Oscilloscope Floating using UPS

[johansen]

fairly certain my old TEK 475 oscope survived being floated at +/- 170vdc at 120KHZ.. no problem, with a cut ground pin.

doesn't mean the waveform was accurate, due to the resistance, inductance, and coupling of the 10 foot long oscope leads... but, it didn't blow up anything.

I'm fairly certain I've walked into the road without looking... no problem.

With your 475, what were the voltages across the transformer windings and "nearby" PSU components. Were they rated for that voltage? Were they subtly damaged if the rating was exceeded?

Tek used a fairly high quality transformer.. with i think, a shield inside between primary and secondary.

i would expect them to work fine at even much higher voltages.

[tggzzz]

I've just received a new-old toy[1], a THS720A handheld mains/battery scope with isolated inputs (Sorry, pdenisowski, not R&S )

At first glance the BNC connector looked surprisingly dirty, but the OP might be interested in the reason it isn't.



As you can see, the "outer shield" is plastic, not metal.

[1] no I don't need it. TEA syndrome.

I can tell it's plastic.  What do the probes look like if you do not mind?

Metal bit at the blunt end is covered with plastic, so that small fingers would be needed to touch metal. I received P6117, but he normal type would be P5102

https://w140.com/tekwiki/wiki/File:Tek_p5102_2.jpg

[JJ_023]

fairly certain my old TEK 475 oscope survived being floated at +/- 170vdc at 120KHZ.. no problem, with a cut ground pin.

doesn't mean the waveform was accurate, due to the resistance, inductance, and coupling of the 10 foot long oscope leads... but, it didn't blow up anything.

I'm fairly certain I've walked into the road without looking... no problem.

With your 475, what were the voltages across the transformer windings and "nearby" PSU components. Were they rated for that voltage? Were they subtly damaged if the rating was exceeded?

Tek used a fairly high quality transformer.. with i think, a shield inside between primary and secondary.

i would expect them to work fine at even much higher voltages.

Up to what voltage would you be comfortable with and not worry about damaging the oscilloscope?

[johansen]

fairly certain my old TEK 475 oscope survived being floated at +/- 170vdc at 120KHZ.. no problem, with a cut ground pin.

doesn't mean the waveform was accurate, due to the resistance, inductance, and coupling of the 10 foot long oscope leads... but, it didn't blow up anything.

I'm fairly certain I've walked into the road without looking... no problem.

With your 475, what were the voltages across the transformer windings and "nearby" PSU components. Were they rated for that voltage? Were they subtly damaged if the rating was exceeded?

Tek used a fairly high quality transformer.. with i think, a shield inside between primary and secondary.

i would expect them to work fine at even much higher voltages.

Up to what voltage would you be comfortable with and not worry about damaging the oscilloscope?

What voltage do you expect the transformer to arc over internally?

Im talking about ancient crt. The whole case is aluminum.. dont touch it.

[tggzzz]

fairly certain my old TEK 475 oscope survived being floated at +/- 170vdc at 120KHZ.. no problem, with a cut ground pin.

doesn't mean the waveform was accurate, due to the resistance, inductance, and coupling of the 10 foot long oscope leads... but, it didn't blow up anything.

I'm fairly certain I've walked into the road without looking... no problem.

With your 475, what were the voltages across the transformer windings and "nearby" PSU components. Were they rated for that voltage? Were they subtly damaged if the rating was exceeded?

Tek used a fairly high quality transformer.. with i think, a shield inside between primary and secondary.

i would expect them to work fine at even much higher voltages.

Up to what voltage would you be comfortable with and not worry about damaging the oscilloscope?

Whatever is in the oscilloscope specification. Beyond that you have to do a thorough reverse engineering job, and hope you haven't missed something. Or you can gamble.

[MrAl]

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)

Hello,

You have no idea what you are talking about.
How can I be so sure?
I already mentioned this but you, as usual, fail to understand.
I have worked with a LOT of tech's who ALL used scopes with the ground pin cut off, and NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.  Let me repeat that so MAYBE you will read it and understand this time.

NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
Not ever, over years and years of working with them in several different companies.  That's three, four, five companies.  Plus, we're talking about a LOT of technicians, and who were not reprimanded in any way by the upper management.  In fact, they were instructed to do it this way.
How can this be?  It be, period.

I am not trying to force anyone to do this, just stating some facts about this.  If a given reader does not like this idea, that's ok with me.  If you feel it is dangerous, then don't do it.  Don't like it?  Then don't do it.  But don't spread misinformation.

Some people have been killed. Your shouted assertions don't change that.

Here’s an anecdote about someone that "was WELL AWARE of the danger and took a lot of precautions to prevent shock . . . Bottom Line: He died anyway". https://groups.io/g/TekScopes/message/10754

Also, mentioning a CRT scope may be irrelevant to you, but not to me.  You're welcome to talk about any kind of scope you like however, feel free.

Why, exactly, is the display type relevant to the points being made? Repeated assertions are not a reason.

Now if you want to go ahead and keep bitching about this, go right ahead, and I'll start to ignore what you have to say also.

From your responses (i.e. not answers) you have already started that

I do not see *too* much difference between using a scope and using a volt meter or ammeter.  You have to know a little about how to do it with either kind of instrument.  Heck, don't use a toaster.  Don't use a space heater.  Hide under the covers until you grow old and die of natural causes (ha ha). You can get killed just crossing the street.
Life is dangerous, no way around it.

This reminds me of that movie, don't know if anyone here has seen it.  It's named:
"A Million Ways To Die In The West".
Really funny with a dash of old west history.  Highly recommended.
The point is, there are a million ways to die in just about any era.

The bottom line might be if you don't feel comfortable doing it, then don't.  That goes for a volt meter too though, or any other kind of test equipment that can be live sometimes.

In my old rock and roll band days, we were performing outside one day for a big big party and I got stung by the microphone.  I was standing on the grass and the mic had the plug in the outlet in such a way as to make the chassis of the amplifier hot.
That has killed people too from what I have read.  So don't become a singer, you might get killed (ha ha).

[johansen]

Dont do this at home.

I didn't even feel a tingle. Warm dry steel toed boots, clean dry floor.

[MrAl]

OSHA standard 1910.334 - Use of equipment, section (3) Grounding-type equipment specifically prohibits cutting the ground pin:

Attachment plugs and receptacles may not be connected or altered in a manner which would prevent proper continuity of the equipment grounding conductor at the point where plugs are attached to receptacles. ...

and later in the section, explicitly prohibits the use of adapters that interrupt the ground conductor.

Assuming this was in the USA, MrAl and his 'tech' friends exposed the company to potentially massive liability, and should all have received a formal written warning for the first offence of interrupting a grounding conductor, escalating to retraining and even dismissal for repeated offenses.

Even ignoring the fact that OSHA regulations are written in blood, i.e. are codified in response to serious or fatal industrial accidents, or serious negligent harm to employee health, to attempt to protect all employees in similar industries, so should be obeyed even if (and especially if) you do not understand the rationale behind them, the fines OSHA  would have imposed if even one disgruntled (ex) employee had reported cutting ground pins, + the disruption from the resulting surprise inspection to confirm the report, would almost certainly have exceeded the cost of providing isolated differential probes to all techs that actually needed to make floating scope measurements.

Cutting ground pins, like drunk driving, may once have been socially acceptable even though it was and is prohibited/illegal.  It is no longer acceptable, and any colleague or supervisor that proposes or performs it is an active danger to your personal safety.  If management wont listen and act, gather evidence and report it to OSHA or your equivalent national workplace safety authority.

Shoulda, coulda, woulda, mighta, but they did not.  Why not?  Why didn't they do anything when they came around one day?
What they did do was make everyone wear safety glasses, that's about it.

I don't think there is anything in this world that would stop some people from cutting the ground pin, or using a piece of test equipment with a three to two prong adapter.  For me though, I don't have to anymore, and that's why I always recommend a battery powered scope now.  They are not too expensive for the frequency we are talking about.  In fact, you can get one for under $40 or even less.  I got one brand new for $25 some years ago, it was the kind that uses that ARM chip, and only has one input.  Comes with one probe but no batteries you have to get your own batteries.

