Connecting an Oscilloscope to the primary side of a SMPS

[OutThere]

so the primary side of the SMPS is a hot ground and the scope is at earth ground. Should i use an isolation transformer for the o'scope? i have one that i plug SMPS's into. it has a passthrough ground, i can disconnect the ground. in general, is a pass-through ground good on an isolation transformer since the ground goes back to the circuit breaker and is connected to the neutral.

[nctnico]

What you need is a CAT rated differential probe.

Another good option is to power the circuits in the primary side from a low voltage lab supply. That way you can check whether the switched chip is doing what it is supposed to do and whether the switching transistors still work BEFORE applying dangerous voltages to the PSU.

[bdunham7]

A while back in a similar discussion I posted this:

https://www.eevblog.com/forum/testgear/probing-smps-with-oscilloscope/msg3152256/#msg3152256

Unfortunately the photos didn't go where they should have and I haven't sorted them, so read carefully and understand before you do any of this.

DO NOT float the scope.  That is an insane idea and totally unnecessary.  You do have to interrupt the DUT ground in most cases otherwise the isolation transformer is of no use.  You should do all of your probe connections with the power off and then all measurements with power on should be totally hands-off.  Start with only one ground clip, then if you need to add more only connect them all to the exact same spot.

[Faringdon]

The trouble with pass-though earth on an isol transformer, is that you can then only hook your scopeprobe ground to that node to which the pass through earth connects, on the secondary side of the isol transformer. If you do cut the pass-through earth, then putting a 1MEG resistor between isol txformer sec side "neutral" and earth is a good idea.

The problem with isolation transformers is that with BCM Flyback PFC stages at the front end, the isolation transformer leakage will ring with the input filter, so you cant use isol transformer with them.

The problem with diff probes , eg TA041, is that you get dreadfully noisy scope shots when you probe eg low value sense resistors.

If you know exactly what you are doing, you can float the scope (cut off the earth), and use it like that, but be very careful, dont touch anything metal on the scope when you do it......turn off the mains when you move the probe onto the circuit....ensure eveything is discharged....when you float it, you can then use eg a home brew coaxial probe to cleanly scope sense resistors on the primary side of the SMPS.......and get much less noise on your scope shot.

[bdunham7]

If you know exactly what you are doing, you can float the scope (cut off the earth), and use it like that, but be very careful, dont touch anything metal on the scope when you do it......turn off the mains when you move the probe onto the circuit....ensure eveything is discharged....when you float it, you can then use eg a home brew coaxial probe to cleanly scope sense resistors on the primary side of the SMPS.......and get much less noise on your scope shot.

Yikes!  For current measurements, I'd really recommend getting a current probe instead, or a better diff probe or an isolated scope.  There's also no guarantee that your scope can withstand ~400V from its ground to its power neutral.  As for clean scope shots, doing repairs and hobby-level stuff, there's often no need for such detail.  If you are in the research and design area and have such poor toolset and budget that you have to resort to floating scopes and homemade probes, then I don't know what to say.

[tggzzz]

If you know exactly what you are doing, you can float the scope (cut off the earth), and use it like that, but be very careful, dont touch anything metal on the scope when you do it......turn off the mains when you move the probe onto the circuit....ensure eveything is discharged....when you float it, you can then use eg a home brew coaxial probe to cleanly scope sense resistors on the primary side of the SMPS.......and get much less noise on your scope shot.

Dangerous bollocks. Pure and simple. (For the uninitated, have a look at the posting history of Faringdon/Treez, his employment history, and the opinion many people have expressed on this forum).

Read this anecdote about a professional engineer that took considerable precautions: https://groups.io/g/TekScopes/message/10754 Key sentence: "This guy was WELL AWARE of the danger and took a lot of precautions to prevent shock . . . Bottom Line: He died anyway."

