Power line voltage out of phase with ambient 60hz elec - Page 1
Forum section: electronics / beginners

author: hilltop
author_rank: Contributor
timestamp: December 08, 2013, 06:58:48 pm
content: I just bought my first oscilloscope and have been tinkering with the measurement of different signals just to learn the ropes and familiarize myself with the scope.One of the first things I noticed was the ambient 60hz "noise" that is picked up when a probe is connected to the scope. (I am in USA, so it's 60HZ 120V mains power here) It is obviously originating from the electrical lines in the building, and I can verify this by holding the probe in my hand and placing my other hand nearer or farther from an electrically powered object or cable - the amplitude of the noise increases or decrases respectively.Then I probed the mains voltage to see how clean the incoming power is. Next, I viewed the aforementioned 'noise' signal on one channel and the mains power on the other channel.What I saw was surprising and I don't understand the reason: The mains voltage shows a clean 60hz sine wave. The 'noise' signal is a jagged sine wave, still obviously 60hz but with a lot of static. However, the two signals are out of phase, by what appears to be approximately 90 degrees if my measurement is correct. I would have expected the phase of the mains power to match the phase of the noise. I was concerned my scope had some sort of delay between sampling the two channels, so I switched the inputs and still saw the exact same phase difference. Attached is a screen capture showing the two signals (red = mains, yellow = noise).A little more research shows that transformers can cause phase shift between the input and output sides. (TI pdf:http://www.ti.com/lit/an/slaa122/slaa122.pdf) Is this what I'm observing? or is something else in play?Screenshot from scopeThank you to anyone who cares to kindly explain this to me. If I'm missing a basic point, please help me understand it or at least guide me in the right direction.Best wishes to all!Ryan

author: IanB
author_rank: Super Contributor
timestamp: December 08, 2013, 07:40:33 pm
content: At a guess, the ambient 60 Hz signal is being capacitively coupled from the wiring in the walls and the capacitance in the transmission path is leading to the phase shift. I don't think a transformer would be doing it since there is a tight magnetic circuit between the primary and the secondary.(By the way, it is not wise to measure the mains voltage directly with an oscilloscope. It would be better to measure through an isolation transformer.)

author: moepower
author_rank: Contributor
timestamp: December 09, 2013, 08:36:53 am
content: Yep, I wouldn't be probing the mains either. If you're not careful, you can make the whole case "live" and trip off your earth leakage device if you don't have isolation.

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 09, 2013, 09:45:04 am
content: The only time you will not see a phase shift is when there is a connection without any inductance or capacitance, it's just that it is mostly too small to notice if there is a wire connection between the 2 points. You can demonstrate it to yourself by connecting both channels together with a capacitor and then touching one of the channels. So yes, it is perfectly normal.