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Author Topic: Viewing AC Line with Oscilloscope  (Read 1089 times)
W6EM
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Posts: 1860




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« Reply #15 on: October 09, 2018, 06:17:43 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.

...
Martin - G8JNJ

Martin:  Not to belabor things, but here in the US, the neutral is tied to earth/ground at the electrical panel.  Normally, only a few volts, if any potential at all on it, and indicative of serious wiring/grounding problems if there is.  Perhaps in the UK, the neutral isn't grounded in the residence so as to have more of an offset from ground.  The utility does bring in the supply transformer neutral, and it is also grounded by the utility but not very well, unless part of what is referred to as a common neutral (CN).  The CN is multi-grounded and contiguous back to the high voltage substation.  About the only way to see significant neutral voltage is via an open neutral path at some point ahead of the service panel which would allow the neutral to float and swing, based on 120-N-120 service drop supply.

The reason that I suggested only the hot leg and no connection to the neutral at all, even for his voltage divider, was to enable a GFCI (if he had chosen to use one) to rapidly trip, should something go wrong.  If the neutral were to be used in his measurement, any fault from hotleg to neutral would not be detected by the GFCI.  Only the branch circuit breaker.
 
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K4SAV
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« Reply #16 on: October 09, 2018, 06:40:43 AM »

There is a standard, safe way of making this measurement.  Put one scope probe on one side of the AC line and another scope probe on the other AC line.  Connect the scope ground leads to each other and to nothing else.  If your scope does differential just measure the differential.  If it doesn't, then invert the signal from one probe and add the two. 

Jerry, K4SAV
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G8JNJ
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« Reply #17 on: October 09, 2018, 10:07:14 AM »

The reason I suggested looking at the differential output is that it's a UPS.

So unless you know that one side of the output is connected to the same ground point as the scope you can't guarantee the reference point.

Also beware that if the scope is running from the mains and the UPS is unplugged from the mains, there won't be a common reference.

Regards,

Martin - G8JNJ



 
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W6EM
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« Reply #18 on: October 09, 2018, 12:10:52 PM »

The reason I suggested looking at the differential output is that it's a UPS.

So unless you know that one side of the output is connected to the same ground point as the scope you can't guarantee the reference point.

Also beware that if the scope is running from the mains and the UPS is unplugged from the mains, there won't be a common reference.

Regards,

Martin - G8JNJ



 
Excellent points.  He'd only have a reference if the scope itself was grounded to the UPS ground reference.
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W6EM
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« Reply #19 on: October 09, 2018, 12:14:42 PM »

There is a standard, safe way of making this measurement.  Put one scope probe on one side of the AC line and another scope probe on the other AC line.  Connect the scope ground leads to each other and to nothing else.  If your scope does differential just measure the differential.  If it doesn't, then invert the signal from one probe and add the two. 

Jerry, K4SAV
Jerry, I think you meant the probe ground leads for each probe.  True, if they're isolated from the frame of the scope, but the ones I have used shielded probe wire and the ground leads were connected to the grounded scope cabinet.  Perhaps not nowadays.
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G8JNJ
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« Reply #20 on: October 09, 2018, 01:15:39 PM »

Quote
Excellent points.  He'd only have a reference if the scope itself was grounded to the UPS ground reference.

Even then I have known UPS's with floating outputs when running unplugged, one side is only grounded on the output when running on mains (which provides the grounded and 'live' polarities). The chassis often actually sits at 1/2 supply potential when the UPS is unplugged due to the Y capacitor configuration in the input and output AC filter networks acting as a capacitive potential divider.

You have to be very careful when dealing with UPS's, especially in server rooms. If some kit is still supplied from the mains and some other kit is running from a UPS on battery, you can get very high peak voltages between racks because the two supplies are not synchronous. If you are running with 3 phase incomers you need to be especially vigilant.

Regards,

Martin - G8JNJ
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KE2KB
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« Reply #21 on: October 11, 2018, 08:20:02 AM »

When I measured the output of the UPS on battery, there was still ground references through the devices plugged into it.
My PC and Astron RS-35M are both bonded to my radio earth ground, which runs to the same ground rod system used by the electric service, so there is definitely continuity between the scope's ground and the UPS ground.
That said, I suppose that for safety purposes, it might be best to plug the scope into the UPS while making the measurement.

