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Author Topic: Viewing AC Line with Oscilloscope  (Read 2086 times)
KE2KB
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Posts: 1085




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« on: October 07, 2018, 09:09:15 AM »

Hi;
I just purchased a Siglent SDS1104X-E oscilloscope, and one of the things I would like to use it for is to look at the 120VAC line voltage my poco is supplying me, as well as that which is generated by my portable generator, and my APC UPS.

I don't need this fixture to be calibrated - just safe.

There are two methods I am considering:
A) Step-Down transformer
B) Resistor Divider

The Step-Down transformer method would provide the best safety, but how much high frequency noise will get through it?
I don't believe I have any wall-wart transformers that provide an AC output, so I have nothing to go on at the moment.

If I do need a resistive divider, I would go with 10:1.
I would want to protect myself and the scope from any failure that would cause line voltage to appear at the output terminals.
My thoughts are to use a 25V MOV with the mains being fused to a very low current.
Then I may want to add wiring fault detection, in case the receptacle I plug into is wired incorrectly.
I can build the same circuit used by those $5 testers sold at DIY stores, or I could buy that device, take it apart and wire it into my circuit.
Or I could just use the $5 wiring tester I already own before I plug in my divider.

I will have three terminals for output.
V-out
Neutral
Ground

I probably don't need the neutral, but my thinking is that I might want to check for noise between ground and neutral as well as from line to ground.
I would of course never attach my scope's ground clip to the neutral terminal - only the ground. I am well aware of the dangers of using neutral as ground reference for a scope (which is of course grounded through its EGC)

I intend to build this fixture into a standard non-metallic junction box, with the output terminals mounted on a blank cover plate (non-metallic). I would put the fuse inside, since I don't expect to be changing it frequently, and if it were to blow, I would need to open the box anyway to check for what caused it to blow.

And finally, should I trust components bought on Ebay, or should I spend a little more and go with Mouser?

Thanks for your input
Frank <KE2KB>
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WA3SKN
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« Reply #1 on: October 07, 2018, 03:01:07 PM »

You are "over-thinking" again!
Most scopes come with a 1Megohm/10 Megohm probe for input.
Expose the 120v metal contacts.  Touch the tip side of the probe to a contact... view results.  The ground return of the probe need not be connected, as the ground return is via the scope.
Either 1 meg or 10 meg setting will work for this, just don't SHORT the wires and you will be fine.
73s.

-Mike.
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W6EM
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Posts: 1941




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« Reply #2 on: October 07, 2018, 04:18:56 PM »

A couple of comments.  First, don't use an MOV.  At least one with an MCOV below 200V.  That is, unless you don't care about it clipping any transients that you are looking for.

Also, even with a 10:1 divider in your scope probe, you will be looking at 12V RMS, or about 34V peak to peak.  Hopefully your scope has a range of around 5V/cm or higher, else you may still want to employ another voltage divider.
 If you are worried about being shocked while doing your measurements, you could insert a GFCI receptacle between the power utility or generator source.... Make your own out of a box and short cord, but make sure that you have a ground path via the grounding conductor (green wire) to ground for the generator.  That way, if you or your apparatus were to cause more than 2 or 3 mA to flow when connected to ground, then the GFCI will trip.  That will only work if you do not connect the neutral as part of your voltage divider ground reference.  Assuming no wiring errors inside the oscope that would ground the neutral internally, you should be OK.  The GFCI is a differential current device and only will trip if ground current flows exclusive of neutral current.

GFCIs have been used as inexpensive protection for generator over voltage by using a zener, mov or transorb to cause ground current and trip the GFCI above a given level.  QST had such a project back a few years......
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KE2KB
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« Reply #3 on: October 07, 2018, 04:35:42 PM »

You are "over-thinking" again!
Most scopes come with a 1Megohm/10 Megohm probe for input.
Expose the 120v metal contacts.  Touch the tip side of the probe to a contact... view results.  The ground return of the probe need not be connected, as the ground return is via the scope.
Either 1 meg or 10 meg setting will work for this, just don't SHORT the wires and you will be fine.
73s.

