Isolation transformer ground question - Page 1
Forum section: electronics / beginners

author: OutThere
author_rank: Contributor
timestamp: December 29, 2022, 08:30:51 pm
content: I have acquired an isolation transformer. its's just the transformer. I have a sheet metal box i am going to install the transformer in. ON my side of the transformer going to install a duplex GFI SOCKET to plug the devices under test into. The AC line side I live in a house with just 2 prong AC outlets. so the GFI has for a ground or if i use a regular 3 wire duplex outlet that has a ground too. i was told to use the center screw as ground on the wall outlet. not sure if the screw which contacts the cover plate which contacts the metal box the 2 prong AC socket is in connects to some sort outer shielding on the 2 wire cable or connects to the neutral wire. so how should i think about grounding in this case or in a perfect case if it was a 3 wire AC outlet

author: Benta
author_rank: Super Contributor
timestamp: December 29, 2022, 09:12:35 pm
content: If your house installation has no ground, there's no reason to do all the things you're planning.The GFIs on the isolation transformer are completely misplaced. What should they do? The output is (hopefully) floating, so how would you get a ground fault? Discard them. Fuse and on/off switch make sense, include those.On the primary side, you have no ground either. That's not a tragedy, it will work without problems. That the metal case is not grounded is not perfect, but no show-stopper. It's not dangerous.

author: py-bb
author_rank: Regular Contributor
timestamp: December 29, 2022, 09:53:02 pm
content: How do your plug sockets not have ground?

author: nightfire
author_rank: Frequent Contributor
timestamp: December 29, 2022, 11:38:53 pm
content: For US specifics, I am no expert.But in the 1950s in germany it was up to code to also install 2-pronged outlets in rooms where there was no other conductor like central heating etc., and you had to obey some distance of I think 3 meters (approx 10 feet) to the next tertminal to avoid touching other metal cases in case of failure.Was called "isolation due to non conducting rooms"Or you have a so called "IT" system, which is isolated against earth- I also would be interested in the backgrounds ans US specifics in this case.

author: OutThere
author_rank: Contributor
timestamp: December 30, 2022, 03:34:13 pm
content: i thought the way a GFI worked was if as long as the current entering was the same as exiting it would not trip. if the current leaving varied it would trip the breaker in the GFI , explain why this would not be good?

author: themadhippy
author_rank: Super Contributor
timestamp: December 30, 2022, 03:39:44 pm
content: weres the current going to escape to if there no path to earth?

author: Ian.M
author_rank: Super Contributor
timestamp: December 30, 2022, 04:16:43 pm
content: Unless the transformer develops an internal fault, and neglecting high frequency noise on the power line which could be capacitively coupled from one transformer winding to another, all current going into one end of a transformer winding *MUST* come out the other, so there can never be a current difference to trip a GFCI directly connected to the secondary. Similarly the transformer blocks any faults downstream of it from tripping any GFCI upstream of it.If you don't have a good earth ground to ground the metal box to, then all circuits and connections inside it *MUST* have double or reinforced insulation and all wiring must be secured so it can never contact the box, even if wire nuts or terminal screws work loose. Personally, I'd use a well-built plywood box with a non-resettable thermal fuse in close contact with the transformer so it isn't a fire hazard, and an on-off switch with no metal parts exposed to the operator.

author: EPAIII
author_rank: Super Contributor
timestamp: December 31, 2022, 02:02:34 am
content: YOU!Quote from: themadhippy on December 30, 2022, 03:39:44 pm

author: themadhippy
author_rank: Super Contributor
timestamp: December 31, 2022, 03:36:11 am
content: Quoteonly if the supply is referenced to earth,not much is gonna flow if the supply is isolated via say an isolation transformer

author: Zero999
author_rank: Super Contributor
timestamp: December 31, 2022, 10:17:16 am
content: Quote from: themadhippy on December 31, 2022, 03:36:11 amThis discussion arises fairly often. Although an RCD doesn't require an earth connection to the device, it still needs the neutral to be earthed, otherwise it won't work. An RCD will not function if it's on the secondary side of an isolation transformer, which is not connected to earth.

