Oscilloscope Floating using UPS

[JJ_023]

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

Thank you for the model suggestions and the advice.  What I really am looking for as far as input is was to explore the following idea.  Which I unfortunately did not articulate well enough.

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.

[magic]

I doubt there would be serious problems running a scope from a UPS, particularly any modern one with switching PSU.

Line trigger will trigger off the UPS output rather than true mains, obviously.

You may not even need a UPS, just enough batteries.

If the UPS is disconnected and doesn't pass mains earth then you are floating the scope, and so the thread has turned into another typical "omg somebody is floating a scope" thread.

On a car you generally don't have the high voltage danger issue, although you could connect grounding clips to spark plug cables for a little fun

[CaptDon]

Worst idea ever!!! Due to the line filter built into the power input of the scope the scope's probe ground will be capacitively coupled and balanced at about 60 v.a.c. above earth ground. A sure way to blow up SMPS units under test or almost anything else the O.P. is working on. Isolation transformer connected to U.U.T. is best answer. Isolation transformers are cheap enough when balanced against the cost of human life or blown up items under test. Isolation transformers can also be easily and cheaply made from a pair of back to back control transformers of proper VA rating.
E.G. 120 to 24 driving a 24 to 120. Don't cut the ground lug and don't use those adapters, just bad unprofessional work ethic.

[JJ_023]

I doubt there would be serious problems running a scope from a UPS, particularly any modern one with switching PSU.

Line trigger will trigger off the UPS output rather than true mains, obviously.

You may not even need a UPS, just enough batteries.

If the UPS is disconnected and doesn't pass mains earth then you are floating the scope, and so the thread has turned into another typical "omg somebody is floating a scope" thread.

On a car you generally don't have the high voltage danger issue, although you could connect grounding clips to spark plug cables for a little fun

Thank you. This is similar to what I was looking for.

I understand your fun suggestion but no grounding the spark plug cables for me. 

Checking the ignition coils can be fun if you do it the proper way. 

[JJ_023]

Worst idea ever!!! Due to the line filter built into the power input of the scope the scope's probe ground will be capacitively coupled and balanced at about 60 v.a.c. above earth ground. A sure way to blow up SMPS units under test or almost anything else the O.P. is working on. Isolation transformer connected to U.U.T. is best answer. Isolation transformers are cheap enough when balanced against the cost of human life or blown up items under test. Isolation transformers can also be easily and cheaply made from a pair of back to back control transformers of proper VA rating.
E.G. 120 to 24 driving a 24 to 120. Don't cut the ground lug and don't use those adapters, just bad unprofessional work ethic.

Thank you for the warnings.  I think you misunderstood the intent of the post.  It is not to create a floating oscilloscope.

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.

[Thunderer]

Under no circumstances, nor new ways of measuring with a scope, the scope should ever be floating. The scope should be solidly grounded, for your own protection, and that of your gear. If you want to float something, it should be the object you want to measure.

Second, the fact that some portable scopes are floating by construction is in line with their construction: if you use the provided probes, you cannot touch the metallic part of the probe. Which is not the case with the bench scopes that we try to make them floating but cutting the ground or powering from an UPS/generator.

STOP inventing dangerous ways to kill people. The argument that you will pay attention on what you do, that you are using the safest methods and/or approaches, again STOP! You do not know who reads your posts and what level of understanding the risks they have.

This kind of topics are like the ones explaining how to do a bomb, but says: do not do it at home! Well, someone will do it, and Lady Luck may not be in the neighborhood.

[JJ_023]

Under no circumstances, nor new ways of measuring with a scope, the scope should ever be floating. The scope should be solidly grounded, for your own protection, and that of your gear. If you want to float something, it should be the object you want to measure.

Second, the fact that some portable scopes are floating by construction is in line with their construction: if you use the provided probes, you cannot touch the metallic part of the probe. Which is not the case with the bench scopes that we try to make them floating but cutting the ground or powering from an UPS/generator.

STOP inventing dangerous ways to kill people. The argument that you will pay attention on what you do, that you are using the safest methods and/or approaches, again STOP! You do not know who reads your posts and what level of understanding the risks they have.

This kind of topics are like the ones explaining how to do a bomb, but says: do not do it at home! Well, someone will do it, and Lady Luck may not be in the neighborhood.

I appreciate you taking the time to respond as well as your concerns. 

