Purpose of isolation transformers - Page 1
Forum section: electronics / beginners

author: dasein
author_rank: Newbie
timestamp: March 15, 2024, 03:48:40 pm
content: What is the point of isolation transformers? Had the idea to try to diy one in my head for awhile after watching the how to not blow up your oscilloscope () if I ever managed to find two identical transformers. I am now in possession of two identical transformers (hefty ones from backup battery ups,) but it seems like plugging test equipment into an isolation transformer is a bad idea? If it is a bad idea to plug the oscilloscope into an isolation transformer and a bad idea to plug test equipment into it, what exactly is the point of this piece of equipment at all?Example cases:- Using oscilloscope to test a circuit powered by wall wart that has an earth connection (plugging wall wart into isolation transformer).- Using oscilloscope to test an arduino powered by a charging laptop (plugging laptop charger into isolation trasnsformer).- Using oscilloscope to examine the DAC in powered benchtop multimeter (plugging multimeter into isolation transformers).Making a post specifically because most other places I have looked have been focused on the safety aspect and just encouraging the person making the post to not mess with mains voltage or use an isolation transformer if they don't know what they are doing. Obviously safety is a huge concern and I am aware of that and that is why I am making this post. If I still don't find that I have a justifiable reason or sufficient knowledge to own much less build one, then I won't. Going down this rabbit hole though I really just want to know more about why these things even exist in the first place. Thanks.

author: DonKu
author_rank: Regular Contributor
timestamp: March 15, 2024, 04:20:21 pm
content: See if this video helps:

author: bdunham7
author_rank: Super Contributor
timestamp: March 15, 2024, 04:41:03 pm
content: Quote from: dasein on March 15, 2024, 03:48:40 pmObviously "isolation transformers" (which is almost all transformers except autotransformers) have been around forever. In the context of using a 1:1 transformer just for isolation on the bench as one typically imagines, they were originally marketed as a service accessory for working on TVs and radios that had a live chassis. The purpose was to mitigate the signficant shock hazard that results from having a large chunk of exposed metal at a dangerous potential.

author: wasedadoc
author_rank: Super Contributor
timestamp: March 15, 2024, 05:27:08 pm
content: Quote from: bdunham7 on March 15, 2024, 04:41:03 pmwith respect to ground and able to source (to ground) sufficiently high current to be dangerous to human life.In the same vein, shaver sockets in UK bathrooms must have an isolation transformer. That a cheap to add tap on the secondary winding can provide 110 Volts for visitors from some other parts of the world is a bonus, not the primary reason for the transformer.

author: dasein
author_rank: Newbie
timestamp: March 15, 2024, 05:52:53 pm
content: Quote from: bdunham7 on March 15, 2024, 04:41:03 pmHow does this translate to modern test equipment such as a benchtop multimeter? In these cases I was under the impression that earth ground was to prevent a floating ground and therefore a live chassis in the case of a fault...

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: March 15, 2024, 07:05:19 pm
content: As described in the previous posts. it is primarily a safety function.It can be thought of in the same way as an "Isolated DC Power Supply" in that neither pos or neg is grounded (Earthed). Obviously in AC the Plus and Minus are changing at 50 or 60 Hz.As Bdunham7 already pointed out. If all the power is run thru a transformer, the power is essentially Floating or Isolated. Until you "reference" the Neg to GND.Lab PS (DC and AC) use this arrangement. The Chassis can be grounded but neither pos or neg is referenced to GND.There are implications both good and bad. Running everything thru an Isolation Transformer, the chassis is not Grounded.In the US there were many radios that had no transformer and one end of the only two wire AC was attached to chassis. Also the two wire plugs were symmetrical. US at that time 110 V AC. I think a popular radio like this was an "All American 5" (5 valves), the heaters of the valves were also fed in series with the 110 AC. They could be a problem especially if operated on the kitchen sink.Today, Oscilloscopes almost all have their Neg connected to GND, there are implications if you "float" a scope.I think Benchtop Multimeters like HP 3456 have "Floating Inputs". Handheld Multimeters are floating because of the battery power.

