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Reading a signal off of one wire?

  1. Aug 10, 2011 #1
    When I put an o-scope probe into my (120VAC) wall socket, with no ground (reference) attached, just a single wire, I get a perfect 60 Hz sine wave, with about 35Vrms magnitude.

    When I plug the o-scope probe into the socket with the ground attached appropriately to the return of the same socket, I get the 120V 60Hz sine wave as expected.

    Why do I get the perfect sine wave (with the smaller magnitude) with no reference attached? Is it capacitive coupling?

    The reason I ask this is because I was chasing down a signal today at work on some equipment and was receiving this signal (unexpectedly) off of a circuit with a known open conductor... This got me to thinking about why.

    Hope that made sense.
  2. jcsd
  3. Aug 10, 2011 #2


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    Science Advisor

    Your body (or even a piece of wire, or the probe itself) is acting as an antenna and picking up the 60 Hz that's in the wiring all around you.

    A friend of mine was working on a circuit and kept on getting an odd 88.5 MHz signal in his circuit, which persisted even when it was powered off. He then realized the campus radio station (which he was incidentally listening to) was at that frequency.
  4. Aug 10, 2011 #3
    Ok so an o-scope is basically a voltmeter, correct?

    Is it measuring a difference between two points?

    If no ground (or reference or whatever) is hooked up, what two points is it measuring between?
  5. Aug 10, 2011 #4


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    Staff: Mentor

    What kind of 'scope are you using? Is it battery-powered, or plugs into the wall? If it's wall-powered with a 3-prong power cord, and the 'scope probe ground is Earth Grounded by the 'scope, then it doesn't matter much if you hook up the ground clip in this measurement, since the Hot lead in your AC Mains wall socket is referenced to Neutral (= Earth Ground at the breaker panel).

    If your 'scope is floating and battery powered, then what you see displayed is due to the capacitance from the 'scope internal ground to external Earth ground (and nearby metal objects).
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