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I bought a new PicoScope USB oscilloscope and noticed that when I touch the tip of the probe with my hand I see a 50 Hz signal of 2 volts on the screen. What is causing this? (Or rather, how is the mains 50 Hz sine wave getting through me to the scope?)

Peter Mortensen
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Pownyan
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    2 Volts on 1 MegOhm is only 2 uA. E-Field coupling from power lines. – sstobbe Sep 14 '17 at 16:38
  • Then don't touch it.... – Trevor_G Sep 14 '17 at 16:55
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    @Trevor of course i won't touch it when making measurements, but I was curious of how the phenomena happened. Isn't this site made for education people? – Pownyan Sep 14 '17 at 18:08
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    lol Sorry..it just reminded me of an old joke.. "Doctor..it hurts when I do this...." ANyways, as @sstobbe mentioned, the scope input is very high impedance and you/we are swimming in a sea of EMI especially mains hum so our bodies pick it up all the time.. when the scope is actually attached to some signal you are measuring, it wont matter much unless the impedance at the point you are measuring is in the order of the scope impedance. – Trevor_G Sep 14 '17 at 18:16
  • See also: [My scope detects a 50Hz signal when the probe is not connected to a circuit, is this normal?](https://electronics.stackexchange.com/q/78920/25328) – Tut Sep 14 '17 at 18:42
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    @Pownyan, perhaps the problem is that you're in Europe; you can fix this by moving to the USA and then it will be a 60Hz signal. – Jason S Sep 15 '17 at 05:28

2 Answers2

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There is a small capacitance between you and the power network. Also there is another, different capacitance between scope's ground and power network. Input impedance is high enough, usually 1Mohm, so you create a kind of voltage divider between power network, oscilloscope's probe and it's ground.

This is a purely capacitive phenomenon. Calling it an "antenna" is a bit misleading and is not any better than others slogans.

Peter Cordes
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    Nice answer. More quantitatively, I think the scope's input acts like a resistor and capacitor in parallel, with the R=1 Mohm resistance being dominant at 60 Hz. The capacitance C of your body with the wall is probably ~1 pF. These two impedances make a voltage divider on V=110 V, and the voltage measured by the scope is iωRCV, or ~40 mV, which is the right order of magnitude compared to what I observe. –  May 25 '20 at 21:11
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You are acting as a big antenna and you are picking 50Hz from the mains.

Chupacabras
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    I figured it would be something like this, but i was surprised that it was as high as 2 volts so i thought it might have been something else to. – Pownyan Sep 14 '17 at 16:45
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    @Pownyan Roughly speaking, if you consider your body to be effectively 1 meter from the PicoScope's input ground, then an ambient electric field of 2 volts per meter will produce 2 volts at an input with sufficiently high input impedance. If you trigger off of a separate, fixed signal synched to the mains, you'll see that as you move around the room with respect to the scope's ground, both *the amplitude and the phase* of your personal signal will change. – uhoh Sep 15 '17 at 02:18
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    @uhoh Why would the phase change? Very few locations are wired with polyphase power and the frequency is much too low for propagation speed to matter. – AJMansfield Sep 15 '17 at 04:13
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    @AJMansfield If the local electric field is uniform and east-west for example, then if you change from east-of-ground to west-of-ground, there will be a 180° flip, which I guess you could call a sign change of the amplitude as well. But in a realistic situation the electric field one is detecting is not necessarily just radiation from electric dipoles, there are inductive loads making AC magnetic fields (motors, refrigerators) that are being converted to electric fields in various ways, and capacitive effects in series with resistance produces phase shifts as well. A mess of possibilities. – uhoh Sep 15 '17 at 04:32