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Offline hussamaldean

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measuring mains with oscilloscope
« on: February 23, 2018, 11:38:40 am »
Hi all
what is the safest way to measure the mains with oscilloscope without ending burning my oscilloscope ?
and is it possible to build differential probe ?


Regards
 

Offline joeqsmith

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Re: measuring mains with oscilloscope
« Reply #1 on: February 23, 2018, 11:49:53 am »
Depending how many harmonics out you need, maybe just a stepdown transformer.   That is all I was using to measure AC power with my scope. 



 
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Offline Gandalf_Sr

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Re: measuring mains with oscilloscope
« Reply #2 on: February 23, 2018, 01:35:46 pm »
what is the safest way to measure the mains with oscilloscope without ending burning my oscilloscope ?
I know the purists will attack me for saying this - and I caution you that this is not recommended by any manufacturer - but the safest way to ensure you don't burn your oscilloscope is to disconnect the ground pin from the input. If you do this, you risk electrocuting yourself!

What most manufacturers do is connect the ground input pin to the ground/shield of the scope probes.  The problem can be that, if a mains outlet is wired incorrectly swapping live and neutral, or if you connect the ground of the scope lead to a live mains signal you will fry the scope because the live will travel through the ground of the scope, through the PCB, and down the ground pin of the power input cord  :-BROKE.

If you disconnect the ground from the input cord, then you've effectively made the ground+signal of your oscilloscope into a differential input because neither shield or signal on your scope probe is referenced to ground.

I keep a spare power cord with the ground disconnected for this very purpose.

WARNINGS
1. Mains voltages are lethal
2. Removing the ground means that the scope ground can be at mains voltage so be very careful in touching the oscilloscope when connecting the probes to mains - in fact, don't touch the scope when the probes are connected to mains is the best advice.
3. Check that your oscilloscope does isolate the power input from the scope ground - test with a meter from live and neutral in to the oscilloscope's ground / shield on front, it should be Meg Ohms
4. Check that your oscilloscope can handle the mains voltages (you may need to make sure the probes are on x10)
If at first you don't succeed, get a bigger hammer
 

Offline hussamaldean

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Re: measuring mains with oscilloscope
« Reply #3 on: February 23, 2018, 01:39:19 pm »
what is the safest way to measure the mains with oscilloscope without ending burning my oscilloscope ?
I know the purists will attack me for saying this - and I caution you that this is not recommended by any manufacturer - but the safest way to ensure you don't burn your oscilloscope is to disconnect the ground pin from the input. If you do this, you risk electrocuting yourself!

What most manufacturers do is connect the ground input pin to the ground/shield of the scope probes.  The problem can be that, if a mains outlet is wired incorrectly swapping live and neutral, or if you connect the ground of the scope lead to a live mains signal you will fry the scope because the live will travel through the ground of the scope, through the PCB, and down the ground pin of the power input cord  :-BROKE.

If you disconnect the ground from the input cord, then you've effectively made the ground+signal of your oscilloscope into a differential input because neither shield or signal on your scope probe is referenced to ground.

I keep a spare power cord with the ground disconnected for this very purpose.

WARNINGS
1. Mains voltages are lethal
2. Removing the ground means that the scope ground can be at mains voltage so be very careful in touching the oscilloscope when connecting the probes to mains - in fact, don't touch the scope when the probes are connected to mains is the best advice.
3. Check that your oscilloscope does isolate the power input from the scope ground - test with a meter from live and neutral in to the oscilloscope's ground / shield on front, it should be Meg Ohms
4. Check that your oscilloscope can handle the mains voltages (you may need to make sure the probes are on x10)
well, this case I will step down transformer
another question
is it possible to use this as current probe for oscilloscope ?
http://statics3.seeedstudio.com/assets/file/bazaar/product/101990028-SCT-013-030-Datasheet.pdf

regards
 

Offline Gandalf_Sr

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Re: measuring mains with oscilloscope
« Reply #4 on: February 23, 2018, 02:07:40 pm »
That can be used as a current probe but it only works up to 1 kHz.  If you want to go into MHz then you need a current probe.  Search eBay for "AM503B" (probe amplifier) and "A6302" which together will allow you to probe up to 20A and 50 MHz.  I own this setup and I got it for around $300 on eBay.
If at first you don't succeed, get a bigger hammer
 