author: Ian.M
author_rank: Super Contributor
timestamp: December 31, 2022, 11:23:51 am
content: North American practice according to the NEC is to earth (ground) the neutral at the main distribution panel (aka: fuse-box or breaker-box) with a link (frequently a special screw in the neutral busbar that bonds it to the earth grounded case) that must *NEVER* be fitted in subsidiary panels.Other parts of the world do it differently and in most cases, earthing the Neutral is the responsibility of the supply authority/utility company and must *NEVER* be done by the customer's wiring/equipment.Exceptions: non grid-connected generators and the like, for which: You own/rent it, *YOU* are responsible for earthing its Neutral so RCDs/GFCIs work etc.

author: OutThere
author_rank: Contributor
timestamp: December 31, 2022, 01:35:07 pm
content: well why do you use an isolation transformer? to protect in case, you become the path for the current 1. your isolated from the main AC. 2.When the GFI sees an unbalance of current, more current coming in on the hot and less or no current on the neutral it pops the internal cicuit breaker

author: OutThere
author_rank: Contributor
timestamp: December 31, 2022, 01:39:07 pm
content: I am using the GFI as an out worldly device attached to the secondary that I plug whatever i am working on into the GFI for additional protection. I am using a switch on the primary side and all the leads are soldered and shrink wrapped except for the leads to the GFI

author: Ian.M
author_rank: Super Contributor
timestamp: December 31, 2022, 01:48:57 pm
content: As has been pointed out, above, that wont work. The GFCI fed by the isolation transformer secondary wont trip due to a downstream fault to ground, unless you defeat the isolation by grounding the end or tap on the secondary feeding the GFCI supply side neutral terminal, before the GFCI. You may be fooled by the GFCI self-test button which *will* trip it, as it connects a resistor diagonally across the GFCI terminals e.g. from load side Line (phase) to supply side Neutral, thus unbalancing its leakage detection current transformer *without* a current to ground.Try it yourself - you are in America so your GFCI should trip below 5mA. Briefly connect a 22K 1W resistor from the GFCI's load side Line terminal to ground and see if it trips! Then remove the isolation transformer and connect the GFCI direct to the supply and repeat - it will trip as expected unless your 'ground' is faulty. Make sure your 'ground' is actually at ground potential - use a socket tester to confirm its at ground potential. Please set up this experiment safely - e.g. wire the circuit with mains rated terminal blocks and a momentary switch rather than holding the resistor in your hand!

author: OutThere
author_rank: Contributor
timestamp: December 31, 2022, 01:56:27 pm
content: I have been told that to upgrade a two-wire outlet to a three-wire is to connect the ground lug to the neutral in the wall outlet IE on the receptacle. Which I believe the ground and Neutral wire end up connected to the same bus bar in the fuse box. so the only way connecting the ground to the neutral would be bad is if in the wall the neutral became open. Don't know if this is CODE or not just something i filed in the back of mind when someone told me this. actually, from my understanding of the reason of a three-wire plug is if you have a piece of machinery and the hot lead shorts to the metal case. nothing would happen till you touched the case and became the path to ground on a two-wire plug. In the three wire the equipment case is connected to the ground lug so in the previous scenario as soon as the hot wire contacted the case it would cause a short and open the circuit breaker.

author: OutThere
author_rank: Contributor
timestamp: December 31, 2022, 02:00:02 pm
content: does the GFI need a ground to make make it's decision on whether or not there is an imbalance of what is coming in to going out

author: bdunham7
author_rank: Super Contributor
timestamp: December 31, 2022, 02:03:59 pm
content: Quote from: OutThere on December 31, 2022, 01:56:27 pmIt is absolutely not code compliant. To convert a two-wire socket to a three-wire version with a ground prong, you would use a GFI socket that is rated for this purpose and then put the sticker on the front that tells you that there is no ground.QuoteNo.

author: Ian.M
author_rank: Super Contributor
timestamp: December 31, 2022, 02:05:34 pm
content: It is not permitted to connect Neutral to Ground anywhere except your main distribution panel and it is no longer permitted*to use the Ground wire as a current carrying conductor for normal operation.Reason: a single broken wire or loose connection must *NEVER* cause the case of any grounded equipment to become live.If you cant grasp these simple concepts, you should avoid working on *ANY* mains voltage circuits - call in a qualified electrician for fixed circuits and take portable devices to a competent repair shop. We don't *EVER* want the next message from your user name to be from your nearest and dearest asking us to explain how you electrocuted yourself and died!* Exception: I believe there is still a grandfather clause for existing electric stoves with 120V mechanical timers on circuits fitted with a 3 pin 240V NEMA outlet (which doesn't have Neutral). If you do *ANY* electrical work on the stove or circuit or move it to another location, you must either rewire both with separate Ground and Neutral conductors, replacing the plug and socket with appropriately rated four pin ones, or remove the 120V timer and fit a 240V one. New installations of such three wire 240V stoves with 120V timers haven't been permitted for many years, so you are vanishingly unlikely to encounter one, except in a museum.