I also think it's ridiculous that you think ideas can't be explored such as this 1 because someone may not understand them and therefore be in harm's way. 

I totally get the idea that people can misinterpret, it's clear that you misinterpreted this particular post, but that is a poor excuse not to discuss concept and ideas to further educate and explore concepts. IMHO

[Thunderer]

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. 

The fact that you did not know/see something, it does not mean it does not exist.

I am pretty sure there are things you never saw (in real life), and yet you believe they exist when you see a photo or a text mentioning them. So why your thinking is so selective?

Again, some people can live while doing properly some dangerous jobs. Yet, by your text, you think anyone can do those dangerous things, while you have no idea what background they have. Just follow some rules... until we make a mistake or decide a shortcut is worthed.

[Thunderer]

I also think it's ridiculous that you think ideas can't be explored such as this 1 because someone may not understand them and therefore be in harm's way. 

Stay safe, buddy! I do safety for a living, that's why I posted what I posted. Clearly, you did not get my message. Again, stay safe!

PS: You asked about concerns, what answers were you expected if the safety disturbs you?!

[JJ_023]

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. 

The fact that you did not know/see something, it does not mean it does not exist.

I am pretty sure there are things you never saw (in real life), and yet you believe they exist when you see a photo or a text mentioning them. So why your thinking is so selective?

Again, some people can live while doing properly some dangerous jobs. Yet, by your text, you think anyone can do those dangerous things, while you have no idea what background they have. Just follow some rules... until we make a mistake or decide a shortcut is worthed.

You're making a point based on someone else's post which doesn't even tie into the main topic of this post. 

Based on your line of thinking you are doing the same exact thing.  You are assuming without knowing.  You don't know what's going to be measured.  It could be an LED connected to a resistor connected to a 5 V battery pack.  Please stay on topic.

[JJ_023]

I also think it's ridiculous that you think ideas can't be explored such as this 1 because someone may not understand them and therefore be in harm's way. 

Stay safe, buddy! I do safety for a living, that's why I posted what I posted. Clearly, you did not get my message. Again, stay safe!

PS: You asked about concerns, what answers were you expected if the safety disturbs you?!

  I appreciate you taking the time.  You make legitimate points, however they are completely off topic.  The concerns are not with safety. 

[Thunderer]

You're making a point based on someone else's post which doesn't even tie into the main topic of this post. 

Based on your line of thinking you are doing the same exact thing.  You are assuming without knowing.  You don't know what's going to be measured.  It could be an LED connected to a resistor connected to a 5 V battery pack.  Please stay on topic.

Buddy, why don't you exactly explain why you'd need separation (floating) if it is not to measure HV signals? You see that most posts are about safety. The answers are as good as the question is.

PS: If you need to measure floating signals, do as I did: build yourself a differential probe (it works even in HV cases if you do it properly). I can help you (with knowledge) to build one. You can buy one too, no obligation to build one.

I will assume something now, and it is based on thousands of similar cases found on forums: one beginner, with not even the basics in possession, asks a question on a forum (or more, the same question). The question is not asked to get real insight on the issue he/she is asking about. The question is asked to get his/her assumption the validation that he/she knew it. That way, the person is highly irritated when the answers do not validate his/her expectations. Most of the time, the person will end up saying that nobody understand him/her, or that nobody knows the answer. The same will repeat on other forums. Bottom line, most people on forums are offtopic or simply dumb, unless they validate his/her expectations.

End of the story. Stay safe, you may need it more than you believe it!

[magic]

Buddy, why don't you exactly explain why you'd need separation (floating) if it is not to measure HV signals?

Already did several times, including to you directly.
Now, why can't you read the thread you are responding to?

[bdunham7]

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.

Of course what you propose will "work", as long as the UPS can be cold-started or you have it connected to line power and then unplug it.  The issues you will run into are probably mostly related to noise, as the inputs are not differential and not designed to have a significant AC voltage on the ground.  The effect of this will vary a lot with frequency and so forth.  At low frequencies and moderate voltages, such as you might find in many basic automotive circuits, this may work just fine.  At RF frequencies or very high voltages, these issues may be more of a problem.  If you're trying to look at high-side gate drive signals on an SMPS, the combined capacitive loading of your scope/UPS combo may be an issue as well.  And, of course, if you try to float the combo up too high you'll have corona discharge issues.  Depending on the exact model of scope, both pure-sine and modified-sine UPS versions will likely work, but the modified-sine may have additional noise issues. 