Offline hussamaldean

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Re: measuring mains with oscilloscope
« Reply #5 on: February 23, 2018, 02:22:53 pm »
That can be used as a current probe but it only works up to 1 kHz.  If you want to go into MHz then you need a current probe.  Search eBay for "AM503B" (probe amplifier) and "A6302" which together will allow you to probe up to 20A and 50 MHz.  I own this setup and I got it for around $300 on eBay.
it is little bit expensive and if I need more than 1KHz, I will use current shunt resistance with 10X probe
 

Offline vk6zgo

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Re: measuring mains with oscilloscope
« Reply #6 on: February 23, 2018, 02:44:45 pm »
what is the safest way to measure the mains with oscilloscope without ending burning my oscilloscope ?
I know the purists will attack me for saying this - and I caution you that this is not recommended by any manufacturer - but the safest way to ensure you don't burn your oscilloscope is to disconnect the ground pin from the input. If you do this, you risk electrocuting yourself!

What most manufacturers do is connect the ground input pin to the ground/shield of the scope probes.  The problem can be that, if a mains outlet is wired incorrectly swapping live and neutral, or if you connect the ground of the scope lead to a live mains signal you will fry the scope because the live will travel through the ground of the scope, through the PCB, and down the ground pin of the power input cord  :-BROKE.

If you disconnect the ground from the input cord, then you've effectively made the ground+signal of your oscilloscope into a differential input because neither shield or signal on your scope probe is referenced to ground.

I keep a spare power cord with the ground disconnected for this very purpose.

WARNINGS
1. Mains voltages are lethal
2. Removing the ground means that the scope ground can be at mains voltage so be very careful in touching the oscilloscope when connecting the probes to mains - in fact, don't touch the scope when the probes are connected to mains is the best advice.
3. Check that your oscilloscope does isolate the power input from the scope ground - test with a meter from live and neutral in to the oscilloscope's ground / shield on front, it should be Meg Ohms
4. Check that your oscilloscope can handle the mains voltages (you may need to make sure the probes are on x10)

I'm not quite a purist, but I don't like this idea.
Without an Earth ( ground ) connection, the external metal parts of the Oscilloscope are free to take up whatever potential is present on the probe's "ground clip".

If you connect that clip to Mains "Active"( the "hot"wire), then touch the scope metalwork at the  same time as you touch some other piece of equipment with an intact Earth lead you will "zap"yourself.

If your Mains circuit is fitted with an RCD device, it will just be a quick shock before the device removes the Mains connection.
If not, you may be badly injured or killed

Sometimes, all we want to do is to have a "quick look", to see if the Mains is there, & is about the right amplitude.

In such cases, just remove the probe ground altogether, & rely upon the return circuit already existing via the 'scope Earth lead & the power point Earth circuit, back to where the Earth & Neutral are bonded at the building power entry.
This of course, will introduce errors if you are trying to look at higher frequency artefacts upon the basic 50 or 60Hz Mains waveform.
Another way is to, again without using the ground clip, use two channels as a " pseudo differential input ".
(Put, say, Ch1 probe on one leg, & Ch2 probe on the other ,& set your 'scope to add the two.

This does have problems, in that it is not always easy to balance the Channel gains, giving rise to errors, but it can be useful.
Obviously, the best ways to do it, are a 'scope designed for such testing, with differential input circuits, or a differential probe with a conventional 'scope.
 
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Offline hussamaldean

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Re: measuring mains with oscilloscope
« Reply #7 on: February 23, 2018, 02:46:05 pm »
what is the safest way to measure the mains with oscilloscope without ending burning my oscilloscope ?
I know the purists will attack me for saying this - and I caution you that this is not recommended by any manufacturer - but the safest way to ensure you don't burn your oscilloscope is to disconnect the ground pin from the input. If you do this, you risk electrocuting yourself!

