CAN YOU EXPLAIN THIS OSCILLOSCOPE PHENOMENON ? - Page 1
Forum section: electronics / testgear

author: Xgentec Jason
author_rank: Contributor
timestamp: October 14, 2020, 12:28:01 pm
content: Set oscilloscope to 20milliseconds at 50millivolt divisions and you clearly see 50hz waveform from UK 240v mains wall sockets.If you now put your hand near the oscilloscope probe the waveform voltage increases in height.?Is this capacitance related ? Is my hand acting as a core material between a primary ac signal and a secondary the probe ? Is this RF related ? or a combination ? I am using a (RIGOL DS1054Z) standard rigol probe on x1I am new to the forum so hi to everyone out thereThis has me a little baffled. Many thanks from Jason in the UK

author: Simon
author_rank: Unknown
timestamp: October 14, 2020, 04:45:18 pm
content: It's because the probe cable is screened so not a lot of exposed probe to pick it up. Once you touch it your body is now the areal.

author: Xgentec Jason
author_rank: Contributor
timestamp: October 14, 2020, 05:11:53 pm
content: Hello Simon. The 50 Hz waveform increases in voltage when moving your hand near to the probe tip (without actually touching the probe tip) ? The question is what causes this voltage increase effect

author: SilverSolder
author_rank: Super Contributor
timestamp: October 14, 2020, 05:17:01 pm
content: You were right in your original post, it is the result of capacitance.You could think of it this way: Your finger is one plate of a capacitor, the probe tip the other plate. As you move your finger closer to the probe, the capacitance increases (smaller distance between plates means more capacitance). When the capacitance goes up, the reactance (impedance or AC "resistance") goes down - so more signal makes it through the "finger capacitor" into the probe, and the voltage seen by the scope goes up.The same thing happens between the mains wiring in the house and your body. So your body picks up the signal from the house wiring (capacitively), and your finger then passes that on to the probe tip (another capacitor). Two capacitors in series, essentially. The capacitance between your body and the house wiring is much bigger than the one between your finger tip and the probe. So the small capacitance is the limiting factor and has the biggest influence on the changes you see when you move your hand/finger around.Makes sense?

author: Xgentec Jason
author_rank: Contributor
timestamp: October 14, 2020, 05:30:09 pm
content: Hi SilverSolder. This was my original suspicion, I wanted to clarify or run this idea past others just to make sure my conclusion was indeed correct. Many thanks for your input.

author: magic
author_rank: Super Contributor
timestamp: October 14, 2020, 05:54:00 pm
content: My thinking too. It should disappear if you ground yourself (by touching the probe's alligator clip, trigger input BNC, mains socket, whatever).

author: Xgentec Jason
author_rank: Contributor
timestamp: October 14, 2020, 06:18:50 pm
content: Hi Magic, Yes you are correct the phenomenon disappears when you touch the ground line of the probe. I should have mentioned this in my original post.I will be running some other tests surrounding this effect and do a video in the future. many thanks.