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When I touch with my bare finger the oscilloscope probe I get a perfect sine ac wave 155V peak to peak!.

This is my first oscilloscope and I don't know if this is a normal glitch.

Do I have a faulty one?

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marked as duplicate by JRE, Ricardo, Daniel Grillo, Matt Young, PeterJ Aug 27 '15 at 23:58

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If you have an unearthed plug or perhaps a miss wired plug or socket you may find that the scope is internally weakly earthed (via a drain resistor or coupling cap) to what they think will be the neutral wire in the mains input. This will cause the scope to have a earth reference to be floating at some fraction of the mains voltage.

The peak to peak voltage of 115V RMS is about 162V p-p. Alternately you may find that the scope is close to the correct ground reference but your body is at close to main potential due to capacitive coupling or resistive leakage to some other mains apparatus in the vicinity. The moderately high input impedance of the scope will allow any stray signals that different from the scope ground reference to be accurately displayed.

You could test by grounding the scope (they often have grounding points on the dash and grounding yourself and will likely see the signal disappear. Alternately you can usually enhance the signal by touching the metal case of plugged in equipment, a florescent tube in a fitting or even holding a insulated mains cable and insulating your self from earthed objects.

When doing measurements it is advisable to minimise such stray signals before trying to trouble shoot circuits even though correct connection of the earth clip will usually remove the artefact in practice, sometimes it is nice to be able to probe with no earth connected

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What you are seeing is normal. There are always electrical fields around you. These can cause your body to have a voltage higher than ground.
Since the oscilloscope shows the difference in voltage between ground and whatever is connected to the input, you will normally see a voltage of some kind when you touch the probe but not ground.

If you touch the probe and ground at the same time you should see an almost perfectly flat line.

Don't worry about the voltage on your body. The oscilloscope takes very little current when measuring the input voltage (think in terms of microamperes.) The voltage induced on your body by the surrounding electric fields can't supply much current at all, and only the extreme sensitivity of the oscilloscope input makes possible to measure it at all.

Your body acts sort of like an antenna, sort of like a capacitor, and sort of like an inductor. Those let it pick up the electric fields. Thing is, your body isn't very good at any of those things so it can't pick up any real power - and, the power lines around you kind of suck at those things, too, so there's not much to pick up.

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Your body acts as an antenna. You "pick up" the 50/60 Hz electromagnetic field emitted by your mains wiring, and a small current is induced in your body. An oscilloscope is excellent at picking up this current and displaying it. It is perfectly normal.

Not sure I've ever seen a 155V peak voltage displayed, though. Usually I only see a few volts maximum.

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The other answers are correct; your body will pick up rf noise and display it on the scope.

In addition, make sure that your oscilloscope probe is set to the same scale as your o-scope channel input. It is common for both to be set to 10X.

If the scope is expecting a 1X signal from the probe but the probe is set to 10X, then the displayed value will be 10X higher than measured.

The probe likely has this type of switch:

enter image description here

The scope needs to be set to match. The method for doing so depends on the scope.

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  • \$\begingroup\$ Excellent point regarding the multiplier. Had not thought of that \$\endgroup\$ – DerStrom8 Aug 27 '15 at 17:10

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