What most manufacturers do is connect the ground input pin to the ground/shield of the scope probes.  The problem can be that, if a mains outlet is wired incorrectly swapping live and neutral, or if you connect the ground of the scope lead to a live mains signal you will fry the scope because the live will travel through the ground of the scope, through the PCB, and down the ground pin of the power input cord  :-BROKE.

If you disconnect the ground from the input cord, then you've effectively made the ground+signal of your oscilloscope into a differential input because neither shield or signal on your scope probe is referenced to ground.

I keep a spare power cord with the ground disconnected for this very purpose.

WARNINGS
1. Mains voltages are lethal
2. Removing the ground means that the scope ground can be at mains voltage so be very careful in touching the oscilloscope when connecting the probes to mains - in fact, don't touch the scope when the probes are connected to mains is the best advice.
3. Check that your oscilloscope does isolate the power input from the scope ground - test with a meter from live and neutral in to the oscilloscope's ground / shield on front, it should be Meg Ohms
4. Check that your oscilloscope can handle the mains voltages (you may need to make sure the probes are on x10)

I'm not quite a purist, but I don't like this idea.
Without an Earth ( ground ) connection, the external metal parts of the Oscilloscope are free to take up whatever potential is present on the probe's "ground clip".

If you connect that clip to Mains "Active"( the "hot"wire), then touch the scope metalwork at the  same time as you touch some other piece of equipment with an intact Earth lead you will "zap"yourself.

If your Mains circuit is fitted with an RCD device, it will just be a quick shock before the device removes the Mains connection.
If not, you may be badly injured or killed

Sometimes, all we want to do is to have a "quick look", to see if the Mains is there, & is about the right amplitude.

In such cases, just remove the probe ground altogether, & rely upon the return circuit already existing via the 'scope Earth lead & the power point Earth circuit, back to where the Earth & Neutral are bonded at the building power entry.
This of course, will introduce errors if you are trying to look at higher frequency artefacts upon the basic 50 or 60Hz Mains waveform.
Another way is to, again without using the ground clip, use two channels as a " pseudo differential input ".
(Put, say, Ch1 probe on one leg, & Ch2 probe on the other ,& set your 'scope to add the two.

This does have problems, in that it is not always easy to balance the Channel gains, giving rise to errors, but it can be useful.
Obviously, the best ways to do it, are a 'scope designed for such testing, with differential input circuits, or a differential probe with a conventional 'scope.
yeah
I scraped the whole idea :D
 

Offline tggzzz

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Re: measuring mains with oscilloscope
« Reply #8 on: February 23, 2018, 03:34:23 pm »
Hi all
what is the safest way to measure the mains with oscilloscope without ending burning my oscilloscope ?

What are you measuring and why? Frequency, amplitude, harmonics, etc?

Quote
and is it possible to build differential probe ?

Possible, yes. But if you have to ask it is not safe!
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline nctnico

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Re: measuring mains with oscilloscope
« Reply #9 on: February 23, 2018, 06:29:34 pm »
what is the safest way to measure the mains with oscilloscope without ending burning my oscilloscope ?
I know the purists will attack me for saying this - and I caution you that this is not recommended by any manufacturer - but the safest way to ensure you don't burn your oscilloscope is to disconnect the ground pin from the input. If you do this, you risk electrocuting yourself!

What most manufacturers do is connect the ground input pin to the ground/shield of the scope probes.  The problem can be that, if a mains outlet is wired incorrectly swapping live and neutral, or if you connect the ground of the scope lead to a live mains signal you will fry the scope because the live will travel through the ground of the scope, through the PCB, and down the ground pin of the power input cord  :-BROKE.

If you disconnect the ground from the input cord, then you've effectively made the ground+signal of your oscilloscope into a differential input because neither shield or signal on your scope probe is referenced to ground.
This is worst idea ever! There is nothing purist about not turning your test equipment into a death trap.

ALWAYS MAKE SURE YOUR SCOPE IS GROUNDED BECAUSE THE GROUND IS THERE FOR YOUR SAFETY!