-Mike.
You are right. My scope has 10V/cm, and switchable 1x/10x probes. The max input rating on the probe in 10x is 300V RMS. Even at 1X the rating is 150V RMS. I just need to be sure the probe is in the 10X position, which is where I normally leave the switch.
I also have a portable GFCI which I can plug in.
Funny thing is that when I was in my 20's and owned a cheap Heathkit scope, I used it to measure line voltage by doing exactly what you recommend, and never thought twice about it.
But now that I'm 30+ years older than that inexperienced kid, I am afraid to do these things. And that's not because I've had any bad experiences. I have never been seriously shocked by AC mains, or blown up any equipment.
I used to work on H.V. equipment that was a lot more dangerous than the AC mains.
And it's not as though I will be making this sort of measurement every day.

Thanks for helping me get my head straight about this.


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K6BRN
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« Reply #4 on: October 07, 2018, 06:42:38 PM »

I'm just going to disagree with Mike.  Thinking is GOOD.

While you CAN get away with touching the 10x probe tip to the hot lead of 120VAC (RMS) mains on SOME scope/probe combinations, it's just an unsafe condition.  They were not intended to withstand the peak voltages often encountered on AC lines during transients (A/C startup, for instance) AND since the only thing between your scope and a LOT of current flow during a fault is a 15 or 20 amp breaker, guess which is going to pop first if something goes wrong.

Do NOT connect the probe ground, especially to the neutral lead of the A/C line, or you may very well see some sparks fly.

This question has been asked and answered before - the safest approach (basic lab safety) is to 1.  Use a high voltage differential scope probe built for this purpose (most expensive route), 2.  Use a high voltage (non-differential) probe (about $70 - it's basically a resistive divider), 3.  Use  a step-down transformer, which "floats" the hot and neutral wires and limits current and voltage both, or 4.  Build your own resistive divider.

Plugging your scope probes directly into an A/C outlet is not a terribly bright thing to do.

Brian - K6BRN
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AC5UP
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« Reply #5 on: October 07, 2018, 08:18:52 PM »

I have never been seriously shocked by AC mains, or blown up any equipment.

Consider this for a moment.............

Could be a completely irrelevant thought, but I've spent many, many hours tooling around the more popular interstate highways of the western persuasion with only two wheels on the ground.  Every spring I'll see at least one goober riding around town on a 250 enduro or similar wearing shorts, tee shirt, sneakers and sunglasses.  I cringe.  That's because I was taught to always dress like you're going to kiss the asphalt at 50-ish miles per hour whenever you swing a leg over a motorcycle.

Because............   Some day.............   You Will.      Even if it's only a beer run, and even if you've never dropped the bike before.

Whenever I work with voltages high enough to make me twitch I assume they're looking for the first chance I give them to do exactly that.
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KE2KB
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« Reply #6 on: October 07, 2018, 08:22:43 PM »

Well, after all said and done, I did connect the 10x scope probe directly to the hot side of the AC line. I left the ground lead disconnected.
Neither I nor my scope suffered any ill effects. I was extremely careful when attaching the probe. I did so on a disconnected cord, then when it was securely connected I plugged the cord into my APC Back-UPS BX1350 and ran tests on AC and on battery.
The results I got were very interesting, and a little confusing.

This is the AC line from the APC Back-UPS BX1350 while running on AC.




This is the waveform when the Back-UPS is unplugged from AC, running on battery.



I understand that the waveform generated by the Back-UPS is an approximated sine wave, but why is the amplitude only half of what the line waveform is?

Note that I took this reading at the end of a 25ft extension cord plugged into a 2nd surge protector strip which is itself plugged into the APC Back-UPS. There was no load on the extension cord - I used a 'dummy plug' which I normally use for my ESD strap (which of course I did not wear during this test).
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KE2KB
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« Reply #7 on: October 07, 2018, 08:31:24 PM »

I have never been seriously shocked by AC mains, or blown up any equipment.

Consider this for a moment.............

