Connecting an Oscilloscope to the primary side of a SMPS  - Page 1
Forum section: electronics / general

author: OutThere
author_rank: Contributor
timestamp: March 19, 2023, 03:04:19 pm
content: 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.

author: nctnico
author_rank: Super Contributor
timestamp: March 19, 2023, 03:41:07 pm
content: 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.

author: bdunham7
author_rank: Super Contributor
timestamp: March 19, 2023, 03:50:15 pm
content: A while back in a similar discussion I posted this:https://www.eevblog.com/forum/testgear/probing-smps-with-oscilloscope/msg3152256/#msg3152256Unfortunately 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.

author: Faringdon
author_rank: Super Contributor
timestamp: March 19, 2023, 04:01:28 pm
content: 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.

author: bdunham7
author_rank: Super Contributor
timestamp: March 19, 2023, 04:25:50 pm
content: Quote from: Faringdon on March 19, 2023, 04:01:28 pmYikes! 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.

author: tggzzz
author_rank: Super Contributor
timestamp: March 19, 2023, 04:50:26 pm
content: Quote from: Faringdon on March 19, 2023, 04:01:28 pmDangerous 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/10754Key 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 inhttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/If you really want to play silly buggers, do this:

author: bdunham7
author_rank: Super Contributor
timestamp: March 19, 2023, 04:58:30 pm
content: 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.

author: dmills
author_rank: Super Contributor
timestamp: March 19, 2023, 04:59:02 pm
content: 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.

author: coppice
author_rank: Super Contributor
timestamp: March 19, 2023, 05:03:38 pm
content: 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.

author: bdunham7
author_rank: Super Contributor
timestamp: March 19, 2023, 05:04:58 pm
content: Quote from: dmills on March 19, 2023, 04:59:02 pmIn 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.

author: bdunham7
author_rank: Super Contributor
timestamp: March 19, 2023, 05:07:52 pm
content: Quote from: coppice on March 19, 2023, 05:03:38 pmThey 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.

author: Faringdon
author_rank: Super Contributor
timestamp: March 19, 2023, 07:40:18 pm
content: QuoteThanks, the Y cap can withstand that, they are always 250VAC.

author: jonpaul
author_rank: Super Contributor
timestamp: March 19, 2023, 08:10:12 pm
content: no need.1. connect any 120 or 240 V low voltage mai,s transformermain>< 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 VERTCH 1 >> probe>> L1 hotCH2 >> probe)>>L2 n'eutMains =CH1- CH2 like a differential ampJon

author: OutThere
author_rank: Contributor
timestamp: March 21, 2023, 03:20:44 am
content: 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

author: ahbushnell
author_rank: Frequent Contributor
timestamp: March 21, 2023, 03:42:55 am
content: Quote from: OutThere on March 19, 2023, 03:04:19 pmI suggest using a Differential Probe. I have a pico TA041 which is 1000Vrms CAT III. 700V common mode DC plus peak AC.picotech.comhttps://www.picotech.com/accessories/active-differential-high-voltage/25-mhz-700-v-differential-probe

author: OutThere
author_rank: Contributor
timestamp: March 21, 2023, 03:47:53 am
content: 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" versionI think I have cleared the SMPS power supply from being bad, just want to understand how it functions. electronics is truly remarkable.

author: OutThere
author_rank: Contributor
timestamp: March 21, 2023, 03:53:03 am
content: so what this guy is doing is dangerous?

author: OutThere
author_rank: Contributor
timestamp: March 21, 2023, 03:54:31 am
content: thanks I will check that outlow 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?

author: OutThere
author_rank: Contributor
timestamp: March 21, 2023, 03:59:58 am
content: Quote from: jonpaul on March 19, 2023, 08:10:12 pmnot 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

author: bdunham7
author_rank: Super Contributor
timestamp: March 21, 2023, 04:38:44 am
content: Quote from: OutThere on March 21, 2023, 03:53:03 amYep. Every metal part of his scope ispotentiallylive, 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.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: March 21, 2023, 04:42:14 am
content: Quote from: OutThere on March 21, 2023, 03:54:31 amNo it would not be the same. One side is grounded on the tek HV probe.

