Complete noob question from not a noob. - Measuring Mains AC with Oscilloscope

[Kyrad777]

Hey everyone!  I just moved up from a standard multimeter to a Siglent SHS806.  ( For those wondering why I chose this, I got it new for next to nothing.)  I want to verify my head is straight concerning a couple of things lest I release the magic fireworks. 

Anyway, here is the question I cannot find a straight answer to. 

I need to measure mains voltage ( and sine wave quality - Oscilloscope noob terminology ) over time, hence needing the oscilloscope.  I'm hitting a wall verifying that pushing 110 AC ( yes, A Yank-ette from the USA here ) through the X10 probe included won't release the magic smoke, or the magic firecracker.

 I'm hesitating because:

1. The probe seems a little small.  It says its rated at 300V ( 1X ) and  600V ( 10X ) but honestly the cable seems a little thin to me.  Its obviously not carrying a huge load, but it just seems like it should be a larger diameter for ( american ) mains.

2.  I am not finding anywhere in software where I can manually set the range to 150VAC.  I'd hate to set to auto and hear ...POP!

3.   In the Meter mode, it has a maximum measure scale displayed of 6 volts.   ( not the oscilloscope mode obviously ).

So these are my Oscilli-Noob questions.  I did search the forum, but I think I may lack the specific terminology to search correctly, and it might be this particular unit's documentation may be confusing.

- Am I paranoid about this?  The Oscilloscope inputs say 300V CAT II, so it seems like this should be fine.

- If not, do I need to use a dedicated probe to accomplish this safely?  I planned on using the probe in X10 mode, and only for about 5 minutes max. I am familiar with electrical safety so electrocution hazard is not an issue.



If anyone who owns one of these particular Siglent SHS 806 ( or 810 ) units ( or if you know the answer regardless ), can you PLEASE tell me if this is just me being overly cautious? or if I'm missing something?

Thanks Everyone!

[radar_macgyver]

You can never be too cautious around line voltages.

That said, the probe should attenuate the voltage by 10x, so the cable's only carrying 11V. The thing to watch for is to never connect the probe ground directly to the circuit under test. The "correct" way to do this is with a high-voltage differential probe. Without one, you can use two channels and the math mode to subtract one from the other. This of course won't work if you're interested in higher frequency components (beyond, say, few 10s of kHz).

[ez24]

Come on - a little explosion can be fun.  Go for it, after all maybe it is just a nuclear power plant behind those wires.  When I did this, the wires exploded like a firecracker.  I still remember this after 40 years, so it can make a good memory maker     (seems I never used that scope again)

Edit - The wire that exploded was the ground lead on the probe 

[tautech]

General advice to Oscilli-Noob's is DON'T do it.

Safety of the user is always the first concern.

The Siglent SHS8** series is quite capable of these measurements but are you?

First, nearly all use of any scope should be with 10X probe settings, this allows safer operation at elevated voltages and also permits use on higher voltages before "voltage de-rating with frequency" constraints apply. Understand also that within switchable probes (in fact most probes) there is a resistive voltage divider and the voltage "seen" by the scope will be the division ratio of the probe.

Now, mains Sine wave quality;
Know that it won't be perfect so why on earth would you need to confirm it?

[wraper]

Come on - a little explosion can be fun.  Go for it, after all maybe it is just a nuclear power plant behind those wires.  When I did this, the wires exploded like a firecracker.  I still remember this after 40 years, so it can make a good memory maker     (seems I never used that scope again)

Why should it explode? It is a portable scope, so it is not grounded.

[DmitryL]

I would start with a simpler device... Putting a hairpin into mains socket is a great fun and a great experience!

[Kyrad777]

Thanks everyone for the responses!

I should have waited until I was a bit rested before writing the original post.  I completely forgot about thew cable only carrying 11 volts via 10X ( Thanks radar_Macgyver ).  I think it might have been more accurate for me to say that I was concerned that a probe less that the width of a pencil would handle that step-down without any consequences ( again, coming from a ton of years with the "plain multimeter 1X probes" way of thinking ).  Its easy to get into a procedural rut with those. 

"wraper" - My initial concern came from Dave's experience plugging a supposedly "Auto" probe into mains and having it explode in his hand.  ( Video 30-something..38 maybe?  I forget. )  Anyway, Thats why I initially tried to manually set the unit in software and then couldn't find a way to do it...  Well, I did find a "CurA=" setting, but again being new to the scope I decided to check before diving in, Hence this entertaining post.