My shack is on an AF/GFCI breaker, so connecting a scope ground lead to the neutral would probably trip it and shut everything down.
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N4MQ
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« Reply #22 on: October 12, 2018, 04:31:46 AM »

You can also set the scope probe to the lowest setting and use the capacitive coupling to an insulated wire to see the line voltage by placing the probe in close proximity.  Not calibrated but the pick up should show the signal of the power line.  I use a spring loaded clip on probe for this, not the needle probe as it has more AREA. woody
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KE2KB
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« Reply #23 on: October 12, 2018, 05:28:47 AM »

You can also set the scope probe to the lowest setting and use the capacitive coupling to an insulated wire to see the line voltage by placing the probe in close proximity.  Not calibrated but the pick up should show the signal of the power line.  I use a spring loaded clip on probe for this, not the needle probe as it has more AREA. woody
It might also be fun to probe around with an open probe. I discovered an interesting pulse while probing my coaxial cables for RF. I had several turns of enameled wire wrapped around the coax and transmitting CW into a dummy load when I saw the pulse at about 4Hz. I couldn't figure it out, but when I went downstairs to the kitchen, I found that the stove's electronic igniter was snapping away. It does that (when at least one burner is lit) often when one or more burners are damp, or the humidity is high.
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AC5UP
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« Reply #24 on: October 12, 2018, 09:12:51 AM »

Hold the probe tip near a compact fluorescent lamp and see what happens..........

Although I like the flatness and color temperature of the light from the gooseneck above my workbench with a CFL, the hash it generates makes sensitive measurements problematic.  Not a fault of the lamp itself but more a case of proximity.  Back off a few feet and it's not an issue.  LED's are quiet but the light quality is harsher so you pays your money and takes your pick.........
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My favorite tool is a ball peen hammer.  Ask me how I think you should fix your radio.........   
KE2KB
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Posts: 1039




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« Reply #25 on: October 12, 2018, 02:24:40 PM »

Hold the probe tip near a compact fluorescent lamp and see what happens..........

Although I like the flatness and color temperature of the light from the gooseneck above my workbench with a CFL, the hash it generates makes sensitive measurements problematic.  Not a fault of the lamp itself but more a case of proximity.  Back off a few feet and it's not an issue.  LED's are quiet but the light quality is harsher so you pays your money and takes your pick.........
I haven't looked at a  CFL yet, but I do have several LED lamps, which, as you said, appear pretty quiet.
If I simply hold the probe up in the air and set the scope to 50mV/div, I can easily see both AM and FM radio stations. I can even see the modulation of one or more of the AM stations. I haven't turned on a radio to see which one it is, but there are towers only a mile or so from me.
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N7EKU
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« Reply #26 on: October 12, 2018, 03:36:16 PM »

Hmm,

Fluorescent light has the same problem as LED in that the actual light is produced only on a specific wavelength or two.  White light is then generated by the various coatings on the tubes or in the epoxy lens material.

Here's one look at some spectra of various light sources:  https://www.youtube.com/watch?v=xZOjVScxKeo

When used as house bulb replacements, LED's also often use switching power supplies just like CFL's.  I think the noise will mostly depend on the brand.

73,


Mark.
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Mark -- N7EKU/VE3
KE2KB
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Posts: 1039




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« Reply #27 on: October 12, 2018, 09:10:51 PM »

Hmm,

Fluorescent light has the same problem as LED in that the actual light is produced only on a specific wavelength or two.  White light is then generated by the various coatings on the tubes or in the epoxy lens material.

Here's one look at some spectra of various light sources:  https://www.youtube.com/watch?v=xZOjVScxKeo

When used as house bulb replacements, LED's also often use switching power supplies just like CFL's.  I think the noise will mostly depend on the brand.

73,


Mark.
That's a very interesting video.
I'm using Phillips LED bulbs; never buy the 'EcoSmart' or other economy brands.
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AC5UP
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« Reply #28 on: October 13, 2018, 10:03:28 AM »

https://www.nature.com/articles/d41586-018-06923-y
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My favorite tool is a ball peen hammer.  Ask me how I think you should fix your radio.........   
N7EKU
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Posts: 1012




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« Reply #29 on: October 13, 2018, 05:30:07 PM »

Very cool,

Thanks for the link!

73,


Mark.
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Mark -- N7EKU/VE3
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