author: freda
author_rank: Regular Contributor
timestamp: March 21, 2023, 06:11:56 am
content: 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 fromthe isolation transformer.

author: Jim from Chicago
author_rank: Regular Contributor
timestamp: March 21, 2023, 05:22:27 pm
content: 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?

author: coppice
author_rank: Super Contributor
timestamp: March 21, 2023, 05:33:57 pm
content: Quote from: Jim from Chicago on March 21, 2023, 05:22:27 pmOld 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.

author: TimFox
author_rank: Super Contributor
timestamp: March 21, 2023, 05:46:24 pm
content: 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.

author: tggzzz
author_rank: Super Contributor
timestamp: March 21, 2023, 05:58:33 pm
content: Quote from: Jim from Chicago on March 21, 2023, 05:22:27 pmIn your case, if your DUT was battery powered, why was it (a) necessary and (b) beneficial to float the scope?More generally, there can be significant capacitance between a scope's case and other conductors. Some nodes don't tolerate excess capacitance, which could lead to erroneous traces and/or DUT damage.Very generally, it is reasonable to walk out into a quiet cul-de-sac road at night without looking. That doesn't mean it is sensible to do it on other roads at other times, does it?Once you have answered that,read thishttps://groups.io/g/TekScopes/message/10754Key sentence: "This guy was WELL AWARE of the danger and took a lot of precautions to prevent shock . . . Bottom Line: He died anyway."

author: shapirus
author_rank: Super Contributor
timestamp: March 21, 2023, 07:20:32 pm
content: Quote from: tggzzz on March 21, 2023, 05:58:33 pmBut what exactly did he do that caused his death? That message doesn't tell the details, which leaves the question unanswered. Did he take a lot of precautions and then touch a metal knob on the scope? Did he take a lot of precautions and then insert two nails with bare hands into the wall outlet? Did he take a lot of precautions and then walk out of the window?

author: coppice
author_rank: Super Contributor
timestamp: March 21, 2023, 07:23:45 pm
content: Quote from: tggzzz on March 21, 2023, 05:58:33 pmThat doesn't seem to say what killed him. I assume he used the oscilloscope's controls, which is pretty hazardous with an, old mostly metal, oscilloscope. It would be interesting to know, though. I've only seen someone hurt by a grounded oscilloscope, when a probe touched something it shouldn't, and he suffered metal vapour deposition on his hand and arm. As I said earlier, safety is a process, not a product.

author: tggzzz
author_rank: Super Contributor
timestamp: March 21, 2023, 07:38:13 pm
content: Those questions are asking for overly specific details that distract from the general lessons. Similar question: what caused the loss of an empire? Answer: a nail was missing from a horseshoe. Accurate, but not helpful.On one level what caused it is that he went into work that morning. On another perhaps his supervisor had set unrealistic expectations, or he had had an argument with someone. Maybe one end of a wire fell out his pocket and touched the scopes' case.None of those would have mattered if he hadn't floated the scope. That's the key point.If the answer to the question is that he touched the controls, some bright spark would latch onto that and claim everything would have been OK if he hadn't touched the controls (and that scope controls shouldn't be touched when the power is on!) Don't believe me? Look at some chains of "reasoning" elsewhere on this forum!

author: bdunham7
author_rank: Super Contributor
timestamp: March 21, 2023, 07:50:13 pm
content: Quote from: tggzzz on March 21, 2023, 07:38:13 pmYour method would make aviation safety investigations quick and simple...

author: Sighound36
author_rank: Frequent Contributor
timestamp: March 21, 2023, 07:57:19 pm
content: For all SMPS work I use a fibre optic full isolated voltage probe with 150Mhz BW, when dealing with small voltages that float on top of larger ones ALWAYS isolate the probe (or use a fully isolated probe) or use a scope with a fully isolated input usually a battery powered scope.

author: shapirus
author_rank: Super Contributor
timestamp: March 21, 2023, 08:36:31 pm
content: Quote from: tggzzz on March 21, 2023, 07:38:13 pmThey don't. The rules are only useful when it is clearly explained what exactly causes a given consequence, so much so that it makes any doubt impossible.The reason why it may be dangerous to do what that guy did is vague, unless it is explained how exactly he got himself killed.In other words, knowing the rules is only half the job of staying safe. Understanding them and the physical effects and processes which are behind those rules make it complete.