Could be a completely irrelevant thought, but I've spent many, many hours tooling around the more popular interstate highways of the western persuasion with only two wheels on the ground.  Every spring I'll see at least one goober riding around town on a 250 enduro or similar wearing shorts, tee shirt, sneakers and sunglasses.  I cringe.  That's because I was taught to always dress like you're going to kiss the asphalt at 50-ish miles per hour whenever you swing a leg over a motorcycle.

Because............   Some day.............   You Will.      Even if it's only a beer run, and even if you've never dropped the bike before.

Whenever I work with voltages high enough to make me twitch I assume they're looking for the first chance I give them to do exactly that.
So that's why most motorcyclists wear their leather even in summer. I have never been on one, and never will, but that's a matter of choice.
I have been in rather dangerous situations though; snow skiing for one. Everyone will wipe out many times, and once in a while it's gonna hurt. I nearly killed myself at Jackson Hole WY when I lost control on a double black diamond slope with icy moguls. My skis weren't tuned very well, and my edges didn't hold me, so I lost it. I got very, very lucky in that I landed in deep, soft powder. But as I went down, I lightly bumped the back of my unhelmeted head. No injury, but it scared the hell out of me, and the next day I bought a helmet.
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AC5UP
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« Reply #8 on: October 07, 2018, 08:38:32 PM »

What you're seeing is very typical of a non-sine UPS intended for temporary duty.  You were also looking at a no-load condition.

Remember your AC theory that a true sine wave has a peak value that lasts only for an instant.  The rest of the time it's either rising or falling, the modified square wave you've observed holds a peak value both + and - for a much longer period of time.  Integrate the area of the waveform and it's not that much different in terms of the Root Mean Square value.

Research basic AC theory and RMS Value on the web and you should find a better explanation than mine.

Most equipment can tolerate an ugly sine wave long enough to get through a power glitch, but a transformer won't do much during the flat peaks.  You may want to try the same with an appropriate load on the UPS like an incandescent lamp in the 100 watt range to see if it makes a difference.
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AC5UP
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« Reply #9 on: October 07, 2018, 08:41:49 PM »

I got very, very lucky in that I landed in deep, soft powder.

Two words:   Sonny Bono
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KE2KB
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« Reply #10 on: October 07, 2018, 09:19:51 PM »

I got very, very lucky in that I landed in deep, soft powder.

Two words:   Sonny Bono

Ouch!
Didn't that happen at Deer Valley Utah?
I have skied there a few times. Best groomed slopes in the USA!
Same thing happened to one of the Kennedys. From what I recall, they were 'playing football' on the slopes.

My Back-UPS was slightly loaded, as I had my PC running at the time I made the measurements. But the PC wasn't doing anything 'strenuous', so not presenting much of a load.
I want to look at the output of the PC's PSU while it's running on the Back-UPS. After all, that's really what matters to the PC.

I think I will still build the resistive divider, as it will make me feel a lot more comfortable if I want to run anything more than a brief test of the line.
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G8JNJ
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« Reply #11 on: October 08, 2018, 01:25:54 AM »

but why is the amplitude only half of what the line waveform is?

Think about how you are measuring the voltage - you have the x10 probe connected to the AC line but what is the ground reference ?

The best way to do this is to use two channels with two matched x10 probes and the scope set to differential mode.

Each probe goes to each side of the AC receptacle.

If you use transformers or almost any other coupling mechanism (other than a resistive potential divider) it is likely to distort the voltage wave form in some way. If you look at the output of the UPS when feeding different types of load, you will most likely find that the stepped pseudo sinusoidal waveform changes shape fairly significantly depending upon the load impedance (power factor) and current draw.

You may also wish to take a look at the harmonic content by using the scope as a spectrum analyser in it's FFT mode.

Regards,

Martin - G8JNJ



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KE2KB
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Posts: 1085




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« Reply #12 on: October 08, 2018, 05:44:55 AM »

but why is the amplitude only half of what the line waveform is?

Think about how you are measuring the voltage - you have the x10 probe connected to the AC line but what is the ground reference ?