Thanks "tautech" for confirming the SHS8 series will handle this.  I am a bit confused why my hesitation on not being able to manually set the range for mains voltage somehow equals "not being ready".  If anything it says that I look at every factor I can before leaping, including asking people with way more years of experience if I'm not familiar with the particular scope. Not terribly important, I just don't see the correlation.

The sine wave question is a bit too complicated to explain on my break.  I'll post in the near future about what I'm dealing with.  Again, the term "Sine wave Quality" isn't 100% accurate for what I'm looking at.  I promise I will elaborate in the near future.  If what I suspect is happening is really happening, it will make for an interesting read.

If not, then a good laugh at me.  : )

ez24, Thanks.  not instilling me with a lot of confidence - ; )

DmitryL - Oddly enough, I saw my cousin do that when I was like 8 years old, Hence my healthy respect for Line level AC.

[Kyrad777]

Oops, Video 94 - "Near Death Multimeter Experience" is the video I was referring to, not 38.

[Kyrad777]

You can never be too cautious around line voltages.

That said, the probe should attenuate the voltage by 10x, so the cable's only carrying 11V. The thing to watch for is to never connect the probe ground directly to the circuit under test. The "correct" way to do this is with a high-voltage differential probe. Without one, you can use two channels and the math mode to subtract one from the other. This of course won't work if you're interested in higher frequency components (beyond, say, few 10s of kHz).

I almost glossed over the other part of your post.  I need to measure mains directly from the socket.  Long story short, I'm having issues with the voltage dropping dramatically when any significant load is applied to this particular circuit.  ( the lights in the room significantly dim when a microwave is used and once in a while it pops the circuit breaker.)  My initial thought was that the old wiring is introducing resistance and when the load increases the voltage decreases ( Hence the dimming lights and the desire to examine whats really going on ).  To clarify, I'm NOT measuring anywhere near the microwave or the microwave itself ( I don't want to be turned into a pile of dust anytime soon ).

Again, coming from the multimeter world, not using the ground ( or both probes ) normally results in no measurement. I only have a 10x probe at hand. Well, back to the drawing board. : )

Side note, Oddly, I have never seen a basic video on the correct procedure for doing this with a 10x probe.  Its surprising that in two days of searching the internet I have not come across at least a tutorial to verify procedure.  It's always mentioned that it can be done in a pinch, but no one seems to want to show it.

Ok, I've dumped on you enough.  Thanks so much for your response.

Anyway, Thank you for your time. I know this is beyond basic, I just want to verify before proceeding. 

[fpliuzzi]

As has been pointed out by radar_macgyver above, the "use two channels and the math mode to subtract one from the other" method of doing differential measurements with a scope is demonstrated in the following video. I used this method for a short period of time before I finally bought myself a LeCroy high-voltage differential probe.



I also found it beneficial to read the application note "Fundamentals of Floating Measurements and Isolated Input Oscilloscopes" from Tektronix. The app-note also mentions the "A minus B” Measurement Technique plus a lot of valuable info about safety (pdf below).

[tautech]

I'm having issues with the voltage dropping dramatically when any significant load is applied to this particular circuit.  ( the lights in the room significantly dim when a microwave is used and once in a while it pops the circuit breaker.) 

Does this symptom apply to other power outlets in your dwelling?

Loose mains connections in your internal or external wiring might be the first suspects.
Note: internal (switchboard/fusebox to outlet) could pose a fire risk and an electrician should be able to sort this for you quickly.
Line voltages can be safely monitored with a DMM for fault finding your mains voltage drop-out in your case.

[Richard Crowley]

So your presenting symptoms indicate dodgy or deteriorating mains wiring. Typically from joint or connector problems. It is not at all clear why looking at the waveform would be important, or why one would need an oscilloscope for this kind of problem. As @tautech observes, this is the kind of thing you can easily troubleshoot with a cheap DMM.  No putting your 'scope at risk for such a mundane task.

[Timpert]

Hmmm...

Tripping your circuit breaker usually is a symptom of drawing too much current from a circuit. Dodgy connections tend to increase circuit resistance, and thus to slightly reduce current draw from things like microwaves. This decreases the chance for a circuit breaker to trip (but increases the risk of fire). Are you sure the microwave isn't acting up? Have you tried this microwave on another circuit in order to see if the problem follows it?

There are testers on the market which will measure an outlet's internal resistance among other things. I don't know if this particular tester is available in the US, but I have played with a Fluke 1653B installation tester, and it allows you to quickly assess your mains condition at various places in the installation. If one of the outlets has a significantly lower short circuit current than all the others in your house, and this can not be attributed to exceptionally long wiring, then you may have a dodgy connection. Or a dodgy breaker, don't rule that one out! In order to spot a dodgy breaker, you can measure the short circuit current right before and right after the breakers (watch out, the incoming connection packs way more punch than an outlet in terms of short circuit current) to see if one deviates. Such an instrument is expensive, so consider borrowing/renting one. Or simply paying the electrician that comes attached to many of them, with the added benefit that (s)he knows how to use it properly!