author: tggzzz
author_rank: Super Contributor
timestamp: March 21, 2023, 08:58:00 pm
content: Quote from: bdunham7 on March 21, 2023, 07:50:13 pmTrueNonetheless, the AAIB reports I've read highlight generic lessons such as that. They also mention any other contributory causes.Example: the glider which was hit by lightning near Dunstable Downs and exploded. That lead to improvements in "plastic" airliners w.r.t. lightning strikes.Or all the airspace infringements by power planes relying on electronic maps that choose not to include significant hazards

author: tggzzz
author_rank: Super Contributor
timestamp: March 21, 2023, 09:04:55 pm
content: Quote from: shapirus on March 21, 2023, 08:36:31 pmFrom that we can deduce you aren't a parent."Don't run into the road without looking" is sound advice. Itreallyisn't necessary to discuss which bit of the car impacts the body, nor what happens to the brain when it is subjected to rotational acceleration. Unless, that is, you have some blancmange handy and are prepared to sacrifice it in the name of science, safety, and fun.

author: shapirus
author_rank: Super Contributor
timestamp: March 21, 2023, 09:32:52 pm
content: Quote from: tggzzz on March 21, 2023, 09:04:55 pmWrong.Quote from: tggzzz on March 21, 2023, 09:04:55 pmIt is. It's not sufficient to be effective enough, this is why I add: "...because you can be hit by a car", because otherwise the child will have a "why?", which, if not asked, will leave the question unanswered, which can lead to ignoring the advice and, worse, maybe trying experimentally what would happen. This short explanation is both simple and sufficient even for a child: it is obvious that getting hit by a car is going to hurt. No further details will normally be requested (in my case, however, sometimes they will be).In times past children were taught using rods and other equipment to follow the rules, so even if they did not fully understand the real reasons why the rules must be obeyed, they at least understood that they would be beaten if they break them. Nowadays those methods are considered inappropriate, therefore children need to get and understand a reasonable explanation, at least, in as many cases as possible.It's the same with electricity. There is no need (in this case) to explain the mechanism of how exactly electricity affects the internal organs and precisely what of that can cause death, but it is essential to explain the possible ways in which a dangerous potential can come into contact with the human body, causing a potentially lethal current passing through it, when the oscilloscope is used in an unsafe way.

author: nctnico
author_rank: Super Contributor
timestamp: March 21, 2023, 09:56:37 pm
content: Quote from: shapirus on March 21, 2023, 08:36:31 pmThe problem is that some are incapable of explaining clearly or comprehending the reasons why so there can be a lot of mismatch between the transmitter and receiver of the message. Adhering to 'Do as I say, not as I do' is a good default assumption.

author: tggzzz
author_rank: Super Contributor
timestamp: March 21, 2023, 10:05:34 pm
content: Quote from: shapirus on March 21, 2023, 09:32:52 pmYou snipped relevant context from my message, and proceeded to a straw man argument.But then you reversed that position and started agreeing (in your last paragraph) with the point I was making. Strange.For amusement I'll note that I taught my daughter to be careful of electricity with an electric fence. I included sufficient information that she understood how and why to do and not to do the test.IFundamentally you have to try to pitch the lesson so that it expands understanding. Pitching too much detail prevents that.

author: bdunham7
author_rank: Super Contributor
timestamp: March 21, 2023, 10:18:09 pm
content: Quote from: shapirus on March 21, 2023, 08:36:31 pmI'd agree it is at least worth discussing, but do we need to know exactly what part of the scope he touched? IIRC, that isn't actually known. What I remember from the original story is that it was determined that the electrocuted engineer was aware of the danger and took some precautions by installing some shielding or whatnot, but didn't do an adequate job and somehow, whether from forgetfulness or clumsiness or very bad luck, came in contact with the high voltage. If your scope is still sitting on a cart, as I believe it was in this case, you likely have not done a good job of shielding and safely floating it.You actually can float a scope safely and it has (and probably still is) done in special circumstances using isolated power supplies and putting the scope out of reach behind adequate shielding. I vaguely remember some story about old Tek 500-series scopes being floated up to very high voltages for some high energy experiments. The problem is that none of this is appropriate in the vast majority of circumstances, certainly not a home lab or even a commercial repair shop. The effort required to do it properly is simply way beyond what it takes to do something simpler and safer.

