Is an oscilloscope supposed to do this? - Page 1
Forum section: electronics / beginners

author: Hextejas
author_rank: Frequent Contributor
timestamp: February 18, 2018, 09:51:54 pm
content: I am trying and learning on a kit scope that I built. So, far it has tested ok on a few things so I am now trying something a bit more difficult.What is strange is that when I disconnect the probe and touch it to my finger it measures 60hz !!How can that be ? The only thing different from the original is that i replaced the probe with a better 10x probe.Am I electrified ?

author: Cliff Matthews
author_rank: Unknown
timestamp: February 18, 2018, 09:55:32 pm
content: This is normal since the inputs are high impedance. Unless you live in the dessert, you will have AC electric fields around you constantly.We are after all "Ugly Giant Bags of Mostly Water"

author: helius
author_rank: Super Contributor
timestamp: February 18, 2018, 10:01:16 pm
content: Quote from: Hextejas on February 18, 2018, 09:51:54 pmIn a word, yes! We're surrounded by EMI (electromagnetic interference) produced by mains electric cables, electric lights, and appliances. The human body acts as a capacitor and couples some of this energy into the scope input. This is also why you should always connect the probe's ground lead if possible to a ground point near the probed signal, to minimize the EMI picked up by the probe.

author: GerryBags
author_rank: Frequent Contributor
timestamp: February 18, 2018, 10:01:32 pm
content: You're acting as a hairy antenna for EMI.

author: Hextejas
author_rank: Frequent Contributor
timestamp: February 18, 2018, 10:50:24 pm
content: Quote from: GerryBags on February 18, 2018, 10:01:32 pmHahahaha

author: Hextejas
author_rank: Frequent Contributor
timestamp: February 18, 2018, 10:56:14 pm
content: Quote from: helius on February 18, 2018, 10:01:16 pmThank you and very interesting and totally unknown and kewl to me. So, in looking for a ground point, should I use the circuit that I am working on, a water pipe, or something else ?Fascinating topic.

author: ataradov
author_rank: Super Contributor
timestamp: February 18, 2018, 11:10:10 pm
content: You need to use the ground of your circuit. Ideally the closest ground point to the signal you are measuring, but for low frequency designs it is not that important, any ground connection will work,And by no means use water pipes. That's a way to fry stuff.

author: helius
author_rank: Super Contributor
timestamp: February 18, 2018, 11:11:35 pm
content: Quote from: Hextejas on February 18, 2018, 10:56:14 pmIf your circuit is battery powered, and everything connected to it is battery powered too, you can connect the ground clip to anything you like in the circuit (but make sure every probe's ground clips are to the same thing!). If your circuit is mains operated, you should connect to its chassis or star ground. If there is a test point nearby to your signal that is also bonded to the chassis (test them for resistance < 2ohm or so) you can use that.https://www.eevblog.com/forum/blog/eevblog-279-how-not-to-blow-up-your-oscilloscope!-63751/

author: Hextejas
author_rank: Frequent Contributor
timestamp: February 19, 2018, 12:06:44 am
content: Quote from: helius on February 18, 2018, 11:11:35 pmThank you for this caution. The link is not good but I found Dave's video on the subject. Scary.

author: timelessbeing
author_rank: Frequent Contributor
timestamp: February 19, 2018, 12:19:24 am
content: Quote from: Cliff Matthews on February 18, 2018, 09:55:32 pmI wish I lived in a banana split. That would be sweet!

author: BrianHG
author_rank: Super Contributor
timestamp: February 19, 2018, 07:12:21 am
content: Quote from: Cliff Matthews on February 18, 2018, 09:55:32 pmActually, once when I went to pick up and purchase a used scope, I usually look for & use the 60hz by touching the probe tip of the scope to test, however at this one place, there was no 60hz. I thought the scope was broken, but, it turns out that the basement dwelling in the apartment of the owner has aluminum lining inside the walls grounded to the AC mains earth. I was literally inside a virtual Faraday cage without knowing it and the scope showed nothing when touching the probe tip. It took a signal generator to convince me otherwise...

author: paulca
author_rank: Super Contributor
timestamp: February 19, 2018, 08:05:54 am
content: It's even more head melting when you look at Maxwell's original premise that a charge creates a electro-magnetic field which spreads out and eventually fills all of space/the universe. Everything is connected in a huge soup of electromagnetic fields interacting across the whole universe.

author: Raj
author_rank: Frequent Contributor
timestamp: February 21, 2018, 04:54:04 pm
content: demo of capacative coupling-there's the while electromagnatic spectrum and also the capacative coupling form electricity.ground yourself