author: Benta
author_rank: Super Contributor
timestamp: December 31, 2022, 07:46:08 pm
content: @OutThere, you're in very deep water here, with a real risk killing someone or burning your house down.Forget those d*mn GFIs, why are you so fixated on them? They will NOT work without ground/PE.Your two-prong isolation transformer will work just fine. Leave it at that.If you absolutely want ground (the correct techincal term is PE = protective earth) in your installation, get a certified tech with license from the utility to make it.Period. Anything else is gambling.

author: james_s
author_rank: Super Contributor
timestamp: December 31, 2022, 07:56:33 pm
content: Quote from: OutThere on December 31, 2022, 01:56:27 pmNO! This is illegal and is absolutely not a good idea, NEVER connect the ground terminal to anything other than a separate dedicated earth ground conductor. Yes in the panel neutral is bonded to ground but that is the ONLY place you are permitted to tie these together. Do not mess with the wiring in your house unless you know what you're doing, which in this case it sounds like you don't.

author: james_s
author_rank: Super Contributor
timestamp: December 31, 2022, 08:00:49 pm
content: Quote from: Benta on December 31, 2022, 07:46:08 pmSure they will, the ground terminal on a GFCI doesn't connect to the electronics in it at all, it's just a passthrough and it is permitted (in the USA) to install a GFCI on a legacy non-grounded circuit. They work because the neutral is bonded to earth in the panel so if a leakage path develops (such as a person standing on the ground touching a bare wire) there will be an imbalance between the current through the hot and neutral of the GFCI and it will trip.A GFCI on the output of an isolation transformer will do nothing though, there is no way it can ever trip unless the isolation transformer fails such that it is no longer isolated. It makes no difference whether you have an earth ground or not, the output of an isolation transformer is isolated so it's impossible to have a leakage current from either side to any external node.

author: tatel
author_rank: Frequent Contributor
timestamp: January 02, 2023, 06:49:05 pm
content: Well, from this thread (and others) I got that an GFDI o whatever the name (it seems that here it's called a differential switch) will not work if placed on the secondary side of a (isolation) transformer. The cause would be galvanic isolation given by most (all) transformers, because there isn't any direct electrical connection between primary and secondary, so there isn't any earthing, wich would be needed for the GFDI to work.I just can't understand it yet. So I searched about GFDIs on the Net and got a picture explaining how these devices work. Please see the linked image.https://i.ytimg.com/vi/l_0l_EbxAZw/hqdefault.jpgI can't see anything earth-related on that picture. I see a toroid with tree independent windings. Two of them are related respectively to L and N. They induce contrary magnetic fields. So, if all goes well, those magnetic fields nullify each other and nothing happens. But, should have any difference, a magnetic field would appear, inducing a voltage/current in the third winding, then a breaker would act, disconnecting L and N. Is that right?If so, should I be working on a SMPS, after first an isolation transformer, then a GFDI, one hand in the pocket, and should a current pass through my body to earth, putting my life in danger, surely the N winding would induce a lesser magnetic field tan the L one, therefore a voltage would appear in the third winding, activating the safety switch, so breaking both connections to the secondary winding of the isolation transformer? What am I missing? If that picture is correct, what has earthing N (be it at the home panel or at the electric company substation), to do with this "would GFDI work" matter?Of course I understand that, working on the SMPS with both hands, a current could flow through my body and still get "back" into N, so GFDI couldn't work, but why this has anything to do with having N at earth potential?As I understand it, it's just a matter of having the same current in and out of the circuit. Should there be any difference (30 mA seems to be the norm here), GFDI should work, yet most (all) of you knowledge guys say it woldn't work. Could anyone explain it to me like if I were Forrest Gump?