[JJ_023]

Buddy, why don't you exactly explain why you'd need separation (floating) if it is not to measure HV signals? You see that most posts are about safety. The answers are as good as the question is.

Again I never stated I needed floating to measure high-voltage signals.  You misinterpreted the whole purpose of this post.  I don't doubt you have a lot of valuable knowledge to share but you can't share it unless you take the time to understand the question that is being asked.

PS: If you need to measure floating signals, do as I did: build yourself a differential probe (it works even in HV cases if you do it properly). I can help you (with knowledge) to build one. You can buy one too, no obligation to build one.

Despite us not understanding one another on this particular subject I would gladly welcome your help or anybody's when it comes down to building a differential probe.  So if that time arises you can expect a PM from me.  Thank you for the offer, hopefully it will still be valid.

I will assume something now, and it is based on thousands of similar cases found on forums: one beginner, with not even the basics in possession, asks a question on a forum (or more, the same question). The question is not asked to get real insight on the issue he/she is asking about. The question is asked to get his/her assumption the validation that he/she knew it. That way, the person is highly irritated when the answers do not validate his/her expectations. Most of the time, the person will end up saying that nobody understand him/her, or that nobody knows the answer. The same will repeat on other forums. Bottom line, most people on forums are offtopic or simply dumb, unless they validate his/her expectations.

End of the story. Stay safe, you may need it more than you believe it!

This is really ironic.  Because this is exactly what you are doing in this regard, despite the fact that you claim you have a lot of experience.  Maybe my original question was not articulated properly enough but I have made several attempts to correct what I am seeking to ascertain as far as knowledge. 

I have zero qualms about looking ignorant or stupid when I ask a question provided I learn and educate myself in the process.  I will at times ask for validation of an idea I may have but I will switch my position in a heartbeat if I am educated on why my position is invalid, and thank somebody for pointing out my faulty thinking.

[tggzzz]

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. 

The fact that you did not know/see something, it does not mean it does not exist.

I am pretty sure there are things you never saw (in real life), and yet you believe they exist when you see a photo or a text mentioning them. So why your thinking is so selective?

Again, some people can live while doing properly some dangerous jobs. Yet, by your text, you think anyone can do those dangerous things, while you have no idea what background they have. Just follow some rules... until we make a mistake or decide a shortcut is worthed.

Exactly. I wonder if he will take note now that two people have independently made the same point. Somehow I doubt it.

I also wonder why he mentioned "These were all CRT type scopes" as if the display technology has any relevance. Mind you, that kind of comment is useful for attentive readers: they might presume that such a misunderstanding is a hint that he misunderstands other more fundamental safety topics.

[JJ_023]

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.

Of course what you propose will "work", as long as the UPS can be cold-started or you have it connected to line power and then unplug it.  The issues you will run into are probably mostly related to noise, as the inputs are not differential and not designed to have a significant AC voltage on the ground.  The effect of this will vary a lot with frequency and so forth.  At low frequencies and moderate voltages, such as you might find in many basic automotive circuits, this may work just fine.  At RF frequencies or very high voltages, these issues may be more of a problem.  If you're trying to look at high-side gate drive signals on an SMPS, the combined capacitive loading of your scope/UPS combo may be an issue as well.  And, of course, if you try to float the combo up too high you'll have corona discharge issues.  Depending on the exact model of scope, both pure-sine and modified-sine UPS versions will likely work, but the modified-sine may have additional noise issues.

This is along the lines of what I am looking for.  Let's assume the UPS can be cold started (I actually don't know of any that can't).

I was thinking that it may induce noise from the inside circuitry but not at low frequencies. 

However, how would the oscilloscope differentiate between mains and a UPS, the unit primarily runs on DC inside?

The other thought of reasoning is.  When people have floated a scope by disabling the ground (for whatever reason they do it) are they subjugated to the same type of noise?

I understand your frequency comments.  As well as high-frequency when getting into the high-voltage space.

As far as your SMPS comment.  My question is.  Would this apply to a battery-powered oscilloscope as well?  Most battery-powered oscilloscopes are similar to desktop versions in the regard that the inputs all share a common ground.

Thanks for your insight.  It's just when you answer some questions new ones develop for me.

