60-Hz signal when I touch scope probe

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I am in lab and I just discovered that when I touch my scope probe to my finger I get a 60Hz sinusoid on the screen. Why? Does it have to do with the ground connections? Ground was connected to a DC power supply.

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Because You live in USA. In Europe we get 50 Hz. ;-)
It's absolutely normal in places where is AC grid. Human body act as antenna and received waves may be seen on scope.

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This is visualization of mains hum appearing when one touches amplifier input terminals, to put it in other words. Kind of EM induction on human body acting as a conductor near mains lines.

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Totally normal.

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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normal...

note that the scoop probe is probably fully isolated,
and a big chance that also your power supply is isolated.

so you think you have ground, but unfortunately that is not the case.

You should try to connect the groudnd of the scoop to the ground connection on your 110Mains socket(if that is at all present in the US)

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Your body is not necessarily broadcasting hum. Visualize the circuit if you were just a block of metal shorting the probe to the floor. It's a loop with a rather large cross section feeding a high gain amplifier. Your scope should show all the local radio stations as well.

To get rid of that you have to minimize the cross section of the loop. One way is to ditch the internal ground reference, instead make a differential measurement using two probes that are twisted together along their length. One probe is grounded near the signal point, the other is attached to the signal.

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Another way is:
Balanced Probe Extends High-Frequency Measurements by Douglas C. Smith.
Uses a combiner; so can use 1 scope channel instead of 2.

"Dare to be naïve." - Buckminster Fuller

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Touching the input grid of a valve amplifier was a good way to tell if the valve was working.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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Perfectly normal - your body is acting as an aerial. If you want to get rid of the signal you need a fully-grounded tinfoil hat... ;)

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Being someone who knows a pitiful amount about electronics the induced 50Hz hum is how I always test that the probe is connected and the Y gain is in the right ball-park. One day I'll learn to use the 1Vpp 1kHz signal on the front of the scope to do the same test :?