author: james_s
author_rank: Super Contributor
timestamp: January 02, 2023, 07:01:55 pm
content: The relation to earth is at the panel, neutral is bonded to earth, so whether you have a ground wire or not, if you are standing on the earth and you touch the live wire, some current will pass through your body to earth instead of through the neutral wire and that will cause the GFCI to trip.The isolation transformer breaks this, it provides galvanic isolation so if you are standing on the earth and you touch the live wire nothing will happen, no current can flow because there is not a complete circuit, you won't get a shock, and the GFCI won't trip. A GFCI with an isolated supply can never trip because the isolation means there can never be an alternate path for the current other than the neutral wire.

author: themadhippy
author_rank: Super Contributor
timestamp: January 02, 2023, 07:46:16 pm
content: Quotemaybe thats always the case in america,but in many parts of the world the neutral - earth link could be anywhere on the network,and making your own link at the panel would break local reguations

author: Ian.M
author_rank: Super Contributor
timestamp: January 02, 2023, 07:54:58 pm
content: +1, as I mentioned back in reply#10

author: james_s
author_rank: Super Contributor
timestamp: January 02, 2023, 08:29:19 pm
content: Quote from: themadhippy on January 02, 2023, 07:46:16 pmYes, the OP is in America, I'm in America, that's the location I'm discussing here. Other parts of the world bond neutral to earth in other ways, but as far as I know it is always done somewhere, and it is never done downstream of the panel anywhere.

author: themadhippy
author_rank: Super Contributor
timestamp: January 02, 2023, 08:41:10 pm
content: but the origin of the the post above yours,that you apeared to be replying to was spain,Quoteapart from were concentric earthing on the consumers side is used,ok it maybe never made it to the states,and its an extremly rare beast in the uk nowerdays,but they do exist.

author: gf
author_rank: Super Contributor
timestamp: January 02, 2023, 09:23:32 pm
content: It is of course possible to turn the output of the tranformer into a private TN system in order to make a RCD work, by declaring one of the two conductor as Neutral and tying it to PE. But this defeats the purpose of anisolationtransformer which is rather supposed to provide an IT system at the secondary side.

author: tatel
author_rank: Frequent Contributor
timestamp: January 02, 2023, 10:05:20 pm
content: Yeah, in Spain I don't know any home that has those devices in, say, the bathroom. Just one at the panel, period. They seem to be 30 mA, not 5. Not really sure about it, but earthing seems to be done at least at the electric company transforming sub-station *and* somewhere into the building, the last being absent in many old buildings, I think. And, of course, the plugs can be inserted into the sockets both ways. I guess these are EU rules, not just spanish rules.When I read abouth earthing, three-prong plugs, sockets with fuses and GFCIs, etc, in UK-US-Australia, etc, I wonder why EU citizens aren't dying by electrocution by the dozens daily... I asked about that to some friends of me, pro electricians, and got answers that (I guess) english-speaking people wouldn't like. Not that I understand technical reasons behind those answers, either.So I'm going to catch (I'm being facetious) one of these friends by the neck and get my home electrical installation renewed and re-certified (I guess somebody had it certified somewhere in time or I wouldn't be getting any service from the electricity provider). I live in an old building fully owned by Ebenezer Scrooge, so I will pay for it, then I will try to get my money back from the landlord (it will be like mission impossible). The only thing I know for sure is that earth line is non-existant on the entire building, which is some 150 years old. This has made me afraid of things like having an electric water heater or using the oscilloscope I bought last year. I can solve this problem by just throwing money at it, so let's go for it. That said, neither me nor any neighbour got electrocuted in more than 20 years I'm living there.But, for the life of me, I can't understand why a GFCI wouldn't work after an isolation transformer or, for the case, why would an isolation transformer would spare my life from electrocution. So I'm guessing I don't understand how the earthing works. Your answers seem to imply that a circuit is closed between two earthing points, one of them being me and the other at the sub-station-panel-wathever?I was under the idea that all the earth is more or less at the same potential everywhere and that it is enough to be the path between two points at a different enough potential -say, line and earth- to get pretty darn good toasted. This misunderstanding can't be really solved by just throwing money at it. Could you guys give me any pointers?

author: gf
author_rank: Super Contributor
timestamp: January 02, 2023, 10:49:37 pm
content: Touching two points with different potential only toasts you if the loop impedance of the closed circuit is low enough so that enough current can flow through your body. This is not granted if you touch only one conductor of the secondary side of an isolation tranformer, since (due to the isolation) there is a very high impedance between the other conductor and earth. If you touch both conductors you still get toasted, of course.Edit: Not the voltage toasts you, but the current flowing through your body.

