Electronics > Beginners
Oscilloscope Mains AC Sinewave Testing
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The Electrician:

--- Quote from: scatterandfocus on September 12, 2019, 04:37:52 pm ---I'm interested in this topic too.  I tend to get some sporadic power noise in my audio recording chain.  Should it be ok to drop the voltage down to around 1V using a transformer and voltage divider and capture the output with a soundcard?

--- End quote ---

As long as the peak voltage applied to the sound card doesn't overload the card there should be no problem.  60 Hz is well within the usual audio band.  :)
scatterandfocus:
Thanks.  I was planning to pickup a filament transformer anyway for looking at component I-V curves on a scope.
rstofer:

--- Quote from: n5yzv on September 12, 2019, 04:13:47 pm ---Thank you!  I had no clue which one to get.  Is there a way to monitor AC mains over a time period and replay?   Again, using Rigol DS1054Z with all licenses.

--- End quote ---

That's why Dranetz invented the Power Disturbance Analyzer.  We used these all the time when checking datacenter power.

The newer stuff I have never used:

https://www.tequipment.net/dranetz/power-measurement/power-quality-analyzer/

The older stuff I have used:

https://www.ebay.com/itm/Dranetz-646-3-Disturbance-Analyzer-6463/401621712464

A scope is the wrong tool even if it can save a trace.
The Electrician:
If a scope is the only tool one has to make do.  :)

Here's what I get if I display the derivative of the grid waveform along with the waveform.  The grid is (obviously) in yellow, the derivative is pink.  Remembering what we learned in Trig class, the derivative of the sine function is the cosine function, so the derivative trace should also have a sinusoidal shape, but the scope derivative trace is quite distorted, showing that the grid waveform is not a perfect sine wave.



Here is the result of turning on infinite persistence and waiting a couple of hours.  The saved pixels are gray, and the derivative trace shows a lot of high frequency action, probably due to switching off things in my neighborhood, producing an arc at the switch when it opens.



I'm going to leave this on for 24 hours or so and see what I get.
rstofer:
The derivative always shows noise.  That’s why it isn’t used in analog computing.  We keep integrating unti the derivatives disappear.  Unimportant...

So, there is harmmonic distortion along with some noise.  What does it mean?  Does it even matter when everything is using switching supplies that are causing the noise in the first place?  FCC Part 15 labels come to mind.

In the mainframe era we used 400 Hz motor generators to guarantee relatively clean supply while allowing for smaller transformers inside the equipment.

I would question every implication of power problems that didn’t also dim the lights.  A simple flicker shouldn’t bother anything.  Large motors starting might cause a sag but it won’t be subtle.  The waveform certainly won’t look like a sine wave with a wee bit of distortion.
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