For safe ways to achieve what the OP wants, see the references in https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

If you really want to play silly buggers, do this:

[bdunham7]

If you absolutely must float your scope, get an isolated scope.

https://spwindustrial.com/tektronix-tps2024b-digital-storage-oscilloscope-200-mhz-2-gs-s-4-isolated-channels-tft-color-display-battery-powered-certificate-of-traceable-calibration-standard/?

If this too much money, then consider just not doing whatever it is that you think requires floating your scope.  Or find a less-expensive isolated scope--they are much less common but they do exist in various forms. 

[dmills]

Yea, NEVER float the bloody scope, people, even ones who thought they knew exactly what they were doing have died that way.

There are a few scopes with actual isolated cat III or cat IV inputs that can be used for this sort of thing, see for example the R&S®RTH1002 and friends that are Cat IV 600V (Even input to input), or I know Tek made something similar, but a current probe or Rogowski coil are cheaper and the way to go for most lab stuff. 

If I was doing the hot side of switchers routinely, the boss would buy me the R&S with its power electronics option without even batting an eyelid, because I would simply be refusing to touch that shit without the right tools, end of the day it is less then a months salary.

[coppice]

On a related topic, what has happened to the transformer market? Has the dominance of switched mode supplies driven 50Hz/60Hz mains transformers to boutique status and prices? Its years since I've bought such a transformer. I've just been looking for one to use for bench top isolation and the prices make me wince. There is quite a bit of copper in them, but I don't remember every recoiling at the price before.

[bdunham7]

because I would simply be refusing to touch that shit without the right tools, end of the day it is less then a months salary.

In a first-world professional context, that's the issue.  No sane employer would want to incur the liability of using ad-hoc methods known to be dangerous just to save the cost of an isolated scope.  That said, there are plenty of employers that are not particularly sane, so good luck there.

[bdunham7]

On a related topic, what has happened to the transformer market? Has the dominance of switched mode supplies driven 50Hz/60Hz mains transformers to boutique status and prices? Its years since I've bought such a transformer. I've just been looking for one to use for bench top isolation and the prices make me wince. There is quite a bit of copper in them, but I don't remember every recoiling at the price before.

They might cost a bit more now due to inflation and particularly copper prices, but I don't remember that decent transformers were ever all that cheap other than surplus items.

[Faringdon]

There's also no guarantee that your scope can withstand ~400V from its ground to its power neutral.

Thanks, the Y cap can withstand that, they are always 250VAC.

[jonpaul]

no need.

1. connect any 120 or 240 V low voltage mai,s transformer

main>< 120V pri>< 12V (eg) sec>< scope.

Voila..isolation and good replica of mains voltage scaled down.

Most small,mains 50/60Hz  transformers can give 400..1000 Hz BW.

2. setup 10x or 100x probes on 2,ch scope use CH1- CH2 or ADD/I VERT

CH 1 >> probe>> L1 hot
CH2 >> probe)>>L2 n'eut

Mains =CH1- CH2 like a differential amp

Jon

[OutThere]

yeah  i was thinking of something like that . i also use a load lamp.  i have a 2 channel power supply 50v each channel.  I read somewhere it's bad to run SMPS on lower than normal power. I have a variable transformer somewhere but that runs on the AC lines

[ahbushnell]

so the primary side of the SMPS is a hot ground and the scope is at earth ground. Should i use an isolation transformer for the o'scope? i have one that i plug SMPS's into. it has a passthrough ground, i can disconnect the ground. in general, is a pass-through ground good on an isolation transformer since the ground goes back to the circuit breaker and is connected to the neutral.

I suggest using a Differential Probe.  I have a pico TA041 which is 1000Vrms CAT III.  700V common mode DC plus peak AC.

picotech.com
https://www.picotech.com/accessories/active-differential-high-voltage/25-mhz-700-v-differential-probe

[OutThere]

first off, yeah I like fixing stuff too. i got a 52in 14 year old LCD TV off of craigs list, that keeps cycling on and off. the next week got 2 mowers to add to my collection.  REPAIR EVERYTHING. second thanks for taking the time to create this when you originally did. pic number 2 looks like you are using the case ground which I assume would be for the secondary cold side. what i was thinking and lead to believe for the primary side of the SMPS with a hot ground I would use the negative of one of the filter caps?  so how would you hook up your scope for this ?    I read through what you posted, still not sure I guess I need the " see spot run" version   I think I have cleared the SMPS power supply from being bad, just want to understand how it functions. electronics is truly remarkable.

