seeing mains when measuring with oscilloscope - Page 1
Forum section: electronics / testgear

author: hussamaldean
author_rank: Unknown
timestamp: September 14, 2018, 07:23:08 am
content: Hi allI am not measuring the mains at allI want to probe a circuit which is being fed by PSU from UNI-T, laptop charger with buck converter and 9V power supply from a tp-link router. however, when I want to see the PWM signal on the mosfet driver and mosfet, what I am seeing is 70V RMS with 50Hz which masks everything I am trying to measureis there something wrong with my circuit psu or oscilloscope ?I tried to float the oscilloscope, psu nothing works and I am starting loosing my mindregards

author: Kleinstein
author_rank: Super Contributor
timestamp: September 14, 2018, 10:46:38 am
content: Floating the scope is a rally bad and dangerous idea ! It takes special gear (isolating box and remote control) to do that in a safe way.If needed the DUT should be operated trough an isolating transformer.Just in case one misses the obvious: one usually needs a ground connection from the scope to the DUT.

author: tggzzz
author_rank: Super Contributor
timestamp: September 14, 2018, 10:50:49 am
content: Quote from: hussamaldean on September 14, 2018, 07:23:08 amIf you float a scope, consider yourself lucky that you haveonlylost your mind. The other things you could lose include the scope and your life.Besides that, if you understand the physics of that test setup, you will understand why you almost certainly won't be able to see what you (think you) want to see.See the references inhttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/they indicate how to do this kind of thing safely.

author: taydin
author_rank: Frequent Contributor
timestamp: September 14, 2018, 11:24:49 am
content: I remember reading in a TEK website that, if a scope is used without it being connected to the mains earth through the power cable, the scope is not guaranteed to meet its datasheet specs. So unless you are making the most trivial measurement, you always have to wonder whether the scope is measuring the signal properly.

author: hussamaldean
author_rank: Unknown
timestamp: September 14, 2018, 01:17:49 pm
content: guysI am not trying to measure the mains with my oscilloscope what I am seeing on the output of the DC jack from psu or laptop charger is the 70V with 50HzI am not measuring the mains at all

author: macboy
author_rank: Super Contributor
timestamp: September 14, 2018, 01:19:38 pm
content: You can try the "poor man's" differential probe. You use two identical probes a 2+ channel scope with a A-B (or Ch1-Ch2) trace. This is cheap/free but surprisingly tricky to get right.You must adjust the probes for best CMRR. This is done by first connecting them both to the cal signal square wave, and adjusting compensation on each individually (using individual channel waveforms). Then enable the A-B trace, and if necessary adjust the gain on one channel ("VAR" vertical) to match the other, then adjust adjust the compensation on only one of them to minimize the displayed waveform.To use this probe, you connect the two ground clips to a common ground point on the DUT, then use use the two probe tips to take a differential reading. Also note that although the differential signal may be small, the vertical or V/DIV setting must be set appropriately for the individual signals w.r.t. ground. The differential signal of interest may be small compared to the large common mode component. You can't just set the V/DIV higher to make the differential (A-B) signal bigger. To set the vertical or V/DIV correctly, enable the A, B, and A-B (or Ch1, Ch2 and Ch1+Ch2) traces, connect across the component(s) to measure, adjust the vertical so that the A and B signals do not saturate/exceed the display, then you can turn those traces off and view only the A-B trace. Every time you change vertical, you must change both channels to match each other, and you may need to slightly tweak the variable gain on one channel to minimize the common mode junk.All good analog scopes and some older digital scopes did this summation in the analog domain. All newer digital scopes can do the summation digitally via the math function.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 14, 2018, 02:24:49 pm
content: Quote from: hussamaldean on September 14, 2018, 01:17:49 pmOK, so you are seeing a lot of 50Hz on theDC outputof two different power supplies?Perhaps there is sufficient 50Hz leakage to one leg of the PSU output, but not the other, to make a 70v signal visible when looking at it with the high input impedance of the 'scope.Try putting about a 1k resistor across the terminals.This should not be a big load for the normal DC output, but will load the leakage current down a lot.Another possibility is some kind of artefact of a DSO, if that is what you are using.Look at the PSU outputs with a DMM.If the display on your 'scope is real, it should give you a similar result on its AC range.

author: tggzzz
author_rank: Super Contributor
timestamp: September 14, 2018, 02:52:06 pm
content: Quote from: hussamaldean on September 14, 2018, 01:17:49 pmThat doesn't changeanyof the points about why you really shouldn't float a scope.

author: ogden
author_rank: Super Contributor
timestamp: September 14, 2018, 02:53:34 pm
content: Take mains power splitter, plug both - test supply and scope there so you are sure that your device and test equipment are both powered from same phase and ground as well. Use not only tip of the probe. You shall connect ground (crocodile) clip of the probe to negative terminal of the DC output you measure

author: hussamaldean
author_rank: Unknown
timestamp: September 21, 2018, 01:35:12 pm
content: Hi allafter spending days diagnose what the hell is going on, it turns out that a near by faulty LED driver. since I don't have earth here and the neutral is not balanced at all (Welcome to Iraq ), I had to made my own earth and the LED is connected to the same line that I am using it to power my lab. so everything now is ok and I have no issue since thenregards