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# Using an Oscilloscope to Understand 120 VAC Split-phase Household Power Supplies

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Can you believe there are techs out there who use an o-scope to look at the output from a sound mixer and conclude this is how Line voltage works? The output of a sound mixer is the output of a summing amplifier (Google it). It is an electronically modified signal. To look at this output and assume that Line voltage behaves this way is the height of stupidity. I call it "Idiot with an O-scope syndrome."

Household power supplies in North America use what's called a split-phase system. The transformer on the pole outside the house takes grid power and steps it down to 240 VAC from end to end on the secondary winding. The secondary winding has a center-tap in it which splits this 240 VAC into two 120 VAC voltages from either end to the center tap. This center tap is defined as Neutral and it is tied to Ground in the circuit breaker box inside the home. The two 120 VAC voltages are 180 degrees out of phase with each other and it is this very antiphase relationship that creates the voltage difference of 240 vac between L1 and L2.

There's a lot of disinformation and tech myths out there about 120/240 split-phase household power supplies. You may have even seen videos online claiming that the split phases are in-phase with each other. This is complete hogwash and I prove it to you in this video.

I show the proper phase relationship (180 degrees) between Line 1 to Neutral and Line 2 to Neutral right at the circuit breaker box using an oscilloscope.

I challenge anyone to show differently and to clearly show how you're measuring.

Nice video!   Very informative.

Watching this being filmed I had to keep reminding myself "He knows what he's doing!" as he kept poking his fingers all around that box and I kept thinking of all those videos on YouTube you see where things go horribly wrong.

240vac is pretty powerful, but fortunately, so is knowledge.

2 hours ago, DurhamAppliance said:

Nice video!   Very informative.

Domo, Brother Durham!

22 minutes ago, Mrs. Samurai said:

Watching this being filmed I had to keep reminding myself "He knows what he's doing!" as he kept poking his fingers all around that box and I kept thinking of all those videos on YouTube you see where things go horribly wrong.

Yep, you ain't getting rid of me that easily, baby!

Didn't you say you can do this stuff  blindfolded?   Now that would certainly be a video full of drama and excitement.

That's a damn good idea for a video! That'll be my next one (and probably my last) which I'll dedicate to you. Farewell, sweet world!

Hello, Samurai, my damn good friend. I was hoping I could impose on you to offer a video showing the phase difference between the camel dung and the lime juice that I use to generate the electricity needed to power my Ameedican Army-surplus Apple Macintosh? I am having difficulty with the voltage regulation from the battery and suspect the problem could be phase related. Since you are obviously a master in such matters, I would be most grateful if you would be so kind as to advise me on this matter.

Allahu Akbar!

Moostafa