Jump to content
Electronics-Lab.com Community

50Hz when you touch Oscilloscope probe


Recommended Posts

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


Link to comment
Share on other sites

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?

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

  • 2 weeks later...

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.

Link to comment
Share on other sites

this problem is prevelent even with 74 series . a cct rigged up on a prototype had misarably failed due to noice pick-up. perhaps with proper precautions towards poer decoupling at each IC and by keeping poer extention boards, etc away from the project prototype, and by sticking the specific metal sheet supplied with the bread board, beneath and keeping at earth potential, we may suceed in preventing the power pickup problems.  all of us are aware of antisattic wrist straps with earthing cwire etc . these are to be specifically used when working CMOS digital ICs and in locations of low relative humidity.              (  B series, now a days are ruggedized by protection diodes across every pin  to prevent failure, induction is stil a problem) . whether one measures 50/60Hz or a hormonic depends perhaps on the amount of induction and amount of noice generated by devices adopting triac conrollers- even after all the reglations reg power purity.

hope with adequate precautions one could still prototype and test on bredboards.
anxious to track the success of test rig ups.

sarma

Link to comment
Share on other sites

Inside Your Scope there are two capacitors vith value about 10nF connected between Mains lines and case of Scope for filtering purposes. they Create some AC voltage on Scope Common. Scope Shows voltage between common and probe. When You tuch probe You ground it by Your Body capacitance so voltage difference is created. To avoid this simply ground the scope common.Scope must have spetial coonnector for Grounding

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
 Share

×
  • Create New...