[bdunham7]

I was thinking that it may induce noise from the inside circuitry but not at low frequencies. 

However, how would the oscilloscope differentiate between mains and a UPS, the unit primarily runs on DC inside?

Sure, even a so-called low frequency inverter that uses a large transformer still typically has a HF switching primary so there may be some 50kHz or so noise in there.  The difference is that when connected normally and grounded, the ground components of the scope are at 0V and not changing, thus there is no electric field at those points.  Floating and applying a changing voltage to the scopes ground points changes that.

When people have floated a scope by disabling the ground (for whatever reason they do it) are they subjugated to the same type of noise?

As far as your SMPS comment.  My question is.  Would this apply to a battery-powered oscilloscope as well?  Most battery-powered oscilloscopes are similar to desktop versions in the regard that the inputs all share a common ground.

Floating a scope by just breaking the ground has widely varying results, some of which depend on the nature of the scope's PSU and how it is internally coupled to the ground point.  As far as the performance of a cheap scope that just happens to be powered by batteries, IDK--probably crap.  Good ones that I'm familiar with--Fluke Scopemeters and Tektronix TPS scopes--take this into account and perform fairly well, although the TPS and the newer, better Scopemeters actually do not have common grounds for the inputs. 

[JJ_023]

Sure, even a so-called low frequency inverter that uses a large transformer still typically has a HF switching primary so there may be some 50kHz or so noise in there.  The difference is that when connected normally and grounded, the ground components of the scope are at 0V and not changing, thus there is no electric field at those points.  Floating and applying a changing voltage to the scopes ground points changes that.

This makes perfect sense to me.  Thanks for the concise explanation.

Floating a scope by just breaking the ground has widely varying results, some of which depend on the nature of the scope's PSU and how it is internally coupled to the ground point.  As far as the performance of a cheap scope that just happens to be powered by batteries, IDK--probably crap.  Good ones that I'm familiar with--Fluke Scopemeters and Tektronix TPS scopes--take this into account and perform fairly well, although the TPS and the newer, better Scopemeters actually do not have common grounds for the inputs.

I’ve actually used a fluke handheld.  They do perform well.  I guess a better question would be how are they accomplishing this if they are truly different from a mains powered oscilloscope running essentially on a battery?

I know some of the better scopes do not have common grounds but that is usually not on a common case occurrence from what I understand.  Is this a correct assumption?

Again thanks for explaining what may seem rudimentary to you.

[Vovk_Z]

why are we second guessing them here in a beginners subforum?

So, the only logical answer to this and all other similar questions here in Beginners subforum is 'No'.
And logical answer to the same question given in other than Beginners subforums is 'Yes'.

[tggzzz]

The only logical answer is to learn from the experts that base their recommendations on solid engineering theory and practice.

In this case, suitable starting points are Tektronix'
"Fundamentals of Floating Measurements and Isolated Input Oscilloscopes"
https://www.tek.com/en/documents/application-note/fundamentals-floating-measurements-and-isolated-input-oscilloscopes
and
"Floating Oscilloscope Measurements … And Operator Protection"
http://www.tek.com/dl/51W_10640_1.pdf
both of which discuss various techniques.

For other starting points, see the "Praxis and Safety" refs at https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

[magic]

When people have floated a scope by disabling the ground (for whatever reason they do it) are they subjugated to the same type of noise?

As far as your SMPS comment.  My question is.  Would this apply to a battery-powered oscilloscope as well?  Most battery-powered oscilloscopes are similar to desktop versions in the regard that the inputs all share a common ground.

Floating a scope by just breaking the ground has widely varying results, some of which depend on the nature of the scope's PSU and how it is internally coupled to the ground point.  As far as the performance of a cheap scope that just happens to be powered by batteries, IDK--probably crap.  Good ones that I'm familiar with--Fluke Scopemeters and Tektronix TPS scopes--take this into account and perform fairly well, although the TPS and the newer, better Scopemeters actually do not have common grounds for the inputs.

It depends a lot on how the scope is grounded to the DUT. You never truly float a scope (leave its ground completely unconnected) - it's always connected to the DUT somehow, either with alligator clips at the probes or through mains earth cabling if you omit those and both devices are earthed. Otherwise the two devices have no common point of reference and the waveform has unknown DC offset and may jump wildly with any picked up noise.

