Osciloscope noise on low voltages - Page 1
Forum section: electronics / testgear

author: andyB2022
author_rank: Regular Contributor
timestamp: December 17, 2022, 02:09:33 pm
content: Hi, I'm trying to measure with my Rigol DS1054Z the inrush current of various electronics using a Chinese 100A / 100mV shunt. The only problem that I have is that the noise is terrible and blinds away the important measurement. If I short the prode with the GND clip, the noise is gone, if I leave all four channels disconnected, noise is gone, most of the noise is 50Hz mains...Tip of probe shorted with GND clipProbe is the original one, used on 1X division.Probe in midair.I've tried to use High Resolution Aquire mode but I feel like I'm missing some high peaks when we are talking about inrush...The inrush you see above is a H4 12V 55W car lamp. As it seems, inrush current might be ~14.8A. (14.8mV)For those who know those lamps better than I do, does this current make any sense to you? Again cuz of High Res I feel like I'm missing somewhat of a higher peak.SetupThe scope is NOT EARTHED because I measure AC stuff quite often with it. Neither is the Riden power supply as my house does not have any kind of earthing system. Some of the noise comes from the SMPS inside the Riden of course...Thank you!

author: tggzzz
author_rank: Super Contributor
timestamp: December 17, 2022, 02:33:57 pm
content: Quote from: andyB2022 on December 17, 2022, 02:09:33 pmStop right now before you damage the scope or yourself by "inappropriate" choice of where to clip the probe's shield.Use the proper class of probe.FFI see the references in the "safety" part ofhttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/QuoteWhat is the green/yellow wire in your plug connected to?Be aware that "earth" and "ground" are fictions, but ones that have some use in some circumstances.

author: alm
author_rank: Super Contributor
timestamp: December 17, 2022, 03:35:09 pm
content: Quote from: andyB2022 on December 17, 2022, 02:09:33 pmDo you also see the noise with the probe connected across the shunt without connecting it to the power supplies? The difference between shorting the tip to the ground lead and connecting a 1 mOhm resistor between them should be fairly small, other than the loop area between the tip and the ground lead. I think this might be common mode noise. How does it look using two probes, connecting their ground leads together, connecting the tips on both sides of the shunt and configuring the scope in CH1-CH2 (or CH2-CH1) mode, as a cheap differential probe?Quote from: andyB2022 on December 17, 2022, 02:09:33 pmThis is an antenna that is capacitively coupled to mains through the EMI input filter, so no surprise there.Quote from: andyB2022 on December 17, 2022, 02:09:33 pmAre at least the grounds on the power plugs of the power supply and the scope connected together?

author: andyB2022
author_rank: Regular Contributor
timestamp: December 17, 2022, 06:18:35 pm
content: Quote from: alm on December 17, 2022, 02:09:33 pmThe noise does reduce a bit when the probe is connected to the shunt.No, the power supply and the scope have no ground connection, total "isolation".Using the math function on my scope is really laggy and subtracting CH2-CH1 in order to counter attack noise is not a solution in my case.Quote from: tggzzz on December 17, 2022, 02:09:33 pmI can clip the alligator of the probe to either live or neutral without damaging the scope. I know it's not safe, but a good differential probe is more than the scope itself

author: tggzzz
author_rank: Super Contributor
timestamp: December 17, 2022, 06:53:15 pm
content: Quote from: andyB2022 on December 17, 2022, 06:18:35 pmAre you aware of he ways your techniques can damage/destroy the scope and damage/destroy your circuit? Because botharepossible. Hint: start by considering the effects of "stray" capacitances. They can also introduce significant noise.I presume you didn't read the examples given, including“floating” the scope: to emphasise the previous bullet point, you should never disconnect the scope’s protective mains earth. Too often you find posts to the effect that “I floated a scope and lived to tell the tale (so it can’t be that dangerous)”. Here’s an anecdote aboutsomeone that took precautions and still killed themself.There's an English proverb: "you can lead a horse to water, but you can't make it drink".I've done my duty. I hope your example and confidence doesn't lead other people into danger.

author: bdunham7
author_rank: Super Contributor
timestamp: December 17, 2022, 07:06:13 pm
content: Quote from: andyB2022 on December 17, 2022, 06:18:35 pmAside from being grossly unsafe, floating things like this usually creates a lot of noise for a variety of reasons probably not worth getting into. Those devices are not designed to work without grounds and they really aren't isolated like battery-operated devices would be.