author: james_s
author_rank: Super Contributor
timestamp: January 03, 2023, 05:00:50 am
content: Quote from: tatel on January 02, 2023, 10:05:20 pmI think what you're missing is that electrical potentials are always relative to each other. The potential of the earth is not an absolute value, it is a value relative to some other node that is referenced to it. For example if you take a 12V battery and you tie the negative terminal to a rod driven into the earth then you can call the earth "ground" and you will measure 12V between it and the battery. If on the other hand the battery is just floating without either terminal connected to the earth, you will measure 0V between either terminal and the earth, the battery is floating, it has no connection to the earth so there is no voltage relative to the earth.An isolation transformer is exactly the same, the secondary is floating, just like a battery. If you are standing on the earth and your body is at the same potential as the earth you can touch either side of the isolation transformer secondary and you won't get shocked because there is not a complete circuit and no current can flow. To pass a current you need a circuit, a complete, circular path connected between two nodes that have a potential relative to each other.

author: tatel
author_rank: Frequent Contributor
timestamp: January 03, 2023, 09:09:49 am
content: OK, got it. So the reason an isolation transformer makes it safe to touch just one exposed part of the secondary side, is that the non-galvanic connection to the primary side, wich is the related to earth one, has high impedance. This means the relation to earth has been broken on the secondary side. To get electrocuted by touching just one exposed part of the secondary, a current must flow from that secondary to earth throug me, but it can't happen since their potentials aren't related (=0V=no current can flow). No electrocution then.Should I touch two exposed parts at different enough potential on secondary side, a current would flow trough me, without going to earth, so the GFCI couldn't see any difference between L and N windings, so it wouldn't break.Thank you very much, guys.

author: Ian.M
author_rank: Super Contributor
timestamp: January 03, 2023, 11:04:51 am
content: What's worse is that as soon as your test equipment introduces a ground (e.g. you start probing the primary side of the PSU with an ordinary scope probe), that isolation and the limited safety it provides goes away, leaving you with ground referenced mains voltage that can electrocute you with a single touch, but due to the action of the transformer, wont trip any upstream RCD/GFCI protection.RCDs/GFCIs have saved a *LOT* of idiots from receiving Darwin awards, and a lesser number of smart careful people from unexpected electrocution.Are you certain you would fall in the 'smart careful people' category, as you need to be one if you use an isolation transformer on your test bench!

author: Zero999
author_rank: Super Contributor
timestamp: January 03, 2023, 12:03:15 pm
content: Quote from: Ian.M on January 03, 2023, 11:04:51 amAn insulation monitoring device can provide a warning when that happens.There are off the shelf devices available, or if it's just for hobby use, build your own from a couple of neon lamps and high voltage resistors.https://docs.rs-online.com/e9c5/0900766b812cdc34.pdfIf an audible, as well as visual warning is required, the neon can be substituted for an AC input opto-coupler connected to a BJT to activate a small piezo buzzer and LED.

author: tatel
author_rank: Frequent Contributor
timestamp: January 03, 2023, 01:03:36 pm
content: Quote from: Ian.M on January 03, 2023, 11:04:51 amI'm careful enough not to use my benchtop scope unless the DUT is a batery powered one. I'm careful enough to scratch my not-so-deep pockets to do what the landlord should do if *he* were smart. I'm careful enough not to put an electric water heater in a house where electrical installation has no earth wire. I'm careful enough to watch Dave's video about "How not to blow your scope" and to follow related threads on the forum for more than a year to know what "not to do unless you really know what are you doing".But, above all, I'm stubborn enough to continue asking again and again until being sure I'm getting right any important concept that crosses my way. Those who ask may look stupid once. Those who don't ask, afraid to look stupid once, remain stupid forever.Now that I get how an isolation transformer and a GFCI work, I can begin to think about what should I choose to do. I'm very grateful to both gf and james_s for telling me what I was missing.That said, thank you for the "advice". Do you realize that giving real knowledge, like gf and james_s do, is much better than just calling names and saying things without giving any knowledge?