[OutThere]

so what this guy is doing is dangerous?

[OutThere]

thanks   I will check that out      low voltage or high voltage, so what do they accomplish? ok i clicked on the link, sort of understand now. I have an old Tek 465B, says the probe is only good up to 25mhz. I know a long time ago when there were CRT MONITORS i WAS LOOKING AT A PROBE TEKTRONIX HAD. IT WAS A HIGH VOLTAGE PROBW, WOULD THAT BE THE SAME?

[OutThere]

no need.

1. connect any 120 or 240 V low voltage mai,s transformer

main>< 120V pri>< 12V (eg) sec>< scope.

Voila..isolation and good replica of mains voltage scaled down.

Most small,mains 50/60Hz  transformers can give 400..1000 Hz BW.

2. setup 10x or 100x probes on 2,ch scope use CH1- CH2 or ADD/I VERT

CH 1 >> probe>> L1 hot
CH2 >> probe)>>L2 n'eut

Mains =CH1- CH2 like a differential amp

Jon

not sure i follow seems like u are using an isollation transformer?  u lst me after the 12volts  .pretend i am a 7 year old you aretelling this too

[bdunham7]

so what this guy is doing is dangerous?

Yep.  Every metal part of his scope is potentially live, and perhaps other things as a result.  If he were to use the isolation transformer on the DUT (television in this case) that would not be the case.

[ahbushnell]

thanks   I will check that out      low voltage or high voltage, so what do they accomplish? ok i clicked on the link, sort of understand now. I have an old Tek 465B, says the probe is only good up to 25mhz. I know a long time ago when there were CRT MONITORS i WAS LOOKING AT A PROBE TEKTRONIX HAD. IT WAS A HIGH VOLTAGE PROBW, WOULD THAT BE THE SAME?

No it would not be the same.  One side is grounded on the tek HV probe. 

[freda]

one other thing to note,  the isolation transformer limits the power drawn from the mains. It is the DUT that is uncertain, so firstly that is what you want running from
the isolation transformer.

[Jim from Chicago]

This thread is troubling me because I have been directed in the past by a very experienced EE to float my scope for certain measurements. However in my case, the DUT was not powered by mains. It was powered by a supply giving it 48V. Is floating the scope still ill-advised in this case?

[coppice]

This thread is troubling me because I have been directed in the past by a very experienced EE to float my scope for certain measurements. However in my case, the DUT was not powered by mains. It was powered by a supply giving it 48V. Is floating the scope still ill-advised in this case?

Old scopes could be really nasty when floated, as they had metal cases. The only metal you can touch on most modern scopes is the BNC connectors, and perhaps a USB socket or two. Everything else is encased in plastic. However, if the ground side of one scope probe manages to be raised to a substantial voltage all the BNCs and anything attached to a USB port, do so too, with no RCB or other protection there to quickly disconnect the power for you.

Safety is not a product. Its a process. If you float a scope, and use the process some people advocate here - set things up, turn on the power without touching anything, observe, turn off the power, rinse and repeat with various setups - a floating scope is potentially as safe as any other setup you might use, especially if you shield yourself from the setup, so you are isolated from any spectacular results. However, that doesn't really work with most modern instruments. You have to configure them every time you turn them on. You might just be pushing plastic buttons on a plastic panel, but its not ideal.

[TimFox]

I remember many companies and engineers who should have known better floating a scope (on a 2-pin to 3-socket "UL-listed" adapter without connecting the ground wire or tab) to measure a voltage or waveform with respect to something other than chassis ground, back in the day of analog CROs.
I haven't seen a company recommend it recently.