Some bench meters have a metal case too.
It is true though that there has been more attention to the three prong plug over the last decades than before that.

[MrAl]

Dont do this at home.

I didn't even feel a tingle. Warm dry steel toed boots, clean dry floor.

Wow now that's guts.

[xavier60]

This might not be useful to know. Some mains powered equipment that have a SMPS, will run on DC.
My HP 54645a DSO will run on 85VDC at 0.9A.
There is a chance that this could damage some equipment.

To somebody more experienced this might make sense.  Could you please elaborate in more complete thoughts so that I could understand this better. 

Thank you.

If I wanted to adapt my DSO to battery operation, not necessarily for the purpose of making measurements referenced to hazardous voltages, I would power it from an SLA battery boosted to 85VDC using a boost(flyback) regulator.

[JJ_023]

If I wanted to adapt my DSO to battery operation, not necessarily for the purpose of making measurements referenced to hazardous voltages, I would power it from an SLA battery boosted to 85VDC using a boost(flyback) regulator.

[/quote]

Why would your approach be preferred over utilizing a UPS (where there is nothing to build and it is a finished product in its own case)?

Could you please explain how you would run a mains oscilloscope on 85VDC (that voltage doesn't seem like it would be enough)? 

Thank you.

[johansen]

If I wanted to adapt my DSO to battery operation, not necessarily for the purpose of making measurements referenced to hazardous voltages, I would power it from an SLA battery boosted to 85VDC using a boost(flyback) regulator.

Why would your approach be preferred over utilizing a UPS (where there is nothing to build and it is a finished product in its own case)?

Could you please explain how you would run a mains oscilloscope on 85VDC (that voltage doesn't seem like it would be enough)? 

Thank you.
[/quote]

85 is probably enough. A brick of aa batteries could power a scope for a long time.

[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)

Hello,

You have no idea what you are talking about.
How can I be so sure?
I already mentioned this but you, as usual, fail to understand.
I have worked with a LOT of tech's who ALL used scopes with the ground pin cut off, and NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.  Let me repeat that so MAYBE you will read it and understand this time.

NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
NOT A SINGLE FRIGGIN ONE OF THEM EVEN GOT A SHOCK.
Not ever, over years and years of working with them in several different companies.  That's three, four, five companies.  Plus, we're talking about a LOT of technicians, and who were not reprimanded in any way by the upper management.  In fact, they were instructed to do it this way.
How can this be?  It be, period.

I am not trying to force anyone to do this, just stating some facts about this.  If a given reader does not like this idea, that's ok with me.  If you feel it is dangerous, then don't do it.  Don't like it?  Then don't do it.  But don't spread misinformation.

Some people have been killed. Your shouted assertions don't change that.

Here’s an anecdote about someone that "was WELL AWARE of the danger and took a lot of precautions to prevent shock . . . Bottom Line: He died anyway". https://groups.io/g/TekScopes/message/10754

Also, mentioning a CRT scope may be irrelevant to you, but not to me.  You're welcome to talk about any kind of scope you like however, feel free.

Why, exactly, is the display type relevant to the points being made? Repeated assertions are not a reason.

Now if you want to go ahead and keep bitching about this, go right ahead, and I'll start to ignore what you have to say also.

From your responses (i.e. not answers) you have already started that

I do not see *too* much difference between using a scope and using a volt meter or ammeter.  You have to know a little about how to do it with either kind of instrument.
<irrelevant rant omitted>

Yes, we realise that you don't understand the differences and similarities, and that you haven't been injured or killed (yet).

We don't understand why you think a scope's display technology is relevant. Please enlighten us.

[xavier60]

If I wanted to adapt my DSO to battery operation, not necessarily for the purpose of making measurements referenced to hazardous voltages, I would power it from an SLA battery boosted to 85VDC using a boost(flyback) regulator.


Why would your approach be preferred over utilizing a UPS (where there is nothing to build and it is a finished product in its own case)?

Could you please explain how you would run a mains oscilloscope on 85VDC (that voltage doesn't seem like it would be enough)? 