author: Ian.M
author_rank: Super Contributor
timestamp: January 03, 2023, 01:40:28 pm
content: If you feel offended by my comment, think about those two categories again.I'm assuming you are smart and careful, because you *ARE* asking these questions, not just going ahead and putting yourself into danger, but even smart careful people can make mistakes, so when you are confident you have the necessary knowledge and understanding, *PLEASE* don't let yourself get complacent about the hazards of working on devices under test fed by an isolation transformer. If you've *EVER* tripped a RCD (GFCI) while holding or touching something wet or metallic, you are probably one of the lucky guys who's been saved by one, but downstream of an isolation tranformer you don't have that extra layer of protection.

author: tatel
author_rank: Frequent Contributor
timestamp: January 03, 2023, 02:20:16 pm
content: Quote from: Zero999 on January 03, 2023, 12:03:15 pmThank you for that information. However for use after an isolation transformer, those devices in all cases seem to have in some way connection to earth. This would be defeating the isolation transformer, wouldn't it? As said before, I now have to think about isolation transformer being worth to have or not.I have read somewhere that to properly fully check an SMPS, I would need a dual channel scope. So, no matter if there is an isolation transformer, I could easily blow something. In that case, having an isolation transformer could be useless.All in the room is wood, so I don't really fear so much becoming the path for a current to flow to earth. I think there it's a much higher probability of becoming the lesser impedance path between line and neutral. In that case, not just the isolation transformer but also any GFCI, no matter where, could be useless.Then I would like not just me, but also my equipement to be as safe as possible.Last but not least, the lesser the bulk, the more clarity.So I'm beginning to think that having a couple of micsig DP1007 probes could be the simpliest way to go. Not the cheapest one, however, but that's secondary. Any thoughts?

author: tatel
author_rank: Frequent Contributor
timestamp: January 03, 2023, 02:36:37 pm
content: Quote from: Ian.M on January 03, 2023, 01:40:28 pmUnderstood. Excuse me if you think I've been rude in any way. I'm on this forum to learn and I really appreciate any answers, unless I feel really insulted. And, after seeing how bullets fly between our primma donnas in the threads like the zoom quirk, or how much some developers could despise reports about regressions, I'm not sure I'm thick-skinned enough to be here. So I've probably been so touchy that I feel compelled to apologize again.

author: Ian.M
author_rank: Super Contributor
timestamp: January 03, 2023, 02:48:44 pm
content: Apology accepted. It was *NEVER* my intention to personally insult you or your intelligence, and I unreservedly apologise for giving you the impression that I was. You are still here and talking sense, so I assure you, you are thick skinned enough to benefit from the technical content here without getting drawn too far into personality conflicts. Engineers and good technicians frequently tend to be excessively blunt and a significant number fall short in social skills, because the profession self-selects for people who are more comfortable with dealing with complicated things than with complicated people. However that doesn't justify deliberate targeted rudeness on our part.Personally, for working on SMPSUs, I would prefer at least one differential probe with HV isolation and also an isolation transformer. That combo decreases your risk, as a RCD (GFCI), and no isolation, wont protect you effectively from high energy DC shocks e.g. from the SMPSU's primary side DC bus reservoir capacitor. If you can afford it, get a second isolated probe. The isolation transformer also increases your safety when working on devices where the PSU is exposed and maybe even on the same board as the circuit you are working on, even if the setup has the PSU's secondary side grounded.One thing to be aware of if you are in an older building with whole house RCD protection (rather than per circuit protection) is you should have at least *some* bench lighting with a maintained supply, or enough natural daylight at your bench, as if you trip the RCD, and all the lights go out while holding a test probe in place, you can easily cut yourself or shock yourself on a charged capacitor, or accidentally knock stuff off the bench because there is no longer enough light to see where your hand is. If you do have per circuit protection, make sure that at least one light is on a different circuit and is switched on whenever you are working.