author: shapirus
author_rank: Super Contributor
timestamp: March 21, 2023, 10:24:22 pm
content: Quote from: bdunham7 on March 21, 2023, 10:18:09 pmThat's the problem! We do not even know if it's touching the scope that killed him. Maybe he touched the hot wires that he was going to probe? Maybe something else? There's more than one way to get yourself killed.

author: ogden
author_rank: Super Contributor
timestamp: March 21, 2023, 10:26:15 pm
content: Quote from: Jim from Chicago on March 21, 2023, 05:22:27 pmStill not advised even if considered safe. You can't trust fast transient/signal waveforms you measure such a way, not to mention that circuit may function differently when connected to scope ground "antenna" which is capacitively coupled to mains/earthing.

author: tggzzz
author_rank: Super Contributor
timestamp: March 21, 2023, 10:27:52 pm
content: Quote from: bdunham7 on March 21, 2023, 10:18:09 pmJust so.It is possible, but very difficult to do correctly, very easy to miss something, and the consequences are very serious.The picture I posted is intended to emphasise how difficult it is to do correctly. The chance of an inexperienced amateur doing it safely are close to zero. That doesn't mean they will come to harm, any more than walking into a road without looking means you will be knocked down.This is a common problem, so common that safe solutions are readily available - so use them!

author: Jim from Chicago
author_rank: Regular Contributor
timestamp: March 22, 2023, 07:41:29 pm
content: Quote from: tggzzz on March 21, 2023, 05:58:33 pmMy DUT is not battery powered. When I said my DUT is not powered by mains, I guess I meant that it's not directly powered by mains. I have a variable DC power supply which is plugged into mains, and that power supply is set to 48V which then powers the DUT. So there's nothing on the DUT itself which is over 48V. Within the DUT there is isolation so that the return for the 48V is a floating ground, and I needed to capture waveforms which are referenced to the 48V return. Originally when I was first doing this testing, I was not floating the scope, but then my mentor directed me to float it because he thought that earth-grounding the 48V return could be causing erroneous results. I'm not sure what his reasoning was behind that. It doesn't make much sense to me. But regardless I just want to make sure I'm safe because I'm actually supposed to take more measurements with the exact same setup in the near future.After reading through the various threads/articles on this issue, I do realize that floating the scope is a generally bad practice, so I will bring this up to my mentor soon.

author: Jim from Chicago
author_rank: Regular Contributor
timestamp: March 23, 2023, 08:23:41 pm
content: Well I brought this issue up to my mentor (an EE with ~30 years experience) and he said he doesn't have a problem with floating scopes. I showed him the documentation from Tektronix which warns against the practice but he basically thinks they're being overly cautious and trying to sell more of their differential probes. As I type this, I have a floated scope in front of me. My only solace is that the voltages on my DUT should not exceed ~48V. Regardless...is this a red flag that I should I be looking for a different job?

author: TimFox
author_rank: Super Contributor
timestamp: March 23, 2023, 08:32:17 pm
content: You can be injured with 48 V: see the electrocution literature.https://www.merckmanuals.com/professional/injuries-poisoning/electrical-and-lightning-injuries/electrical-injuriesI have seen CROs floated by much less than that, when a differential voltage waveform was needed with respect to a lower DC voltage, but I would never recommend it to anyone.I also knew employees of an electrical utility who claimed they could hold a 120 V wire, but I definitely don't recommend that!

author: tggzzz
author_rank: Super Contributor
timestamp: March 23, 2023, 08:38:58 pm
content: Quote from: Jim from Chicago on March 23, 2023, 08:23:41 pmI'm an EE with >40 years experience. No, it wasn't 1 year repeated 40 timesDoes that make my statements better than those from a 30 year EE? Of course not.A lot will depend on the details of the nodes to which you attach the shield. A 48V 1Mohm node is probably less problematic than a 48V 1mohm node. But if that node is slewing fast w.r.t. the bench/ground, then the capacitance between the scope case and bench/ground might destroy the UUT.In other words, "the devil is in the details", and nobody here can provide a definitive answer.

