Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!swrinde!ucsd!nosc!crash!simpact!cmkrnl!jeh
From: jeh@cmkrnl.uucp
Newsgroups: sci.electronics
Subject: Re: polarity
Message-ID: <1991Apr30.100031.9@cmkrnl.uucp>
Date: 30 Apr 91 17:00:30 GMT
References: <11864@jarthur.Claremont.EDU> <1991Apr23.152315.22825@news.larc.nasa.gov>  <1991Apr29.132059.9403@news.larc.nasa.gov>
Distribution: usa
Organization: Kernel Mode Consulting, San Diego CA
Lines: 52

In article <1991Apr29.132059.9403@news.larc.nasa.gov>, kludge@grissom.larc.nasa.gov ( Scott Dorsey) writes:
> In article <2226@gold.gvg.tek.com> grege@gold.gvg.tek.com (Greg Ebert) writes:
>>In article <1991Apr24.111307.2296@dcs.simpact.com> jeh@dcs.simpact.com writes:
>>>you lost me here.  The hot side goes from +84 V to -84 V and back, referenced
>>>to neutral, for a peak-to-peak swing of 168.  There is no "DC component"
>>>anywhere.
>>Almost. 120V RMS sinusoids are actually 340 v p-p, or 170 v peak. You are
>>absolutely correct about there NOT being a DC component. If there *was*
>>a DC component in your AC service, it would quickly saturate every
>>inductive load. Hmmmm, sounds like a nasty prank ;-) .
> 
> 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 was afraid that was what you were trying to say.  

Tthe 120 VAC power waveform is NOT offset.  (Where on earth did you get
the idea that it was???)  The zero voltage point (where no current flows,
assuming purely resistive loads) is at the center of the wave, not the bottom. 
To think of it another way, the hot side of a 120V outlet both "pushes" and
"pulls" wrt to the neutral side.  

And the wave looks the same, discounting IR losses in the neutral side, whether
measured from hot to neutral or from hot to ground (it had damned well better,
since ground and neutral are bonded together at the building service entry
point). 

About one minute's experimentation with a DC-coupled scope will demonstrate
these facts.  You will see that the hot side goes both positive and negative
with respect to neutral or ground.  

(Make DAMNED sure to use a 10:1 probe and that the outlet in question is wired
correctly before doing this.  And do not try to test the "inverse waveform"
with the scope neutral hooked to the hot side of the power line and the probe
to power line neutral or ground!  "Thank you for observing all safety
precautions."  On second thought, I hesitate to recommend this experiment to
anyone who so obviously misunderstands the basics of AC power.  If you get
yourself killed, don't come complaining to me about it.) 

The other 120 VAC leg in your house is just the inverse of the first so you
can get 240V between them. 

As I said before, the two "legs" are just the ends of a 240V secondary winding
on a local distribution transformer, and your house neutral lead is this
winding's center tap.  With this arrangement there is NO WAY the zero voltage
point on either leg could be anywhere but the center of the waveform.  There is
simply nothing in the system that could possibly produce an offset such as you
describe -- no matter what you call it.  

	--- Jamie Hanrahan (x1116), Simpact Associates, San Diego CA
Internet:  jeh@dcs.simpact.com, or if that fails, jeh@crash.cts.com
Uucp:  ...{crash,scubed,decwrl}!simpact!jeh