author: bdunham7
author_rank: Super Contributor
timestamp: January 03, 2023, 02:52:43 pm
content: Quote from: tatel on January 03, 2023, 02:20:16 pmA while back I posted a simple explanation of how to use an isolation transformer to probe an SMPS. My post and photos appears to have gotten a bit scrambled, but it should be understandable. A lively discussion follows which reveals some disagreement. I"m not persuaded and I would still do it this way if I didn't have an isolated scope. The bottom line is that this type of work should be done carefully and mostly hands-off, meaning you make all your connections on an unpowered DUT and then don't touch it while making measurements.https://www.eevblog.com/forum/testgear/probing-smps-with-oscilloscope/msg3152256/#msg3152256The trick in using a multi-channel scope in this context is to remove the ground clips from all but one of your probes. One and only one grounding point, although you can vary its location in the circuit--which is the primary reason for doing all this in the first place. Some may protest that this results in some signal integrity issue of one sort or another, but then they tell you to use a cheap differential probe which is typically much worse in this regard. I find that with a bit of thought and experimentation, it works just fine.As far as 'blowing up' something, an isolation transformer usually provides a much lower fault current if you do make a serious mistake. Mine allows me to vary the input voltage (variac) and measure leakage current as well as limiting the fault current to a few amperes. I'm unlikely to 'blow up' my oscilloscope, I'll just pop a breaker.Differential probes have CMRR issues, the Micsig ones have big CMRR issues. I have one, the DP10013, and it works and is handy for some situations. It probably would be OK for a lot of SMPS issues and it could be handy for adding a channel where you just can't manage to set up the measurements you want with any one particular ground point. However, I really don't see how using such a probe without an isolation transformer improves safety, since the DUT is then live with full mains power and a large fault current. As I stated in the earlier post, RCD/GFI is a nice additional safety feature--like airbags in a car--but they should not be your primary plan. There are cases where they won't trip and in any case there is still plenty of stored energy in some SMPS primary capacitors. If you use differential probeswithan isolation transformer, then you can avoid introducing a low-impedance ground (differential probes are not galvanically isolated and will have leakage current) to the DUT and thus keep it 'touch-safe', at least in theory and with respect to a single point.

author: Zero999
author_rank: Super Contributor
timestamp: January 03, 2023, 04:48:37 pm
content: Quote from: tatel on January 03, 2023, 02:20:16 pmSome connection to earth is necessary to monitor if either side is connected to earth.No, it doesn't defeat the isolation. The secondary is connected to earth via two 150k resistors which limit the current to a safe level if there's a fault. In this case it'll be under 1.5mA. I think I originally designed the circuit for 120V or made an error, because the current should be limited to under 1mA, so change R1 & R2 to 270k for 240V.

author: tatel
author_rank: Frequent Contributor
timestamp: January 03, 2023, 05:40:25 pm
content: Quote from: Ian.M on January 03, 2023, 02:48:44 pmNo worries, man. I really appreciate your words. Let's go aheadQuoteI'm not in a hurry to work with SMPS. It's true that one died after just brushing the dust from inside. It bothers me because I'm used to clean them anytime I got a salvaged PC and never had any problems. So I'm willing to do a complete forensic work and even to try and repair it, just to know what happened, to learn something, and to have fun. But, I do have spares and other, more urgent projects, so this is going directly to the backburner for the time being.Quote from: bdunham7 on January 03, 2023, 02:52:43 pmThis post from you and the one in that thread (and that whole thread indeed), are really interesting. Somehow it got under my radar. I have to read carefully the whole thread, for the time being, I've just skimped through it.You have reminded me that I could get an isolated Owon VDS1022i which I do really like. I have old, small laptops available that can run WinXp/Win7/Linux. I use them for OBD work on my car. That, and the Owon, could be a nice thing for automotive work, while also being good enough to check what's the matter on a PSU with it's 5 mV/div IIRC. It would be about the price of a MicSig DP1007 probe, and I would get much more use of that equipement, I think.Fact is, I'm going to buy a chinese benchtop lathe (not the infamous mini-lathe). A couple of MicSig probes + isolation transformer (I would think about 400VA) could easily be half the price of that lathe. Let alone the variac. I have to choose, and the need for that lathe is quite a bit more pressing.I'm not planning to do much work with SMPSUs, I'm much more focused about DUTs that could be powered from a battery. So that approach with no differential probes looks quite atractive to me. Then I could get the differential probes later if really needed.Now I'm going to read carefully that thread. But, what you guys would do if having that isolated scope? I'm trying to have the different options clear. I'll quite probably post more questions after reading it.Thank you all.

author: tatel
author_rank: Frequent Contributor
timestamp: January 03, 2023, 05:50:07 pm
content: Quote from: Zero999 on January 03, 2023, 04:48:37 pmNow that's really interesting, because it seems to do something like the incandescent bulb method, or better, increasing safety. Incandescent bulbs are pretty rare around here lately. I guess I'll have to look at it closely. Thank you very much for the tip

author: Zero999
author_rank: Super Contributor
timestamp: January 03, 2023, 06:14:14 pm
content: Quote from: tatel on January 03, 2023, 05:50:07 pmThe lamp in my schematic is neon, not incandescent. A neon will light with a tiny current, whilst an incandescent takes a relatively large current. The neon could be replaced with a high efficiency green or white LED and a small bridge rectifier.Incandescent lamps are used to limit the current through a power supply, so it doesn't blow up if there's a fault.