The most problematic case is when people try to use a floating scope as a poor man's differential oscilloscope: connect its ground to some sensitive and/or high impedance node in the DUT in order to take measurements with respect to that node rather than ground. In such case noise current coupled into oscilloscope ground through its transformer causes the node's voltage to fluctuate and DUT operation may be disrupted and/or other non-disrupted signals will appear to fluctuate (with respect to the "weak" reference node) even though it's the reference node which fluctuates. Such "measurement" may show problems which simply don't exist or didn't exist before the oscilloscope was connected to the device.

OTOH, grounding the scope to some ground, quasi-ground or power supply rail tends to work much better because such nodes are able to absorb scope's ground noise. I suspect that the most common real world use case for floating scopes (and the reason why safety nuts are going nuts) is grounding the scope to a "hot ground", i.e. the negative of rectified mains. I have done it once or twice when repairing SMPS and observed no unusual behavior, it simply worked as if the hot ground were ordinary ground.

UPS powered scope in a car may be uncharted territory so far, but based on the above I think it has a decent chance to work if you don't try to be clever and ground the scope and the UPS to the car's chassis. (You could also earth the whole car to some available metal structure).

[Thunderer]

Despite us not understanding one another on this particular subject I would gladly welcome your help or anybody's when it comes down to building a differential probe.  So if that time arises you can expect a PM from me.  Thank you for the offer, hopefully it will still be valid.

Of course, I may even ship you a PCB for the probe I built.

[JJ_023]

The only logical answer is to learn from the experts that base their recommendations on solid engineering theory and practice.

In this case, suitable starting points are Tektronix'
"Fundamentals of Floating Measurements and Isolated Input Oscilloscopes"
https://www.tek.com/en/documents/application-note/fundamentals-floating-measurements-and-isolated-input-oscilloscopes
and
"Floating Oscilloscope Measurements … And Operator Protection"
http://www.tek.com/dl/51W_10640_1.pdf
both of which discuss various techniques.

For other starting points, see the "Praxis and Safety" refs at https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

Thank you for sharing these links and information it is greatly appreciated and has contributed to my knowledge base.  I have read both links as well as 90% of the links on "Scope Probe Reference Material".  I will finish the rest within the next day or so.

[JJ_023]

It depends a lot on how the scope is grounded to the DUT. You never truly float a scope (leave its ground completely unconnected) - it's always connected to the DUT somehow, either with alligator clips at the probes or through mains earth cabling if you omit those and both devices are earthed. Otherwise the two devices have no common point of reference and the waveform has unknown DC offset and may jump wildly with any picked up noise.

Is this a typo ?  if you omit those and both devices are earthed

I do get the gist of this and it makes sense to me.

The most problematic case is when people try to use a floating scope as a poor man's differential oscilloscope: connect its ground to some sensitive and/or high impedance node in the DUT in order to take measurements with respect to that node rather than ground. In such case noise current coupled into oscilloscope ground through its transformer causes the node's voltage to fluctuate and DUT operation may be disrupted and/or other non-disrupted signals will appear to fluctuate (with respect to the "weak" reference node) even though it's the reference node which fluctuates. Such "measurement" may show problems which simply don't exist or didn't exist before the oscilloscope was connected to the device.

Thank you for expanding on this it makes sense.

OTOH, grounding the scope to some ground, quasi-ground or power supply rail tends to work much better because such nodes are able to absorb scope's ground noise. I suspect that the most common real world use case for floating scopes (and the reason why safety nuts are going nuts) is grounding the scope to a "hot ground", i.e. the negative of rectified mains. I have done it once or twice when repairing SMPS and observed no unusual behavior, it simply worked as if the hot ground were ordinary ground.

I can definitely see the dangers associated with the negative of rectified mains.  What would be the purpose of taking such a measurement with an oscilloscope when you can take one with a DMM?

I have done this countless times with a DMM and never had a need for anything more.

UPS powered scope in a car may be uncharted territory so far, but based on the above I think it has a decent chance to work if you don't try to be clever and ground the scope and the UPS to the car's chassis. (You could also earth the whole car to some available metal structure).

1st of all that was just a possible application given as a possible scenario.  In that particular situation both the UPS and oscilloscope would be outside the car.   Also since the scope is going to attach to a grounding point on the car I am not sure I understand your point. 

Could you possibly elaborate?

Thank you for your lengthy response.