Electronics > Beginners

Oscilloscope Mains AC Sinewave Testing

(1/5) > >>

n5yzv:
I've been googling around for a few days, and I'm seeing mixed answers, so please forgive me is this post seems redundant.

What I've got:  Rigol DS1054Z with all licenses, 1/10x probe and various outputs of 120v AC (Utility, UPS's, multiple inverters)

From what I gathered, I wanted to use a transformer do buck the voltage down to a lower/safer voltage.  I picked up a crap halogen 120v->12v transformer on Amazon.  (I'll ignore this as when I probe it, it is a nasty sinewave).  So, after giving up on it, I cautiously made a isolated pigtail and probed it (two prong, 1 hot, one neutral (verified))  With this I get a sinewave.  Great.  But, when probing the utility (mains) the top of the curve is flat.  A friend, who knows his way around an oscilloscope, but who is also very busy, noted I am "starting to overlaod and limiting the analog to digital converter range and it no longer tracks the input".

So, I think, in my words. the voltage or frequency is high enough it is pushing the limits of the probe/scope input and distorts the image on the screen as it is out of range.  I think.

So why do I want this in the first place?  I am a Ham operator and in IT.  I regularly run across shitty power.  I diagnose this on experience, with no real scientific proof.  The scope would allow me to really *know* what is going on.  I figure learning the scope with a purpose is a good way to get started. 

So my goal is to do it right, safe and accurate.  I know the method I did is not on the safe side, so I would like improvements there. 

Attached are two images.  One utility one a pure sinewave inverter.  The last image is of an APC ups.  This is why I don't like entry level ups's.

Thanks!

Mark


[/img][/img]

Zero999:
The flat top on the mains is probably caused by switched mode power supplies, without power factor correction. They draw most of the current, at the peak voltage, so it's not surprising it's flattened.

Don't connect the neutral to the 'scope earth. There's a risk a high current could flow through the oscilloscope or the GFCI could trip. This is because the neutral may have a great enough voltage, with respect to earth, to cause a considerable current.

tggzzz:
You should, as a matter of urgency,

* look at the praxis/safety references at https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/
* if you are ever tempted to disconnect a protective mains earth, have a look at https://www.eevblog.com/forum/projects/isolation-transformer-for-scope/msg2259465/#msg2259465
Why? To save your equipment and you from loud noises followed by deathly silence.

BTW, your friend is almost certainly wrong in this case; I see no evidence of overload. Nowadays mains voltage sine waves often have a "flattish" top.

If you want to improve power, use a UPS or consider whether a ferroresonant transformer might help in that specific case.

n5yzv:
Yeah, I read allot about earth ground.  I've done quite a bit with electrical and grounding, so I am being very cautious of that mistake.  Removing ground to a device is normally a very bad idea.

Regarding improving the power, I'm learning to do this mostly to prove power issues.  I also have customers choose low quality UPS's with no real clue of how poor the output power is when on battery.  Most of the UPS's I recommend are higher end, more costly, inverter style ups, which are on DC/Inverter power 100% of the time where as the AC is charging on the back end.

So with testing 120v AC and overall safety, should I just get a decent quality Active differential probe??

n5yzv:
Good point on Neutral to Ground.  I did a quick test with a voltmeter to check first, nevertheless that current can show up when unexpected.   As I just posted before, I think the better route for testing is a Active Differential High(er) voltage Probe.

Navigation

[0] Message Index

[#] Next page

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