author: Faringdon
author_rank: Super Contributor
timestamp: March 23, 2023, 10:18:15 pm
content: QuoteNo, it means you are well suited because you think about this stuff.

author: tggzzz
author_rank: Super Contributor
timestamp: March 23, 2023, 11:04:56 pm
content: Quote from: Faringdon on March 23, 2023, 10:18:15 pmYes. But with the proviso that you are allowed to think, question, and refuse to do something.If that is "career limiting" in that company, you should consider it to be an opportunity rather than a problem.

author: nctnico
author_rank: Super Contributor
timestamp: March 23, 2023, 11:22:50 pm
content: Quote from: Jim from Chicago on March 23, 2023, 08:23:41 pmYou better refer your mentor to the user manual of the oscilloscope which likely says it must be grounded. Not just for safety but also to return HF noise into ground. IOW: your oscilloscope likely isn't designed for work without a ground connection. So ground your scope. Both for safety AND for optimal measurement results. Personally I'd be extremely wary about any information coming from someone who suggests to float a scope (or any piece of test equipment). I'd follow the manual for the piece of test equipment to the letter where it comes to safety. One of the things you should imprint in your mind is that experienced people leave out important details in their instructions because they (wrongly) assume that everyone is aware of those details.Regarding your measurement: if you measure a floating DUT (powered from an isolated bench supply), then it doesn't matter which ground point you choose as long as you use the same ground point for ALL test equipment connected to the DUT.

author: jonpaul
author_rank: Super Contributor
timestamp: March 23, 2023, 11:28:08 pm
content: float the DUT/SMPS on an isolation xformer.Scope is grounded.Done this since 1970s no issuesJ

author: vk6zgo
author_rank: Super Contributor
timestamp: March 24, 2023, 12:12:26 am
content: Quote from: bdunham7 on March 19, 2023, 05:07:52 pmJust about any sizeable city back in the day had at least one transformer rewinder, so if a relatively obscure OEM transformer failed, it was often cheaper to get it rewound, than try to get one from the other side of the world.Some of these rewinders also produced their own line of new transformers.All that expertise had disappeared now, & the only cheap transformers are generic ones from the PRC.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 24, 2023, 12:49:10 am
content: Quote from: bdunham7 on March 21, 2023, 04:38:44 amI certainly wouldn't do it.Back when BJTs were "king" in SMPS, I repaired more of them than I like to remember, & if I absolutelyhadto probe the thing for waveforms, I floated the DUT, not the 'scope.You might have noticed in the video that most of the actual fault finding was done using a DMM on the Continuity or Ohms ranges, with the 'scope just being used to check the correct pulses appeared, so he could have just "buttoned things up" & seen whether the TV worked.I found that after fixing a few SMPS, most of the time, I did my testing without the need to apply power.I have seen 'scope floating done, but this was in a specific situation where testing needed to be phase to phase on a 3ph system.The 'scope was placed on a dry, wooden "Technician's stool" to avoid inadvertent earthing.To do the actual tests a primary team of three was involved: -One to observe, another to stand by & yell loudly in case of trouble, & yet another standing with his hand on the main breaker handle ready to open the circuit if he heard a yell.There were several others who were doing other work nearby, but "keeping a weather eye" out for trouble.Measuring 50Hz phase to phase "hands free" in such a situation is relatively easy, as the 'scope can be set up to "line trigger", & the volts/div set for a convenient range in advance.The "three people" setup was rigidly adhered to, however, & unless you can reproduce such conditions, "floating" a 'scope is averybad idea!To bring things into context----'scopes with isolated inputs were rare & specialised in those days, & differential probes cost as much as the original oscilloscope.

author: David Hess
author_rank: Super Contributor
timestamp: March 27, 2023, 11:01:43 pm
content: If you cannot use a suitable differential probe, then use the isolation transformer on the device under test and leave the oscilloscope grounded.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: March 28, 2023, 11:58:26 pm
content: Quote from: vk6zgo on March 24, 2023, 12:49:10 amWe used a "USB" scope that had Ethernet. We used a Ethernet to fiberoptic coupler and operated the scope on the computer from another room. This was for a MW class system.