The safe way to measure mains is to use a CAT rated differential probe which has been designed to measure the voltage levels you want to measure.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline Gandalf_Sr

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Re: measuring mains with oscilloscope
« Reply #10 on: February 23, 2018, 06:36:08 pm »
GROUNDS ARE OVERRATED! just kidding  :-DD
If at first you don't succeed, get a bigger hammer
 
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Offline BillB

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Re: measuring mains with oscilloscope
« Reply #11 on: February 23, 2018, 06:43:29 pm »
You can pick up a Micsig Oscilloscope 1300V 100MHz High Voltage Differential Probe kit DP10013 for around $140 shipped.  Is that too expensive?
 
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Online tautech

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Re: measuring mains with oscilloscope
« Reply #12 on: February 23, 2018, 08:22:42 pm »
You can pick up a Micsig Oscilloscope 1300V 100MHz High Voltage Differential Probe kit DP10013 for around $140 shipped.  Is that too expensive?
Bill, they as cheap as shit and why most don't have one for their own protection and the same for their hard earned scope escapes me !  :-//

Buying a scope is just the start, if you want to use its full capabilities AND safely, expect to have to spend more than was out-layed on the scope !
It has always been this way.  :(
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Offline Phil_Tech

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Re: measuring mains with oscilloscope
« Reply #13 on: October 25, 2019, 07:14:41 am »
I hadn't heard of the Micsig probes until I read this post - so thanks for the tip. :-+
I've just purchased one now on eBay for AUD 210 from local supplier in Melbourne, was cheaper than I could find from overseas sellers.
Should be a nice addition to the test gear... :)
 

Offline Terry01

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Re: measuring mains with oscilloscope
« Reply #14 on: October 25, 2019, 09:28:26 am »
what is the safest way to measure the mains with oscilloscope without ending burning my oscilloscope ?
I know the purists will attack me for saying this - and I caution you that this is not recommended by any manufacturer - but the safest way to ensure you don't burn your oscilloscope is to disconnect the ground pin from the input. If you do this, you risk electrocuting yourself!

What most manufacturers do is connect the ground input pin to the ground/shield of the scope probes.  The problem can be that, if a mains outlet is wired incorrectly swapping live and neutral, or if you connect the ground of the scope lead to a live mains signal you will fry the scope because the live will travel through the ground of the scope, through the PCB, and down the ground pin of the power input cord  :-BROKE.

If you disconnect the ground from the input cord, then you've effectively made the ground+signal of your oscilloscope into a differential input because neither shield or signal on your scope probe is referenced to ground.

I keep a spare power cord with the ground disconnected for this very purpose.

WARNINGS
1. Mains voltages are lethal
2. Removing the ground means that the scope ground can be at mains voltage so be very careful in touching the oscilloscope when connecting the probes to mains - in fact, don't touch the scope when the probes are connected to mains is the best advice.
3. Check that your oscilloscope does isolate the power input from the scope ground - test with a meter from live and neutral in to the oscilloscope's ground / shield on front, it should be Meg Ohms
4. Check that your oscilloscope can handle the mains voltages (you may need to make sure the probes are on x10)

THE OSCILLOSCOPE WILL BE FINE.....BUT YOU MY FRIEND....WILL BE SMOKED!!  :-BROKE

We'll throw it in beside you when we bury you. Your headstone will read " he saved the scope.. but the driver is TOAST"!   :palm:   :clap:

Too funny!  :-DD
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Offline Gyro

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Re: measuring mains with oscilloscope
« Reply #15 on: October 25, 2019, 09:34:57 am »
what is the safest way to measure the mains with oscilloscope without ending burning my oscilloscope ?
I know the purists will attack me for saying this - and I caution you that this is not recommended by any manufacturer - but the safest way to ensure you don't burn your oscilloscope is to disconnect the ground pin from the input. If you do this, you risk electrocuting yourself!

What most manufacturers do is connect the ground input pin to the ground/shield of the scope probes.  The problem can be that, if a mains outlet is wired incorrectly swapping live and neutral, or if you connect the ground of the scope lead to a live mains signal you will fry the scope because the live will travel through the ground of the scope, through the PCB, and down the ground pin of the power input cord  :-BROKE.

If you disconnect the ground from the input cord, then you've effectively made the ground+signal of your oscilloscope into a differential input because neither shield or signal on your scope probe is referenced to ground.

I keep a spare power cord with the ground disconnected for this very purpose.