author: bdunham7
author_rank: Super Contributor
timestamp: March 15, 2024, 07:18:27 pm
content: Quote from: dasein on March 15, 2024, 05:52:53 pmI'm not sure how your two sentences are related.A modern bench multimeter will be "guarded" and the "in-guard" region--which includes all the inputs--will be isolated from anything else including ground. That is, except for parasitic capacitive coupling and resistive leakage. The "out-guard" region, if it has one, will be referenced to ground. There will be an isolation system between the two.An earth ground does prevent the chassis from floating and that is an entirely separate function. Removing the earth reference from the mains power with an isolation transformer is not directly related to breaking the protective earth ground connection. You can do either or both, except doing the latter without the former is probably a bad idea.

author: eddo
author_rank: Newbie
timestamp: October 08, 2024, 03:49:23 pm
content: there is a little thing which is bugging me and I have no definitive answer. I have a safety transformer 230VAC->230VAC conforming to IEC 61558 (https://www.tme.eu/Document/bb7651643ff3fce400f23ff9a881686a/tr28.pdf).The primary, obviously, has got two connectors. On the secondary, besides the two connectors, there is also a third one identified with a ground symbol. No electrical connection to any other pole.Should I assume it's just a service connector, isolated and placed there for some strange reason? No idea how I am supposed to use it.of course it's not meant to be connected to the ground of the mains.Has anybody got an idea why is the IEC standard placing that?Thanks

author: Phil1977
author_rank: Frequent Contributor
timestamp: October 08, 2024, 04:01:30 pm
content: Many isolation transformers have a PE-connection for the core and some better also have a "shield winding".Connecting it to PE reduces the common mode leakage current. Both windings of the transformer have some parasitic capacitance against each other. Usually you can't avoid some unwanted current flow over this capacitance.If you implement a shield winding that´s connected to PE between the windings, then you don't directly reduce the capacitance, but then the leakage current from mains flows into the shield and into PE. The leakage current to the secondary winding can be heavily reduced that way.There is already too much written about the safety aspects of using or not using an isolation transformer. It only increases your safety if you exactly know why it does. Please do not use it for any activity that feels risky but you don't know why. And please never float any test instrument like a scope.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 08, 2024, 04:23:35 pm
content: This topic also keeps confusing me.Obviously, when working on low voltage equipment, you just reverence everything to mains protective earth, just as your oscilloscope, and you can measure like a happy critter.But now suppose I am designing a triac light dimming circuit that is mains voltage connected.The simplest way to get a little bit of extra safety in it is an extension cord with two neon bulbs in it, that connect either neutral (green bulb) or the phase (red bulb) to Mains PE. This way you just reverse the plug if the wrong neon bulb lights. Another very simple and logical is a hefty two pole switch, so you can easily disconnect the whole circuit from mains voltage. I like that special switching box that Big Clive has. With this method, I also don't use the GND clip of my oscilloscope at all. I only rely on the GND / PE connection of the scope itself, which is connected to the same extension cord as the measurement setup (but without the extra disconnect switch). This obviously only works for low frequency stuff, but is good enough for a light dimmer.But now add an insulation transformer to the mix. What does it add?The first thing you are likely to do is to connect your GND probe of your oscilloscope to the secondary circuit, which means the whole thing is bypassed. On top of that, your RCD does not protect you anymore, as the return current just goes back to the transformer.For the uC controlled light dimmer I designed 20 yeas ago, I designed most of the software while the triacs were controlling some small 12V light bulbs, and at some point I also made an isolator with opto couplers to program the uC. It worked, but I was not comfortable with that.One advantage of a "safety transformer" I can see is when working on equipment with rectified mains voltage, such as for example frequency inverters. This lets you choose the "GND" of the rectified voltage as a "zero reference" for your scope.Also related: Sometimes I think about modifying one of the GND leads of my oscilloscope and put a fuse in series with it. The smallest (mains voltage rated) fuses I could find are 20mA, and that is likely to give some extra protection, and at the least it prevents you from smoking your probes with 10A though the GND clip. (This is obviously also not ideal for higher frequency measurements, just for doing on higher voltage measurements).

author: Phil1977
author_rank: Frequent Contributor
timestamp: October 08, 2024, 05:06:53 pm
content: The isolation transformer is most useful together with a variac.Take the example of the triac-circuit: You can place your scope GND where ever you need it and you can turn on the variac to only 40-50V AC for the first tests. Then you can turn up the voltage with the probes placed and you don't need to touch anything except the variac handle.I would not like to use a variac without an isolation transformer for test activities.

