Oscilloscope Floating using UPS

[JJ_023]

So, this is more of a thought experiment to learn from. 

If you took a name brand scope, for example RIGOL MSO5074 and wanted to make it a floating oscilloscope.  For whatever the reason one may want to do that. Could the following method be employed?

Taking a battery backup UPS that is fully charged.  Unplugging it from mains and securing the plug in a way that would prevent accidental contact with anything.  Then plugging the scope and running it off the UPS without any other connections to the UPS.

Would this work and what would be some potential concerns in doing this?

Thank you in advance for any discussion on the topic.

[CountChocula]

I think it might work*, but it would be dangerous. If one of your ground leads gets connected to mains hot, your 'scope is now referenced to 110 or 220V. If your body has a path to earth and you come into contact with the chassis, it will be an… electrifying experience.

Better to float the DUT using an isolation transformer, IMO.


* Might because, while on battery power, most UPSs output at best an approximation of the mains sine wave, and so your 'scope might end up picking up all kinds of noise from it, to the point of being unusable. Not sure, though—I've never tried it.

[JJ_023]

I think it might work*, but it would be dangerous. If one of your ground leads gets connected to mains hot, your 'scope is now referenced to 110 or 220V. If your body has a path to earth and you come into contact with the chassis, it will be an… electrifying experience.

I am aware of this. Thank you for mentioning that.

Better to float the DUT using an isolation transformer, IMO.

Even better would be differential probes.  But that is not the thought experiment I am conducting.  Besides the dangers associated with doing something like this I am wondering if it would work.

* Might because, while on battery power, most UPSs output at best an approximation of the mains sine wave, and so your 'scope might end up picking up all kinds of noise from it, to the point of being unusable. Not sure, though—I've never tried it.

This would be my main concern. I am fully aware that it is an approximate sine wave.  However the oscilloscope internally primarily runs on DC and the oscilloscope itself is designed to handle mains which is not necessarily all that clean at times.  So I am not sure if that would affect the oscilloscope or not.

[Andy Chee]

I’ve used an oscilloscope from a petrol inverter generator before, though not for the specific purpose of floating isolation. YMMV.

[JJ_023]

I’ve used an oscilloscope from a petrol inverter generator before, though not for the specific purpose of floating isolation. YMMV.

Could you elaborate on any issues with that set up?

[MLXXXp]

This would be my main concern. I am fully aware that it is an approximate sine wave.

There are UPSs available that output a true sine wave. They are usually more expensive than an equivalent with the more common stepped approximation output but aren't hard to find.

Just one example of many:
https://www.apc.com/ca/en/product/BR1000MS/apc-backups-pro-1000va-600w-tower-120v-10x-nema-515r-outlets-sine-wave-avr-usb-type-a-+-c-ports-lcd-user-replaceable-battery

[Andy Chee]

Could you elaborate on any issues with that set up?

I do use a small power line conditioner as standard practice with any sensitive electronics connected to the generator, which would’ve cleaned up the noisy mains considerably as far as interference and “squareness” is concerned.

https://www.poweronaustralia.com.au/wp-content/uploads/2014/07/Sola-200-210-Brochure.pdf

https://cdn.hackaday.io/files/21242908817728/Oneac_CB_Series.pdf

[pdenisowski]

* Might because, while on battery power, most UPSs output at best an approximation of the mains sine wave, and so your 'scope might end up picking up all kinds of noise from it, to the point of being unusable.

This is the main application (in my experience) of a line trigger (i.e. the scope triggers on the peaks of the AC mains supply waveform).  If you do have noise that's somehow synchronized with the AC power, using a line trigger will case the noise to appear stationary on the scope display.  There's an example of this in my video on RTB2000 triggering

https://youtu.be/rg6CECPGbAQ?t=454

[pdenisowski]

If you took a name brand scope, for example RIGOL MSO5074 and wanted to make it a floating oscilloscope.  For whatever the reason one may want to do that. Could the following method be employed?

Taking a battery backup UPS that is fully charged.  Unplugging it from mains and securing the plug in a way that would prevent accidental contact with anything.  Then plugging the scope and running it off the UPS without any other connections to the UPS.

Would this work and what would be some potential concerns in doing this?

