Products > Test Equipment

Osciloscope noise on low voltages

(1/4) > >>

andyB2022:
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 clip
Probe 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.


Setup

The 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!

tggzzz:

--- Quote from: andyB2022 on December 17, 2022, 02:09:33 pm ---The scope is NOT EARTHED because I measure AC stuff quite often with it.

--- End quote ---

Stop 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 of https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/



--- Quote ---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...

--- End quote ---

What 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.

alm:

--- Quote from: andyB2022 on December 17, 2022, 02:09:33 pm ---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...

--- End quote ---
Do 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 pm ---Probe in midair.

--- End quote ---
This 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 pm ---The 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...

--- End quote ---
Are at least the grounds on the power plugs of the power supply and the scope connected together?

andyB2022:

--- Quote from: alm on December 17, 2022, 02:09:33 pm ---Do 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?

--- End quote ---

The 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 pm ---Stop right now before you damage the scope or yourself by "inappropriate" choice of where to clip the probe's shield.

--- End quote ---
I 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 :)

tggzzz:

--- Quote from: andyB2022 on December 17, 2022, 06:18:35 pm ---
--- Quote from: tggzzz on December 17, 2022, 02:09:33 pm ---Stop right now before you damage the scope or yourself by "inappropriate" choice of where to clip the probe's shield.

--- End quote ---
I 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 :)

--- End quote ---

Are you aware of he ways your techniques can damage/destroy the scope and damage/destroy your circuit? Because both are possible. 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 about someone 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.

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod