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Testing AC Line Quality with an Oscilloscope - Generator power quality

ronsimpson

Well-Known Member
Most Helpful Member
filament transformer ...…… they appear to be essentially identical
Good work. I was expecting 10 or 20khz bandwidth. What you show is beyond 100khz. I think I will make one.
I want to not put capacitance on the secondary winding. (1:1 probe) So a 10:1 probe should be good.
I do work at 100khz to 300khz and would like good bandwidth and the 60hz cores might not do that but even 20khz bandwidth has many uses.
I will modify a transformer to get 100:1 or 10:1. My head hurts if I have to work out 17:1. lol
 

The Electrician

Active Member
Good work. I was expecting 10 or 20khz bandwidth. What you show is beyond 100khz. I think I will make one.
I want to not put capacitance on the secondary winding. (1:1 probe) So a 10:1 probe should be good.
I do work at 100khz to 300khz and would like good bandwidth and the 60hz cores might not do that but even 20khz bandwidth has many uses.
I will modify a transformer to get 100:1 or 10:1. My head hurts if I have to work out 17:1. lol
Yes, the bandwidth is surprisingly wide, but I didn't want to claim too much. :( Split bobbin transformers aren't as good as the one shown, but the typical cheap filament transformer is usually concentric wound, and the results will be as I've shown. The first time I ever did this test, I was taken aback at just how wide the bandwidth is; it's a crappy old filament transformer, after all!

It's a good thing to know and provides a convenient, low cost way for the person with a new scope who wants to look at the grid waveform to do so safely.
 
Yes, the bandwidth is surprisingly wide, but I didn't want to claim too much. :( Split bobbin transformers aren't as good as the one shown, but the typical cheap filament transformer is usually concentric wound, and the results will be as I've shown. The first time I ever did this test, I was taken aback at just how wide the bandwidth is; it's a crappy old filament transformer, after all!

It's a good thing to know and provides a convenient, low cost way for the person with a new scope who wants to look at the grid waveform to do so safely.
Please forgive the question if it's uneducated as I'm not in your league when it comes to electronics.
But, are you saying it's "better" to use a filament transformer for the type of measurements I'm seeking as opposed to using two channels of the O-scope for doing it?

I ordered a 117v to 25v filament transformer and it arrived yesterday. But since I still have to make connections to live 120AC to the transformer, using the probes still seems safer......but again, you are the expert. Thanks.
 

The Electrician

Active Member
The primary of the transformer is specifically intended to be connected to the grid, and the common mode rejection is better.

The transformer also provides galvanic isolation--there is no pathway from the grid to your scope except through a magnetic field in the transformer. Any high voltage spikes (up to 1000 volts or so. Lightning could arc over between windings :() on the grid will not be applied to your scope. Give it a try.
 

ronsimpson

Well-Known Member
Most Helpful Member
are you saying it's "better" to use a filament transformer
Better? It is hard to measure better.
It is certainly safer.
Using two channels has some opportunities for errors. Both probes need to be of the same kind. Should use the same length of coax. The gain of each channel needs to set the same. Use the (-) not the (+) mode.

I was making prototype wall-wort power supplies. I got a box full of empty plastic boxes that plug into the wall. In some of these boxes I made circuits for testing the power line. (plug into the wall and then into a scope) There is no exposed wires.

What I was doing was testing power line communications. I was looking at 100khz and 250khz tones on the power ling. I built a circuit that isolates me from the line and then removes 50/60hz but passes high frequencies. (50khz to 20mhz) I needed to lock a scope onto 0.5 volts of 100khz and not see 120 volts of 60hz. It turns out there is lots of communications on the high voltage lines at 1mhz to 20mhz that we are not to know about.
---edited---
What I built looks like this but with coax comming out.
120194
 
I noticed that with my scope, both the waveforms from the A channel and the B channel look identical when viewed separately.

Why do I need to use 2 channels when I can see the exact same sine wave using just one probe?

Is it possible the waveforms could be different in some situations? Why wouldn't the waveform of the Neutral = the waveform of the Line (Hot)
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I noticed that with my scope, both the waveforms from the A channel and the B channel look identical when viewed separately.

Why do I need to use 2 channels when I can see the exact same sine wave using just one probe?

Is it possible the waveforms could be different in some situations? Why wouldn't the waveform of the Neutral = the waveform of the Line (Hot)
You've not read the posts - you're not using two channels, you're using two probes on ONE channel in differential mode.
 
You've not read the posts - you're not using two channels, you're using two probes on ONE channel in differential mode.
Or...maybe I read the posts...and didn't comprehend them.

Maybe I'm misunderstanding the way to connect the probes?
My scope doesn't have a place to connect two probes on one channel. My only option is to connect one probe to channel A and another to channel B (or C or D since there are 4 channels)

That said, I have little experience using scopes and bought this on a whim as a hobby addition to my "tool" collection so it's very possible that I don't even know how to properly connect it. Truth be told. I am NOT an electrician or have any formal electronics training or experience. So a little assistance would be appreciated here.
My areas of 'expertise' are engine building and Information technologies. Jack of many trades, master of few.
Meantime, I'll go to youtube and watch more videos on using an oscilloscope. I already watched a dozen or so.

IGNORE SETTINGS. I JUST CLEANED THE FACE OF THE SCOPE AND ALL SETTINGS ARE RANDOM

HitachiV1050F2.jpg
 
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Nigel Goodwin

Super Moderator
Most Helpful Member
You switch both inputs to one trace, using the Mode switch - set it to ADD at the bottom. This then adds both inputs together and gives a single trace. Then just to the right of the Mode switch is a button labelled CH2 INV, pressing that inverts CH2, so the trace then shows the DIFFERENCE between the two.
 

ronsimpson

Well-Known Member
Most Helpful Member
Assuming you live where line=120vac and neutral = near 0vac.
If there is no load in your house then neutral = 0vac. Load causes voltage loss in the wires. This causes neutral to have a small voltage. Probably less than 5 volts but we don't know.

Your scope measures voltages from line to scope-ground, which should be the same a green wire ground.
Your load only seen line to neutral. It does not see ground at all.
The problem is your load is looking line-neutral while the scope is looking line-ground.
To make a good test the scope needs to see line-neutral.
Probably: Your scope will see 120vac on line-ground and will see 3 vac on neutral-ground. (I just make up the "3" it could be larger or smaller)
Use two probes. Line on A and neutral on B. The scope has a mode where you see A+B or A-B. The A-B should give you line-neutral where ground is not in the equation.
There is a button in the bottom center of the scope. "CH2 INV" This inverts ch2 or what I called "B". You want to add A and -B.
I can not see well in the picture. I do not know if it is under "MODE" or "DISPLAY" But there is a way to add A and B. (ch1 + ch2) You want to (ch1+ -ch2).
Probably MODE is X/Y which you do not want. Maybe DISPLAY. In add ch1 & -ch2 mode there will be only one trace!
----edited---
You might also want to push "20mhz limit" button. This will remove some very high frequency noise that you don't care about.
 

ronsimpson

Well-Known Member
Most Helpful Member
To learn your scope. Connect both probes to the "0.5 cal post in the middle of the scope.
Now look at only ch1. Only ch2. Both ch1 and ch2 which should give you two traces.
Now try to find the ch1-ch2 mode. If you are adding ch1 and ch2 you should get one trace that is twice as big. If you are substracting the you should see a single flat line. (assuming the volts/div knobs are set the same)
 
I'm wondering if maybe my Oscilloscope is working properly? I can't seem to get any clean Sine waves from house AC or Inverter generators.
Here's what I got when connected single channel to a Honda EU2000i inverter generator.
Notice that scary looking spike.....and the really noisy looking curve.....
I find it hard to believe I actually have such noisy home AC and my Honda inverter generator is producing that ugly AC.
That's a $1000 generator so I expect really clean power for that price.

Kinda thinking these new (cheap, ebay) probes are as cheap in quality as their price ($12.00)

Those spikes are pretty serious, no?

HondaEu2000is_Capture.JPG

And here's what I got from a single channel probe of House AC current using single channel....
 
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Here are the specs on the probes I have.

P6100 2PCS Digital Oscilloscope Probe 100MHz BNC Test Lead for Fluke Tektronix

Features and specifications:

Adjustable oscilloscope probe that is compatible with the BNC interface, digital oscilloscopes, virtual oscilloscopes, handheld oscilloscopes, etc.

Model: P6100
Bandwidth: DC-100MHz
Rise time: 3.5ns
Attenuation ratio: 1X/10X
Input resistance: 1MΩ/10MΩ±2%
Input capacitance: 1X:70pF-120pF, 10X:13pF-18pF
Maximum input: 1X-200 working voltage (Vp-p),
10X-600 working voltage (Vp-p)
Compensation range: 14 - 35pF
Operation environment: 0-50℃ ,0-80%RH
Storage environment: -20-60℃ ,0-90%RH
Size:110 ± 2cm
Weight: About 130g/set
 

Reloadron

Well-Known Member
Most Helpful Member
A few things aren't right or at least look peculiar. The last images you posted your scope looks like it is in a delayed sweep mode.
From your scope's user manual:
INTEN Although the sweep on the screen is A sweep it indicates B sweep (delay time sweep) by intensity modulation.

The image sort of reflects an intensified portion. Not sure why that is.

Both images should have a much, much cleaner sine wave, similar to the image I posted.

I have never looked at the output of an inverter type generator but like your house mains voltage I can't believe it would be that bad. Since they use an inverter I would expect a little noise but not to that extent.

This should be your scope's user manual.

If you look at pages 27 and 28 they cover modes:
o CHI Only the input signal applied to CHI is displayed.
o CH2, I X-YI Only the input signal applied to CH2 is displayed.
o ALT CHI and CH2 signals are displayed alternately on consecutive sweeps.
o CHOP CHI and CH2 signals are displayed simultaneously by switching between channels at about 250 kHz rate.
o ADD Displays the algebraic sum of the channel I and channel 2 input signals. If the channel 2 display is inverted (press CH2 display results. CH2 INV), an CHI minus
CH2 INV Inverts the polarity of the channel 2. Useful to comparison of two signals of opposite polarities, and observation of a differential signal of CHI and CH2 along with ADD mode.

Those last few are what we have been getting at. Since the outer shell of your BNC vertical input connectors are at chassis ground using both vertical channels displayed as a single channel can help eliminate a potential shock hazard.

Also page 34 of the linked manual:

CAL O.5V Output terminal of calibration square wave of about 1kHz and O.5V. It has a tip terminal. It is used to calibrate the probe combination .

You may want to try your probes on that since it should be a known signal and see how the display looks. Something isn't right and that needs solved before we worry about looking at signals. Something else to consider is a generator output really only needs to be as good as its intended use or application. I use a few UPS (Uninterruptible Power Supply) which have a less than stellar MSW (Modified Sine Wave) output. Their only purpose is to maintain several electronic devices for a 30 second period until the backup generator is online. Despite the ugly output waveform the supported electronics do fine.

Ron
 
Thanks. Again, very helpful
when I use the .5v connection on AC, I get a modified sine wave that looks like this.....

___.......___.......___.......___
.......___.......___.......___
 

Ylli

Active Member
Without reading through the entire thread, this looks like the lack of a ground. If you are using differential input to the scope, the source still has to have some kind of a ground reference or the noise will get though.

Tie the safety ground and the neutral together and connect the scope probe ground clip to that point. Then connect the scope probe tip the the hot output. Look at it singled ended. Worst that could happen is that you could blow a fuse/pop a circuit breaker in the generator/inverter/wall outlet.
 

Reloadron

Well-Known Member
Most Helpful Member
Thanks. Again, very helpful
when I use the .5v connection on AC, I get a modified sine wave that looks like this.....

___.......___.......___.......___
.......___.......___.......___
That's actually a square wave and what you should see so we can assume the probe(s) are fine. A MSW looks slightly different but really here nor there. With the scope probe ground clip on neutral and the probe tip connected to 120 VAC wall outlet hot you should see a nice relatively clean sine wave.

Ron
 
That's actually a square wave and what you should see so we can assume the probe(s) are fine. A MSW looks slightly different but really here nor there. With the scope probe ground clip on neutral and the probe tip connected to 120 VAC wall outlet hot you should see a nice relatively clean sine wave.

Ron
ok. I've been afraid to connect the ground and probe tip at the same time. I've been leaving the probe ground clip totally not connected to anything except the outer rim of one of the other channels BNC connector.

Tomorrow I will try the connections as you just described....ground to neutral and probe tip to Hot. Will report back.

Thanks again for all your help.
 

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