The best way to do this is to use two channels with two matched x10 probes and the scope set to differential mode.

Each probe goes to each side of the AC receptacle.

If you use transformers or almost any other coupling mechanism (other than a resistive potential divider) it is likely to distort the voltage wave form in some way. If you look at the output of the UPS when feeding different types of load, you will most likely find that the stepped pseudo sinusoidal waveform changes shape fairly significantly depending upon the load impedance (power factor) and current draw.

You may also wish to take a look at the harmonic content by using the scope as a spectrum analyser in it's FFT mode.

Regards,

Martin - G8JNJ




Thanks for the recommendation. Before I do any more line-voltage measurement, I want to build the resistive divider; just to be safe from 'brain farts' I sometimes have, especially late at night... hi.
I was also thinking I could use the integrate function.
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WA3SKN
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« Reply #13 on: October 08, 2018, 04:24:30 PM »

You have a "modified sine wave" UPS system.
You will want a true sine wave system for most all electronics.  There are a lot of harmonics in you UPS output (read that as noise!).  And now you can see why!
Scopes are useful to have and use.
73s.

-Mike.
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W6EM
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« Reply #14 on: October 08, 2018, 05:26:09 PM »

I'm just going to disagree with Mike.  Thinking is GOOD.

While you CAN get away with touching the 10x probe tip to the hot lead of 120VAC (RMS) mains on SOME scope/probe combinations, it's just an unsafe condition.  They were not intended to withstand the peak voltages often encountered on AC lines during transients (A/C startup, for instance) AND since the only thing between your scope and a LOT of current flow during a fault is a 15 or 20 amp breaker, guess which is going to pop first if something goes wrong.

Do NOT connect the probe ground, especially to the neutral lead of the A/C line, or you may very well see some sparks fly.

This question has been asked and answered before - the safest approach (basic lab safety) is to 1.  Use a high voltage differential scope probe built for this purpose (most expensive route), 2.  Use a high voltage (non-differential) probe (about $70 - it's basically a resistive divider), 3.  Use  a step-down transformer, which "floats" the hot and neutral wires and limits current and voltage both, or 4.  Build your own resistive divider.

Plugging your scope probes directly into an A/C outlet is not a terribly bright thing to do.

Brian - K6BRN
A GFCI has what is called an inverse time characteristic.  That is to say, the higher the sensed current magnitude, the less time it takes to trip.  It took me a while to find a curve that demonstrates this, but here is one for a GFCI element on a molded case panel breaker.

  https://apps.geindustrial.com/publibrary/checkout/GES-6200?TNR=Time%20Current%20Curves%7CGES-6200%7CPDF&filename=GES-6200.pdf

As you can see, for currents under 3 mA, the time can be very long.  Whereas, for 50 mA, only about 12 milliseconds.
The devices must trip above 5mA.  At that value of ground fault current, the trip time is approximately 100ms.
I’m not advocating doing anything without safety in mind.
 
As for your claim of transient over voltage on an A/C compressor start, you’ve got your transients upside down.  On start, there is a huge sag in incoming voltage.  If, on disconnection, the contactor opens at anything but a zero crossing, there will be a transient voltage by the motor winding via Lenz’s Law.  And, the transient will be on the open side of the contactor contacts, not the power system side.

Sure, a resistive divider is safer.  As is an HV probe.  And, at an input impedance of 75 megohms and an output impedance of 75K, and an outlay of maybe $50, if he can find one for that, isn’t trivial.  Still, you’re sticking a probe end into a hot outlet.  And no, very likely that if he were to accidentally touch a metal part, a 15A branch circuit breaker probably wouldn’t trip since it would take at least 30A of fault current to get a fast breaker trip.

Power systems these days have lots of current distortion, caused by switching supplies.  And, current distortion creates voltage sine wave distortion.

As for the UPS produced square wave, the more sophisticated UPSs generate pulse trains of varying magnitude and pulse widths to more closely approximate a sine wave.  And, some use low pass filters to help smooth out the output.  Inexpensive UPSs are designed to supply switching mode power supplies, not transformer loads and motors.


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