author: andyB2022
author_rank: Regular Contributor
timestamp: December 18, 2022, 10:50:23 am
content: Thanks @bdunham7 and @tggzzz for your advices.I'm protected by a 6mA RCD in case of anything... I'll try to buy real differential probes.I repeat I'm 100% aware of the danger and I've read the article @tggzzz mentioned. I'm sorry for that guy, RIP.

author: tggzzz
author_rank: Super Contributor
timestamp: December 18, 2022, 11:19:08 am
content: Quote from: andyB2022 on December 18, 2022, 10:50:23 amI presume you need a high voltage differential probe. There are others classes mainly aimed at high impedance high speed signals.I've bought mine cheaply, one via a shop sale, and one at a hamfest. I needed patience to get those prices!Hence one of my pieces of advice to people looking to buy their first scope is to include the price of the relevant class of probes - and to expect they may cost as much as the scope!

author: alm
author_rank: Super Contributor
timestamp: December 18, 2022, 01:02:35 pm
content: A high-voltage differential probe is the right tool for measuring in circuit directly connected to mains, like the primary side of a switch mode power supply. But for this specific problem of measuring current, I think a current probe might serve you better.But for an electronics lab I'd really look into getting a solid ground connection (within your local electrical code). Floating grounds are not much fun with EMI filter that you find in most modern equipment. Differential probes are designed with the assumption that they are connected to a grounded scope.

author: andyB2022
author_rank: Regular Contributor
timestamp: December 18, 2022, 01:10:23 pm
content: My house has no earth system and nothing can be done right now to change that. I do electronics mostly for testing and because it's a hobby.Cheers for your answers guys!Stay safe.

author: tggzzz
author_rank: Super Contributor
timestamp: December 18, 2022, 01:24:58 pm
content: Quote from: andyB2022 on December 18, 2022, 01:10:23 pm"Earth" is a misleading term, for several reasons.The mains lead has an earth connection.That connection connects to a contact in the mains socket on your wall.Is there a wire on that socket?Where does that wire go?There are various "protective mains earth" connection systems in use around the world. I am not an electrician and do not understand the details. The "earth" wire in a mains socket is unlikely to connect to the dirt that supports your house (for want of a better description!). Nonetheless, if connected appropriately, that earth wire can provideauseful function.

author: Fungus
author_rank: Super Contributor
timestamp: December 18, 2022, 01:53:31 pm
content: Quote from: tggzzz on December 18, 2022, 01:24:58 pmI wouldn't say it's "unlikely" - they very often do.

author: tggzzz
author_rank: Super Contributor
timestamp: December 18, 2022, 02:15:11 pm
content: Quote from: Fungus on December 18, 2022, 01:53:31 pmAccepted, but how indirectly and in Romaina and where the OP believes it is not connected at all?

author: joeqsmith
author_rank: Super Contributor
timestamp: December 18, 2022, 04:36:44 pm
content: Quote from: andyB2022 on December 17, 2022, 02:09:33 pmDepending on your requirements, you could amplify the signal across the shunt to get above the scopes noise floor. Video below shows an old 100A shunt that was given to me. You can see my chopper amplifier towards the end. If you are only planning to measure lower currents, you could pick a different value shunt to increase the signal. You may also consider using a different sensor than a shunt. I have a few LEM sensors that can be used to 10s of kHz and several amps. The nice thing about using a clamp, you have no need to float the scope or be concerned about where you are probing.

author: andyB2022
author_rank: Regular Contributor
timestamp: December 18, 2022, 05:02:21 pm
content: Quote from: tggzzz on December 18, 2022, 01:24:58 pmYes in Romania the Earthing of a house / building is done in two main ways, one of them is connection that 3rd connector (represented only by Green-Yellow colors) to "dirt" and to Neutral of the system. We have here TN-C-S system.Quote from: joeqsmith on December 18, 2022, 04:36:44 pmThank you for the info, I'm interested in measuring Inrush currents most of the time with the shunt (most of the time in AC side). A current clamp would be lovely but everything comes at cost... And I doubt there is a clamp that can catch inrush transients as fast as a direct voltage drop across a resistor / shunt.I'll look into your solution.

author: andyB2022
author_rank: Regular Contributor
timestamp: December 18, 2022, 05:13:34 pm
content: This one looks very good.Now I've used a 12V car battery (baiscally no SMPS noise).Max value indicated as an inrush current 47.6mV ~47A. 12V 55W H4 Car lamp.