WARNINGS
1. Mains voltages are lethal
2. Removing the ground means that the scope ground can be at mains voltage so be very careful in touching the oscilloscope when connecting the probes to mains - in fact, don't touch the scope when the probes are connected to mains is the best advice.
3. Check that your oscilloscope does isolate the power input from the scope ground - test with a meter from live and neutral in to the oscilloscope's ground / shield on front, it should be Meg Ohms
4. Check that your oscilloscope can handle the mains voltages (you may need to make sure the probes are on x10)

THE OSCILLOSCOPE WILL BE FINE.....BUT YOU MY FRIEND....WILL BE SMOKED!!  :-BROKE

We'll throw it in beside you when we bury you. Your headstone will read " he saved the scope.. but the driver is TOAST"!   :palm:   :clap:

Too funny!  :-DD

You are replying (unhelpfully!) to a post that is 18 months old and has been well covered.  ::)
Chris

"Victor Meldrew, the Crimson Avenger!"
 

Offline Terry01

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Re: measuring mains with oscilloscope
« Reply #16 on: October 25, 2019, 09:45:22 am »
And your reply to my post is helpful to the thread?  It's nearly the weekend....go on...live a little!  :palm:
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Offline Fungus

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Re: measuring mains with oscilloscope
« Reply #17 on: October 25, 2019, 11:26:33 am »
And your reply to my post is helpful to the thread?

Nope, it's supposed to be helpful to you, personally. Try reading what he said....

« Last Edit: October 25, 2019, 11:34:38 am by Fungus »
 
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Online mansaxel

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Re: measuring mains with oscilloscope
« Reply #18 on: October 25, 2019, 01:04:21 pm »
Hi all
what is the safest way to measure the mains with oscilloscope without ending burning my oscilloscope ?
and is it possible to build differential probe ?


Regards

You need a ScopeMeter 123/124 or similar. The right tool for the right job.

Online AVGresponding

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Re: measuring mains with oscilloscope
« Reply #19 on: October 25, 2019, 01:28:45 pm »
You need a ScopeMeter 123/124 or similar. The right tool for the right job.


I believe you missed out the first step in this process, which is either winning the lotto, or marrying a rich widow...   :-DD
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Offline Terry01

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Re: measuring mains with oscilloscope
« Reply #20 on: October 25, 2019, 02:36:10 pm »
And your reply to my post is helpful to the thread?

Nope, it's supposed to be helpful to you, personally. Try reading what he said....


Aye ok.... you win.   :=\
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Offline Fungus

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Re: measuring mains with oscilloscope
« Reply #21 on: October 25, 2019, 03:26:39 pm »
Aye ok.... you win.   :=\

It's not about anybody "winning", it's about somebody thinking to themselves, "Oh, yeah, so I did...", followed by the formation of a new neuron.

« Last Edit: October 25, 2019, 03:33:36 pm by Fungus »
 

Online mansaxel

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Re: measuring mains with oscilloscope
« Reply #22 on: October 25, 2019, 05:35:02 pm »
You need a ScopeMeter 123/124 or similar. The right tool for the right job.


I believe you missed out the first step in this process, which is either winning the lotto, or marrying a rich widow...   :-DD

Heh, no, the step is "Be present at the launch of the 123 and get one for free." That's what I did.  :-DMM

Offline oz2cpu

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Re: measuring mains with oscilloscope
« Reply #23 on: October 25, 2019, 08:32:29 pm »
here is what I do, when measuring stuff on a live mains:
power the scope via a special low capacity isolation transformator.
it was easy to make:
http://webx.dk/oz2cpu/230V-trafo-isolation-low-capacity/230V-trafo.htm
 
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Offline tggzzz

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Re: measuring mains with oscilloscope
« Reply #24 on: October 25, 2019, 09:32:56 pm »
here is what I do, when measuring stuff on a live mains:
power the scope via a special low capacity isolation transformator.
it was easy to make:
http://webx.dk/oz2cpu/230V-trafo-isolation-low-capacity/230V-trafo.htm

Stop thinking about how that might work, and think carefully about how it might fail.

Have a look at the safety references in https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 


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