author: tatel
author_rank: Frequent Contributor
timestamp: January 03, 2023, 06:31:07 pm
content: Yeah. I meant, it provides visual warning, perhaps somewhat like the incandescent bulb method, where you can know capacitors have been charged. And it would limit the current more. I know it's not exactly the same, sorry. I still need to look at it more closely.The earthing could be to some chassis wich is also earthed from mains, but the secondary side would still have high impedance thanks to that couple of high value resistors, it isn't?

author: tatel
author_rank: Frequent Contributor
timestamp: January 04, 2023, 11:40:13 am
content: Well, it turns out that I was wrong about prices. Eleshop.eu has the MicSig probes cheaper than I thought.I'm leaning towards DP750-100 since it has the most CMRR. 600V CAT II seems to be good enough. However output voltage is the lowest at 1.5V. But I have seen reports about it working with low voltage differential busses:https://www.eevblog.com/forum/testgear/differential-probe-for-offline-pfcd-psu/msg4293502/#msg4293502https://www.eevblog.com/forum/testgear/micsig-dp10007-or-dp750-100-differential-probe/msg4378921/#msg4378921I'm sure there are better differential probes but €177+VAT is already in the upper range of what I'm willing to pay.For isolation transformer, I'm thinking this one should be good enough to power that 300W ATX PSU, and probably other SMPSUs in the future, as long as they get just the load needed to regulate properly (I'm using a 10W 10R resistor right now).https://www.tme.eu/en/details/pfs250_230_230v/safety-transformers/breve-tufvassons/Or perhaps this one, wich is somewhat cheaper and could be combined in the future with a variac into a case. Not sure about that. All AC things here are 220V unless they are tree-phase. Most probably I'll go ahead with just the isolation transformer and a 60W incandescent bulb in series (the only one I have been able to find). Any thoughts?https://www.tme.eu/en/details/pvs250_230_230v/safety-transformers/breve-tufvassons/With this, I think the way recommended by Ian.M gets covered.Now, should I follow bdunham7's recommendation, same isolation transformer. By the way, I got quite a few clues from your posts in that threadNot sure about the probes, however. My GW Instek 1054B accepts 300V CAT 1 max input voltage IIRC and I assume this would be with 10x probes. It's said to be RMS voltage. So I guess this would be enough to work on the secondary side? Anyway I looked for the HV probes you recommended:https://www.ebay.es/itm/332835001398.In the pictures you posted, I see various different probes at same time, i.e, a PP510, the linked High voltage one and still another that I can't identify, which seems to have an alligator adapter.I see you are using the high voltage probe on the primary side and the PP510 on the secondary, but could you elaborate some more about the probes? One or two HV probes? 100x atenuation low voltage ones? Just 10 x attenuation would perhaps be enough on the secondary?Last, the other way -getting an isolated scope (Owon VDS1022i)https://eleshop.eu/owon-vds1022i-usb-oscilloscope.htmlI like this way because it would be the safest one for my benchtop scope, and being so portable I could use it for automotive work, I think. The specs seem to be good enough for most PSU work. Max input voltage is 400V, but p-p this time. Attenuation goes to 500x. Channel isolation would be 100:1 at 50 Hz and 40:1 at 10 MHz.However I'm not sure if differential probes can be used, I looked for Testec TT-SI 7002 probes and I can see the warning "Only connect to a grounded measuring device!". Which wouldn't be the case with this isolated scope. Any comments about probes to use with this device?

author: wasedadoc
author_rank: Super Contributor
timestamp: January 04, 2023, 12:45:25 pm
content: When working on SMPS I isolate it by running from a UPS whose mains plug is not plugged in. No problem to attach grounded scope probe anywhere. I have a 100x probe for looking at transformer primary side.

author: Zero999
author_rank: Super Contributor
timestamp: January 04, 2023, 06:24:20 pm
content: Quote from: tatel on January 03, 2023, 06:31:07 pmThat's the idea. The resistors will limit the current to a safe value, which will reduce the shock and the lamp will provide an indication when either live conductor is earthed.