Thank you.

Most of my work needs 74VDC, so I have built a variable boost converter that I power from my bench PSU, from an earlier thread.
I used this variable boost converter to see what low voltage DC my DSO was able to be powered by.
Since I'm already comfortable with designing SMPS circuitry, I would rather build a dedicated boost converter to power the DSO.

I have seen mains powered equipment wake up at as low as 50VDC because of the wide operating voltage range.

[JJ_023]

If I wanted to adapt my DSO to battery operation, not necessarily for the purpose of making measurements referenced to hazardous voltages, I would power it from an SLA battery boosted to 85VDC using a boost(flyback) regulator.


Why would your approach be preferred over utilizing a UPS (where there is nothing to build and it is a finished product in its own case)?

Could you please explain how you would run a mains oscilloscope on 85VDC (that voltage doesn't seem like it would be enough)? 

Thank you.

Most of my work needs 74VDC, so I have built a variable boost converter that I power from my bench PSU, from an earlier thread.
I used this variable boost converter to see what low voltage DC my DSO was able to be powered by.
Since I'm already comfortable with designing SMPS circuitry, I would rather build a dedicated boost converter to power the DSO.

I have seen mains powered equipment wake up at as low as 50VDC because of the wide operating voltage range.

I understand that explanation.  Thank you.

[Thunderer]

Given that you have a 120Vac system to power the scope, and as the scope has a SMPS PSU, meaning that the 120Vac is rectified you will find around 170Vdc at the SMPS transformer primary.

So, if you apply 170Vdc then you are good to go. If you apply a lower DC voltage (see below), the SMPS will not start, as the UVLO (undervoltage lockout) will do its job.

My Rigol says 100-240Vac input. For 100Vac the DC link in the SMPS is 141Vdc, for 120Vac is 170Vdc. Any voltage under 141Vdc will not be good for the scope PSU, as it will refuse to start.

To power from an UPS (non-sine waveform) it is completely acceptable for the SMPS PSU in the scope. That modified sine will be rectified anyway.

[xavier60]

As well as the UVLO, some offline SMPS's need to detect the presence of AC to run. One I met recently would run on 60VDC, but with only one polarity.
I wouldn't like to see what happens if a PFC was fed with DC.
The main reason I run some things on DC is just for a first test run from a current limited supply. Mostly washing machine motor controllers.

[Thunderer]

The PFC has a rectifier bridge in front of it. Then, the PFC (acting as a booster) may decide to let through if enough DC voltage, or boost if the DC input is lower than expected.



From here: https://www.ti.com/seclit/ml/slup390/slup390.pdf

Power Factor Correction (PFC)
Circuit Basics
Reproduced from
2020 Texas Instruments Power Supply Design Seminar
SEM2400
Topic 2
TI Literature Number: SLUP390

[johansen]

a smps that starts on one polarity only has a half wave rectifier in front of it. i don't even understand how that would be cheaper than a fwb and a smaller capacitor.

[johansen]

Given that you have a 120Vac system to power the scope, and as the scope has a SMPS PSU, meaning that the 120Vac is rectified you will find around 170Vdc at the SMPS transformer primary.

So, if you apply 170Vdc then you are good to go. If you apply a lower DC voltage (see below), the SMPS will not start, as the UVLO (undervoltage lockout) will do its job.

My Rigol says 100-240Vac input. For 100Vac the DC link in the SMPS is 141Vdc, for 120Vac is 170Vdc. Any voltage under 141Vdc will not be good for the scope PSU, as it will refuse to start.

To power from an UPS (non-sine waveform) it is completely acceptable for the SMPS PSU in the scope. That modified sine will be rectified anyway.

if it is pfc corrected, the undervoltage lockout on the chip is likely to be 10-16 vdc, and that voltage is typically fed from two 1/4w resistors in series summing to around 500K ohms. so without enough volts, it won't work..

once the chip starts up, the feedback coil from the inductor supplies the current the control chip needs. so you may find you can start it on 50-100vdc, but once you get it started it will run with far less voltage, perhaps all the way down to 20vdc.

most other smps work the same way, a tertiary coil or the snubber on the flyback transformer powers the brains of the smps. for a flyback, its going to need a much higher voltage than a pfc circuit, which can operate all the way down to just 5volts dc input (to get 99% power factor, the boost converter has to be operating for 99% the area under the half sine. it will of course be severely power limited.

but lots of welding equipment is done this way now, you can get half the output on 120vac as you can 240vac. you'd get a 4th of the output at 60vac if you could make it not fault.

