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dirty waveform

docb

Feb 11, 2010
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I'm looking at the output of a wall powered driver on a scope. The bottom of the square wave, (meaning 0v), is messy, presumably because the output ground isn't tied to the input ground, so it's floating.

I want to use this output to control a device I will make, but not sure what to do to eliminate this noise.
 

Gryd3

Jun 25, 2014
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I'm looking at the output of a wall powered driver on a scope. The bottom of the square wave, (meaning 0v), is messy, presumably because the output ground isn't tied to the input ground, so it's floating.

I want to use this output to control a device I will make, but not sure what to do to eliminate this noise.
what waveform... and how is it 'messy' ?
This could be drift, ringing, or simply an undesirable shape...
More info is most certainly needed here.
 

docb

Feb 11, 2010
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It's a light driver, powered from AC line voltage.
 

Gryd3

Jun 25, 2014
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It's a light driver, powered from AC line voltage.
Again... can you please provide more details.

What is the waveform ?
How is it dirty ?
Do you have a picture?
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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Do a 1-shot capture. What you're seeing is varying waveforms being superimposed.
 

dorke

Jun 20, 2015
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And please show a photo of all the Scope knobs and settings
 

davenn

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Sep 5, 2009
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Now isn't that odd...
I'm not sure what to make of that... It looks like 3 or 4 different shapes superimposed on each other. I would not associate that with a lack of ground between input and output.

in some ways, it looks like ringing
 

dorke

Jun 20, 2015
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What do you want to see on the knobs? I think the screen shows all the settings.

I'm not familiar with that specific scope and the way "settings" are displayed on the CRT.

What are(though a picture is worth thousand words ;)):

1.The trigger setting.
2.Acquisition method.
3.Time/div
 

docb

Feb 11, 2010
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upload_2015-12-23_8-56-47.jpeg


This picture show time div.
Acquisition method is "sample", so maybe that's the issue. If I use "average "it looks much better:

upload_2015-12-23_9-1-48.jpeg
 

dorke

Jun 20, 2015
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View attachment 23928


This picture show time div.
Acquisition method is "sample", so maybe that's the issue. If I use "average "it looks much better:

View attachment 23929

What kind of signal is it?
Is the trigger level about 19V?

No, averaging is a distortion of reality.
Can you use a "single time Acquisition"

What is the ground clip of the scope probe connected to?
 

docb

Feb 11, 2010
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The second image is a single. Ground clip is on negative output of the driver. Other is on positive. Yes, trigger anywhere above about 13 seems ok.
 

dorke

Jun 20, 2015
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Take a single sample,without average.and show it
After the data is in the scope memory ,zoom in the area shown and show it as well.

upload_2015-12-23_9-1-48.jpeg
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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No need to see that in more detail. It's clearly ringing on the negative edge.

Now, what is this actual device, and what sort of load is it powering when you take these readings?
 

dorke

Jun 20, 2015
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No need to see that in more detail. It's clearly ringing on the negative edge.

Now, what is this actual device, and what sort of load is it powering when you take these readings?

Steve,it isn't classical ringing as the final low voltage is at or below the minimum of the "ring" .

Also look at this from post 13.
Not clear at all.upload_2015-12-23_8-56-47.jpeg
 

GPG

Sep 18, 2015
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Could be 1/2 bridge with faulty low side drive.
 
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