

I think what you want to say is that there is a _common mode_, not DC, voltage
component.  That is, the voltages on the lines are not balanced.  That's why
you get a lot of voltage on one side and none on the other.

I think balanced voltages makes more sense, that's generally what you get here
with a 220V outlet.  The advantage is that you don't lose power in the neutral
line, which doesn't serve any useful purpose.
--
Joe Keane, amateur mathematician
jgk@osc.com (...!uunet!stratus!osc!jgk)