[JJ_023]

Given that you have a 120Vac system to power the scope, and as the scope has a SMPS PSU, meaning that the 120Vac is rectified you will find around 170Vdc at the SMPS transformer primary.

So, if you apply 170Vdc then you are good to go. If you apply a lower DC voltage (see below), the SMPS will not start, as the UVLO (undervoltage lockout) will do its job.

My Rigol says 100-240Vac input. For 100Vac the DC link in the SMPS is 141Vdc, for 120Vac is 170Vdc. Any voltage under 141Vdc will not be good for the scope PSU, as it will refuse to start.

To power from an UPS (non-sine waveform) it is completely acceptable for the SMPS PSU in the scope. That modified sine will be rectified anyway.

Most scopes have that same 100-240 VAC Operational parameters.  This is why I question how you could make 1 of these units run on a much lower voltage.  That's the part I was struggling with and it did not make sense to me. 

[JJ_023]

As well as the UVLO, some offline SMPS's need to detect the presence of AC to run. One I met recently would run on 60VDC, but with only one polarity.
I wouldn't like to see what happens if a PFC was fed with DC.
The main reason I run some things on DC is just for a first test run from a current limited supply. Mostly washing machine motor controllers.

So what am I missing because to me it seems contradictory to what you posted earlier.  How would you be able to make it run on that low of a voltage. 

[JJ_023]

a smps that starts on one polarity only has a half wave rectifier in front of it. i don't even understand how that would be cheaper than a fwb and a smaller capacitor.

Is there a way you could elaborate in a more detailed manner so I could grasp the concept.

Thank you.

[JJ_023]

Given that you have a 120Vac system to power the scope, and as the scope has a SMPS PSU, meaning that the 120Vac is rectified you will find around 170Vdc at the SMPS transformer primary.

So, if you apply 170Vdc then you are good to go. If you apply a lower DC voltage (see below), the SMPS will not start, as the UVLO (undervoltage lockout) will do its job.

My Rigol says 100-240Vac input. For 100Vac the DC link in the SMPS is 141Vdc, for 120Vac is 170Vdc. Any voltage under 141Vdc will not be good for the scope PSU, as it will refuse to start.

To power from an UPS (non-sine waveform) it is completely acceptable for the SMPS PSU in the scope. That modified sine will be rectified anyway.

if it is pfc corrected, the undervoltage lockout on the chip is likely to be 10-16 vdc, and that voltage is typically fed from two 1/4w resistors in series summing to around 500K ohms. so without enough volts, it won't work..

once the chip starts up, the feedback coil from the inductor supplies the current the control chip needs. so you may find you can start it on 50-100vdc, but once you get it started it will run with far less voltage, perhaps all the way down to 20vdc.

most other smps work the same way, a tertiary coil or the snubber on the flyback transformer powers the brains of the smps. for a flyback, its going to need a much higher voltage than a pfc circuit, which can operate all the way down to just 5volts dc input (to get 99% power factor, the boost converter has to be operating for 99% the area under the half sine. it will of course be severely power limited.

but lots of welding equipment is done this way now, you can get half the output on 120vac as you can 240vac. you'd get a 4th of the output at 60vac if you could make it not fault.

Just to support what you are saying, on welding equipment the output on 120 VAC is over 50% however the duty cycle is reduced considerably more than 50% on what you would find on 240 VAC.

[johansen]

the boost inductor and the boost switch have a hard current limit. this is likely the majority of the reason a lot of PFC corrected equipment probably won't run at all below 50 volts. the other issue is the tickler to supply current to start up the circuitry.

i was relatively surprised to see my 12v portable hard drive power supply work fine on 60vac, its just a regular 100-240v flyback power supply.