[tggzzz]

You are right to be cautious. Asking questions is the way of avoidign the Dunning-Kruger effect.

For solid information on choosing scope probes, using them effectively and using them safely, see the links at https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

[Timpert]

Just checked the datasheet and the manual, and you should be fine with the supplied probes, although there's an important thing to watch out for: you may only attach the ground lead to a conductor that always carries less than 30 Vrms WRT ground. So that means that the right procedure to measure mains voltage with this scope is to connect the ground leads to safety earth, and use the probe tips to measure the voltages on line and neutral WRT ground. That is because the CAT II rating up to 400 Volts only applies to the probe tip. Using the method from fpliuzzi's post (the youtube video) is fine with the device you have from a safety perspective, assuming you use the probes that originally came with it. If you're using different probes, the CAT II rating and the rated voltage should be printed on the probe itself. Otherwise, don't assume it is safe.

[nour]

Come on - a little explosion can be fun.  Go for it, after all maybe it is just a nuclear power plant behind those wires.  When I did this, the wires exploded like a firecracker.  I still remember this after 40 years, so it can make a good memory maker     (seems I never used that scope again)

 

[ez24]

I missed that the OP's scope is a handheld and it was his first post.   Regardless of his reason in wanting to use it on household AC, this is what he wants to do and asked about this.

From the OP's last post, I almost think he was saying goodbye. I hope not and I apologize if I made fun of his question.

So I will continue ...  I do not have a handheld scope so what I would do is carefully read the manual.  Also post a new thread using the model of the scope in the title and ask owners of this scope to comment.  Also Google "measuring AC with a handheld scope".  Maybe call the company.

So my gut feeling it is ok provided all the settings are set to max on the scope and probe and the scale goes over his AC voltage. 

I hope someone that has a handheld will give the OP a serious response to his question.

To OP,
when I first came to this group, I was insulted and made fun of several times.  That is the nature of these things so stay and learn and take the good with the bad, mostly it is good, fun, and entertaining. 

If and when you do this you can share your experiences and show some photos of the wave forms.  Maybe you could get some of those control devices that work on AC and show those wave forms.
This might be interesting to others.

[tautech]

There is every chance the OP wants to log the mains droop and if this is the case yes the SHS series will do this however the right tool for the job is a logging DMM, which IIRC the SHS in DMM mode can do.

[Kyrad777]

Hey Everyone!

Thanks for all the responses.  I will write soon and explain why I'm leaning toward an oscilloscope for this particular function.  There are more symptoms and clues than I had time to list and explain in the last forum posts.

Just a couple of things..

1.  I don't think my profile lists it on screen, but I'm female.  I noticed a couple of posters referring to me as "he".  I'm not offended, just straightening out mis-information.  It's all just semantics anyway.

 2.  THANK YOU fpliuzzi for the video link.  There was definitely good information in it.  He didn't show actually measuring line voltage in the video.  I'm a visual learner.   I can listen to instructions, but I prefer to see it.  Not sure why no one wants to show this. If anyone knows of a video or web page showing this procedure, please let me know.

tautech - My room ( as far as  can tell ) has 2 separate runs on two separate circuit breakers.   When the circuit breaker blows, one wall still has power.  The weird part is that only one plug on the "bad line" can handle any sort of significant load.  The other three drop down to about 50VAC when a typical computer load is on it. Needless to say most power supplies don't like this at all.   Unless there are are multiple wire runs from the single circuit breaker ( this house was made in the 50's so its possible ) I'm not sure why that is.  All the plugs are new, so its not that.  Also, the wiring leading to the plugs looks new.

There is a reason I want to look at the actual waveform, not so much for the 60hz sine wave, but any noise that might be induced into that, possibly distorting it.  The way some of my video production equipment is acting leads me to believe there might be "dirty power" coming from somewhere.  Its a long story, but I will update when I have a chance and when I can present some facts.

Timpert - The microwave is new.  I have this problem with my computer I use for production also.  I use the microwave to test because its a 1180 watt beast ( not a 1000W model that is actually only producing 300W - to 500W ).  It draws a lot of current, so I tend use it for testing purposes.

ez24 - Im not saying goodbye and I'm not offended, I just didn't understand what the correlation was from what I said and what the response was.  I think its a male / female difference in the way we both think.  Not a big deal.  : )

tautech - I definitely want to log whats going on, I've been fighting with my landlord for several years about this.  Honestly, a visually "dirty" sine wave can potentially make a Non-tech judge understand visually that something is wrong.  Otherwise he or she has to rely on my interpretation of numbers.  I agree that a logging DMM is perfectly acceptable for this.  I just happened to pay about the same amount for the SHS806 I have.

tggzzz - Thanks for the great information.  I will be looking into that as time permits.