Several scope manufacturers (including R&S) sell battery powered oscilloscopes and one of the main applications is making (high voltage) floating measurements.  What you're describing is more or less the same thing - building a battery powered scope.

That said, I'm working on a video discussing isolated and floating measurements, and my "official" recommendation will be "don't float the scope".  There are so many things that can go wrong, many of which are dangerous to the scope, the user, or both.  Differential probes are the way to go in almost all circumstances.

But if one is determined to "float the scope,", a (good) isolation transformer is the best choice.  You also want to make sure there are no LAN, USB, etc. cables connected to your scope.

[JJ_023]

* Might because, while on battery power, most UPSs output at best an approximation of the mains sine wave, and so your 'scope might end up picking up all kinds of noise from it, to the point of being unusable.

This is the main application (in my experience) of a line trigger (i.e. the scope triggers on the peaks of the AC mains supply waveform).  If you do have noise that's somehow synchronized with the AC power, using a line trigger will case the noise to appear stationary on the scope display.  There's an example of this in my video on RTB2000 triggering

https://youtu.be/rg6CECPGbAQ?t=454

Interesting video. Thank you for sharing.

[JJ_023]

Several scope manufacturers (including R&S) sell battery powered oscilloscopes and one of the main applications is making (high voltage) floating measurements.  What you're describing is more or less the same thing - building a battery powered scope.

That was basically my train of thought and was wondering if there would be any effect on the measurements taken by the scope by doing something like that. To me it seems like it would not affect the scope in any capacity but I am not 100% sure that's why I posted.

That said, I'm working on a video discussing isolated and floating measurements, and my "official" recommendation will be "don't float the scope".  There are so many things that can go wrong, many of which are dangerous to the scope, the user, or both.  Differential probes are the way to go in almost all circumstances.

But if one is determined to "float the scope,", a (good) isolation transformer is the best choice.  You also want to make sure there are no LAN, USB, etc. cables connected to your scope.

I agree with you, if you read some of my other replies I am well aware that differential probes are the way to go. And I am well aware of isolation transformers.

My post was more of an idea exploring post on whether a mains oscilloscope could be floated, essentially turned into a battery-powered scope without degrading the functionality of the scope. 

I fully understand being aware of any unwanted connections. 

Thank you for the warnings.

[Smokey]

Like what everyone else said already.  Yer gonna die!  But... If you don't mind dying....
...  you just need a 3-2 plug adapter to float the scope.  Or breaking the ground lug off the cable does the same thing.  No UPS required.

[magic]

I can confirm that the plug adapter of death works. It has a risk though: if you apply really high voltage to the scope's ground for even more certain death, primary to secondary isolation of the scope's transformer may break down. No problems, however, if you simply "ground" it to another mains potential, particularly if it's the same phase.

I have also heard (not tried yet) that modern DSOs with universal switching PSUs can work from DC, and it may not even take a lot of it (tens of volts). Perhaps a battery bank is all you need. I imagine that features like "AC line" triggering may not work...

[MrAl]

Hello there,


I always see people get super paranoid about these kinds of measurements yet they are done every day without incident.

Years ago I used the method of cutting the ground pin off of the scopes line cord plug for literally 100's of measurements.  Many people I knew did the same thing, over and over and over and over and over again.  None of them died.  None of them even got shocked.  These were all CRT type scopes.

The key point here is you just have to pay attention to what you are doing when making ANY measurement of line voltages, 120vac or 230vac, or even 440vac.  It's not limited to scopes.  When you use a regular voltmeter, you have to pay attention to where your hands and feet are, as to what they are touching at all times.  It's either that, or pay someone to do the measurements for you.

These days battery powered scopes are somewhat cheap you can get one now for less than $30 that will do line frequency measurements with ease.  That would be the recommended method for today, but you still have to pay attention to what your hands and feet are touching when you do the measurements.
1.  Don't touch any metal connected to any wire.
2.  Don't stand on concrete or any electrically conductive floor, don't stand in water.

Do some reading on the web for how to make line voltage measurements and precautions, you should be ok.

