50Hz when you touch Oscilloscope probe

[Shahriar]

why when you touch Oscilloscope probe a 50Hz signal will appear?

I think it should be related somehow to  Earth connecttion.

ThanX
Shahriar

 

[faizanbrohi]

no i think it is somewhat your pulse rate .  ;D , like a ecg machine

 

[audioguru2]

Your body is an antenna for picking up 50Hz mains radiation. Same thing happens if you touch an amplifier's input, it produces a loud 50Hz.

That is why the scope's cable and audio cables are shielded.

If your heart fibrillates at 50Hz instead of 50 bpm, then you are having a heart attack.

 

[Shahriar]

Dear Audioguru

1- I Connect the Probe to an Antenna and a 50Hz signal appeared with pick voltage about 2V.
2- when I touched the antenna while it was connected to Probe a 2.2V 50Hz signal appeared.

3- when I touched the probe a 50Hz, 6V signal appeared
4- when I touched the probe a 50Hz, 5.7V signal appeared while I was standing over some papers.

if it was all radiation so what is the roll of those papers, maybe If you examin it, find more useful things.

ThanX
Shahriar

 

[audioguru2]

Hi Shahriar,
Do you have a light dimmer? Turn it on to half then it radiates spikes you can see on a 'scope with your finger or another antenna on its input.

 

[Cabwood]

I understand it's your body's capacitance with ambient that couples you to the mains. A few picofarads. That's enough to deliver a volt or so to a high impedance at 50Hz. It is electromagnetic in nature, of course, but I suggest that "picking up mains radiation" is misleading.

 

[audioguru2]

I think mains hum picked up by your finger or another antenna is produced by  electromagnetic and electrostatic fields. You are surrounded by mains wiring that transmits the fields so that's why I call it radiation.

 

[audioguru2]

Hi Zeppelin,
An elsctrostatic field works with AC or DC.
There is a very small amount of capacitance from nearby mains wiring and your body.
There is a small amount of capacitance to ground at the 'scope probe.
The capacitances are joined in series at your body and make a capacitive voltage divider.

 

[prateeksikka]

hi audioguru ,
thats a strange thing u told.in india we have mains frequency as 50 Hz .Does our body pick only 50 Hz ?
If i am in a country with 30 Hz mains frequency and there i touch a CRO with a fan blowing over me ,what will i get on screen?
30 Hz or nothing?
In other words, is our body tuned to 50 Hz or any mains frequncy noise around us?

 

[audioguru2]

Hi Prateek,
If your home or lab has electrical outlets on each wall and electrical lights on the ceiling then you are surrounded by high voltage AC at the mains frequency. Your body or a piece of unshielded wire will pick it up. It doesn't matter if there is a fan and the frequency doesn't matter.

 

[indulis]

I believe the answer is "leakage current" in conjuntion with a capacitive divider (as mentioned above). If your hands are moist, touch the tip of the scope probe with one hand and the scope ground wire between two fingers with the other hand. Squeeze the ground wire harder between your fingers, and watch the waveform... it will change in aplitude and will always be of the same freqency as the local line voltage. Guess what happens if you do this with battery operated scope? When I say "moist hands", I don't mean with water, I mean with perspiration, which, because of salt content, is a better conductor than plain water.

 

[prateeksikka]

no audioguru,
i think u did not get my question .my question was does our body pick up only 50 Hz or any frequency signal by which it is surrounded say even a 30 Hz or 100Hz .
in other words ,,
is our body an antenna tuned at 50Hz or does it get automatically tuned to any frequency around it?

 

[Shahriar]

Dear Audioguru

A TV tuner is designed to receive a voltage around 400uV Upto 600uV From antenna. then how our body can produce a 6V signal. I think there are some other reasons in addition to electromagnetic in creating such big signal.

shahriar

 

[audioguru2]

Your body isn't tuned. It has a large surface area, is conductive (less resistance than the input of a 'scope), is insulated from ground and therefore picks up any signals that are there.

A TV station is far away so a TV tuner needs good sensitivity by amplifying the signal. Hundreds of volts of mains is very close and is surrounding you, so if you touch a 'scope's input then a few volts are shown. Connect an unshielded piece of wire to the input of a 'scope or a microphone amplifier. You will have the same mains hum.

 

[Cabwood]

Yes, as I said, capacitative coupling. I'm pretty sure there is almost no electromagnetic phenomenon involved in 50Hz mains pickup.

The body's capacitance to ambient electric fields is a few picofarards. When you touch the scope probe, you become a 10pF capacitor in a series circuit - a 50Hz 110V/230 AC signal generator, in series with a 10pF capacitor, in series with a 1MOhm load (the probe) to ground.

That's a cut off frequency of several kilohertz, meaning the 50Hz 'signal' is hugely attenuated.

This is what I believe to be the dominant effect here.

 

[indulis]

As I said... try this with a battery operated/powered scope this will give you a clue!!!

 

[Cabwood]

Indulis - explain please what happens when you do this with a battery operated scope. I'm curious to know.

 

[Muhammad Abu Bakar]

when i put cd4017 or cd4026 on breadboard (from a power supply) it gets self clocked at exact 50Hz. When operating from a battery it doesn't happen.

 

[audioguru2]

Mains radiation is all around you at 50Hz. The long wires and tracks on a breadboard pickup interference.
Maybe the power supply is grounded or has some capacitance to ground then the circuit's wires are an antenna. With the battery then there isn't a ground so the antenna effect from the wires isn't as strong.
Maybe the clock pin on the counter IC doesn't have a low impedance path to ground.
Maybe the power supply has a lot of 50Hz ripple.