I have to get some sleep so I will sign off for now.  I'll update everyone with what I have found as time permits this next week.

Have a great week everyone!
Kyra

[tggzzz]

1.  I don't think my profile lists it on screen, but I'm female.  I noticed a couple of posters referring to me as "he".  I'm not offended, just straightening out mis-information.  It's all just semantics anyway.

Who cares In humans, the differences between one individual and another are much larger and more significant than those between one normal (i.e. average) male and one normal female. Havingg said that, "vive la difference".

Honestly, a visually "dirty" sine wave can potentially make a Non-tech judge understand visually that something is wrong.  Otherwise he or she has to rely on my interpretation of numbers.

They will fight whatever evidence you produce.

Be aware that the mains is no longer a decent sine wave; the tops are somewhat clipped and "less pointy" than they "should" be. To grossly simplify, DC electronic loads have an internal rectifier and capacitor, and current only flows into the capacitor when the mains voltage is higher than that on the capacitor.

[vk6zgo]

Kyra,if you are looking for a "droop" in Mains voltage,& are not interested in high frequency transients,you could use a power transformer,& monitor a low voltage secondary.

You should see a voltage change on the secondary proportional to that on the primary.
This will still show you what you are looking for,but the 'scope will be completely isolated  from the Mains.

The problem ,may be in finding such a transformer.
Older "wall-warts" used transformers,----in fact,some had a low voltage AC output ,so you could either modify a DC one,or if an AC one,use it as is.

[tautech]

Kyra, DMM mode Voltage trend measurement for the Siglent SHS800 series is described on P57 in the user manual.
http://www.siglentamerica.com/USA_website_2014/Documents/UserManual/SHS800_UserManual_UM03008-E03C.pdf

I'd suggest you become familiar with the UI with some LV measurements, say a PSU, before attempting mains unless you're confident you can do it safely.

[ftonello]

Here some reply about the matter.

This is the end of a power cord of a DVD that doesn´t have the 3rd pin (ground). I didn´t used the ground aligator to the neutral just because will be redudant. It is already connected to the mains power panel neutral BAR. But, of course, i previously detected with my multimeter continuity scale, which wire is phase and which wire is neutral!
My oscilloscope has a grounded outlet pin, so, to make simple Phase to Neutral measuring, just use your probe 10x to the Phase wire, (in this case the blue one...)
The other (black wire) is the neutral, that is connected to NEUTRAL BAR-----> that is connected to GROUND BAR (all at the mains breakers box of my house).



This is the plug to connect to the outlet. NBR5410 Brazilian standard for LV(arghhhh  ). Anymway.




The image below is the page of NBR5410 Brazilian standard for LV. In most of the places in Brazil, you can have 127V/220V - 60HZ in outlets.



Before taking measures, your need to know what system does it have in your installation. Otherwise, you are blind about it, and this is potenttialy  dangerous!!!!

In this case (Phase to neutral) you can have a safe measuring of your outlet...(Does anyone desagree with it the results are not accurate?)





By other hand,if you only have an outlet that delivers phase to phase and ground (220V) it is recomended that you use the method shown in the video. (diferential method), or use a 220/127 to 6V transformer, (from a old device), or whatever isolated transformer you have.

[tggzzz]

This is the end of a power cord of a DVD that doesn´t have the 3rd pin (ground). I didn´t used the ground aligator to the neutral just because will be redudant. It is already connected to the mains power panel neutral BAR. But, of course, i previously detected with my multimeter continuity scale, wich wire is fase and wich wire is neutral!

Bear in mind that neutral is very rarely at ground potential, and that large currents can flow if it is connected to earth.

You must read learn and inwardly digest the "praxis" references in https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/ Failure to do so is likely to result in loud noises followed by deathly quiet and foul smells.

[Shadetreeprops]

i know this is been answered, and i am a noob. but i read in my books...NO you should never measure mains AC with a scope, unless you use something called a wal wart...or risk buring your scope out..

but they also said in the book, if you want to measure mains AC is always just best to use the multimeter. so for my sake, if i ever get a scope i will not be putting it on mains AC to see the 60Hz osilation..unless i got a scope, and affored another one. and just wanted to watch it melt..but that would never happen i love my gizmos.