[Vovk_Z]

I can confirm that the plug adapter of death works. It has a risk though:

It has a risk probably if you use to work with grounded instruments. But in some areas we don't have proper grounding at all since ages and it's ok too.
And now we have differential relays. We may use isolation transformers too. So I don't see grounding as an only thing between you life and death.

[Ian.M]

Back in the day of all metal CRO cases and front panels, everybody with the slightest trace of common sense knew that floating a scope was high risk, especially if there were any grounds nearby and a risk of the user touching both.   Also this was before ELCBs/RCDs were in common use so no 'magic' circuit breaker was there to save your misguided ass.

The major hidden hazard with floating a modern plastic cased DSO, is you have absolutely no idea of the creepage and clearance distances between the touchable surfaces of the operating controls and the chassis ground plane of the front panel PCB(s), or other chassis referenced circuits.  To a casual eye, it looks 'safe' with insulating plastic knobs and buttons, but a little conductive dirt buildup on the side of a button or in a knob grubscrew hole could easily be a serious shock hazard.   Then there's the issue of how to handle probes safely when they were never intended to provide more than basic insulation on their grounded parts you've now floated.

Many of the major scope manufacturers have specific technical briefs advising that instruments not designed for floating operation should never be floated, so why are we second guessing them here in a beginners subforum?

[tggzzz]

I always see people get super paranoid about these kinds of measurements yet they are done every day without incident.

Years ago I used the method of cutting the ground pin off of the scopes line cord plug for literally 100's of measurements.  Many people I knew did the same thing, over and over and over and over and over again.  None of them died.  None of them even got shocked.  These were all CRT type scopes.

The key point here is you just have to pay attention to what you are doing when ...

Exactly the same can be said about walking into the road without looking.

(And why did you bring up the display technology (CRT)? That is completely and utterly irrelevant)

[Solder_Junkie]

I had a requirement to measure a DC current switch-on surge by using a low Ohm resistor in series with the load. As the Voltage across the resistor was only a few mV, and I didn’t have a differential probe, I used a battery to power the equipment and used my mains powered scope across the series resistor. The equipment under test was isolated from ground and the mains powered scope was earthed via the mains plug earth pin. The DC Voltage was only 12 Volts, so powering from a battery was not a problem.

No safety issues and no noise pickup. It would have been hard to use a differential probe for this measurement as the peak Voltage was only just over 20 mV and most differential probes appear to have a lot of attenuation for measuring high Voltages.

Only a fool disconnects the earth from mains powered test equipment, it’s there to protect the user in case of failure.

SJ

[magic]

Many of the major scope manufacturers have specific technical briefs advising that instruments not designed for floating operation should never be floated, so why are we second guessing them here in a beginners subforum?

Moreover, much of the equipment that one would probe in such way has specific warnings about hazardous voltages and no user servicable parts inside...

[jonpaul]

Bonjour, bravo to our friends at R&S for the good advise.....

.....my "official" recommendation will be "don't float the scope".

Since 1970s, in HV, SMPS, electronic ballast, medical and avionics design, debug and mfg, we use  mains isolation transformers like Signal DU-1, DU-5 to float the DUT/PSU NOT  the scope!!!

Thus no need for diff probes, or the other kluges mentioned.

Jon

[JJ_023]

Like what everyone else said already.  Yer gonna die!  But... If you don't mind dying....
...  you just need a 3-2 plug adapter to float the scope.  Or breaking the ground lug off the cable does the same thing.  No UPS required.

If you want to be accurate you're the 1st 1 to say that.  I also think you misunderstood the point of the post ( probably my fault for not doing a good enough job explaining).  I am not asking on how you should float a scope. Or why. 

What I am asking is can you take a Mains oscilloscope and turn it into a battery-powered oscilloscope using a UPS that is disconnected from mains.  The byproduct would be a floating scope.  What my question was trying to ascertain was whether it would affect the functionality of the scope by powering it of UPS power supply that is disconnected from the mains.

[JJ_023]

I can confirm that the plug adapter of death works. It has a risk though: if you apply really high voltage to the scope's ground for even more certain death, primary to secondary isolation of the scope's transformer may break down. No problems, however, if you simply "ground" it to another mains potential, particularly if it's the same phase.

I have also heard (not tried yet) that modern DSOs with universal switching PSUs can work from DC, and it may not even take a lot of it (tens of volts). Perhaps a battery bank is all you need. I imagine that features like "AC line" triggering may not work...

I think you misunderstood the intention of the post. However you make a valid point about features like AC line Triggering possibly not working.  That was the purpose of the post to explore ideas like that.  If somebody knows a definitive answer to that it would be greatly appreciated.

[JJ_023]

Hello there,


I always see people get super paranoid about these kinds of measurements yet they are done every day without incident.

Years ago I used the method of cutting the ground pin off of the scopes line cord plug for literally 100's of measurements.  Many people I knew did the same thing, over and over and over and over and over again.  None of them died.  None of them even got shocked.  These were all CRT type scopes.

The key point here is you just have to pay attention to what you are doing when making ANY measurement of line voltages, 120vac or 230vac, or even 440vac.  It's not limited to scopes.  When you use a regular voltmeter, you have to pay attention to where your hands and feet are, as to what they are touching at all times.  It's either that, or pay someone to do the measurements for you.

These days battery powered scopes are somewhat cheap you can get one now for less than $30 that will do line frequency measurements with ease.  That would be the recommended method for today, but you still have to pay attention to what your hands and feet are touching when you do the measurements.
1.  Don't touch any metal connected to any wire.
2.  Don't stand on concrete or any electrically conductive floor, don't stand in water.

Do some reading on the web for how to make line voltage measurements and precautions, you should be ok.

I agree with everything that you are saying.  I measure voltages over 240 at least once a week and feel very comfortable doing so.  Since so many people misunderstood what I was trying to ascertain with this post, I will take the blame for not doing a better job of articulating the question.

The idea was can you take a mains powered oscilloscope and turn it into a battery-operated oscilloscope utilizing a UPS that is completely disconnected from the mains.  The byproduct would be a floated oscilloscope.  However that is not what I'm trying to figure out.  What I am trying to ascertain is how would it affect the operation of the oscilloscope and the measurements taken by it if it is utilized in this manner.  The application doesn't even necessarily have to do with high-voltage.  Maybe you need to check something on an automobile or some other circuit that doesn't have mains power available. 

[JJ_023]

Back in the day of all metal CRO cases and front panels, everybody with the slightest trace of common sense knew that floating a scope was high risk, especially if there were any grounds nearby and a risk of the user touching both.   Also this was before ELCBs/RCDs were in common use so no 'magic' circuit breaker was there to save your misguided ass.

The major hidden hazard with floating a modern plastic cased DSO, is you have absolutely no idea of the creepage and clearance distances between the touchable surfaces of the operating controls and the chassis ground plane of the front panel PCB(s), or other chassis referenced circuits.  To a casual eye, it looks 'safe' with insulating plastic knobs and buttons, but a little conductive dirt buildup on the side of a button or in a knob grubscrew hole could easily be a serious shock hazard.   Then there's the issue of how to handle probes safely when they were never intended to provide more than basic insulation on their grounded parts you've now floated.

Many of the major scope manufacturers have specific technical briefs advising that instruments not designed for floating operation should never be floated, so why are we second guessing them here in a beginners subforum?

Not only do I agree with what you said but I can personally confirm having numerous experiences where dust particles containing metal ( industrial environment) have gotten into places you would never think and either made certain parts of the equipment malfunction or completely shut down the whole machine.

You misunderstood the post. 

The idea was can you take a mains powered oscilloscope and turn it into a battery-operated oscilloscope utilizing a UPS that is completely disconnected from the mains.  The byproduct would be a floated oscilloscope. 

However that is not what I'm trying to figure out.  What I am trying to ascertain is how would it affect the operation of the oscilloscope and the measurements taken by it if it is utilized in this manner.  The application doesn't even necessarily have to do with high-voltage.  Maybe you need to check something on an automobile or some other circuit that doesn't have mains power available.

[JJ_023]

Many of the major scope manufacturers have specific technical briefs advising that instruments not designed for floating operation should never be floated, so why are we second guessing them here in a beginners subforum?

Moreover, much of the equipment that one would probe in such way has specific warnings about hazardous voltages and no user servicable parts inside...

I would disagree with that statement. If there were no serviceable parts then things that got broken would never get fixed.