author: CaptDon
author_rank: Super Contributor
timestamp: October 08, 2024, 06:23:56 pm
content: How can you have posted video links showing the hows and whys and researched the subject and still say you don't understand the value of using an isolation transformer?? In my lab my test equipment is earth grounded 99.999% of the time and the unit under test is floating. There are times when working with mains voltages that the scope must be the 'floating' part and you MUST realize that touching the scope with one hand and touching anything 'earthed' with the other hand will probably kill you. The workaround is using a differential probe but often 'mains capable' differential probes don't have enough bandwidth or suffer from ringing due to long lead lengths when looking at the square pulse shapes of variable frequency drives. To say "I don't understand the value of an isolation transformer" tells us you aren't a knowledgeable or skilled technician and probably a danger to the test equipment and circuits you work on.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 08, 2024, 06:35:29 pm
content: Quote from: Phil1977 on October 08, 2024, 05:06:53 pmI agree that this is the general consensus, but it really is not the issue itself.If you use the neon light method above, andKnow for certainwhich ones are the phase and the GND wires, then this is pretty much equivalent. Note that the "GND" wire and "Protective Earth" are hard connected to each other where power comes out of the ground, (just before the RCD)As a result, if you have identified the "GND" wire, and adjust your variac (autotransformer) to deliver 48V (that's the maximum considered "safe") with the GND wire as reference instead of the phase, then this is very much equivalent to using an isolation transformer, and then connecting the secondary side to PE (a.k.a. Scope GND lead).============Do note that this works for where I live, where one of the wires of the 230Vac mains voltage is always a GND. There may be countries where mains voltages are not referenced to earth, but for example between two phases. This mudball we live on is pretty big still, and there are local differences.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 08, 2024, 06:42:19 pm
content: This is what I mean:Quote from: CaptDon on October 08, 2024, 06:23:56 pmAt the moment you connect your "floating test equipment" to your oscilloscope, it's not floating anymore. And that is just fine for low voltage stuff (<48V) but when you create the GND though your scope, then still all the safety issues apply and it pretty much nullifies the use of the insulation transformer.If I was standing in a mine shaft with my feet in muddy water and handling a 230Vac drill , then It would be a big comfort if that drill is powered though an isolation transformer. In such cases it's useful. But on your bench andpurposefully connecting the GND probe of your scopeto the circuit under test, most of the usage of the isolation transformer is nullified.

author: Phil1977
author_rank: Frequent Contributor
timestamp: October 08, 2024, 06:47:01 pm
content: Quote from: Doctorandus_P on October 08, 2024, 06:35:29 pmIf you connect your scope to such a setup, you most probably trip the RCD.And I just wouldn't want to work "hands on" the neutral wire. I know it should be safe, but it feels totally wrong to touch it.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 08, 2024, 08:03:20 pm
content: Quote from: Phil1977 on October 08, 2024, 06:47:01 pmYes, good argument, it would almost certainly trip the RCD because now any GND current also has a path to PE and around the RCD, so it would likely trip immediately, even from other nearby equipment, because the GND wire also has a finite (nonzero) resistance, PE is likely to have about half of any GND current if the wiring resistance is the same.Quote from: Phil1977 on October 08, 2024, 06:47:01 pmYes, me too. "Don't touch the mains voltage" has been hammered into every child for many decades, at least until those annoying shields started showing up in wall sockets everywhere about 20 years ago. And yet I do not feel any hesitance for touching the PE prongs in wall sockets

author: Analog Kid
author_rank: Super Contributor
timestamp: October 09, 2024, 06:04:35 am
content: Just a little translation service for those of us who don't live in the UKoGBaNI or Australia or one of them other English-mangling places:RCDs (had to look that up: "residual current device") are referred to in North America (and I assume other places as well) as "GFI"s or "GFCI"s (technically, ground-fault circuit interrupters).We call the two conductors (besides ground) "hot" and "neutral".And really, our electricity has been 120 volts (nominal) since forever, despite all those magazine articles and such that specified it as 117 V. or some such. And it's not "220" (here); it's 240 volts.Oh, and "PE" ("protective earth")? That's just called "ground" here.Other than that, it's all good.

author: Phil1977
author_rank: Frequent Contributor
timestamp: October 09, 2024, 06:33:03 am
content: Quote from: Doctorandus_P on October 08, 2024, 08:03:20 pmBut PE is intended to be touched, you regularly touch it if you use your Full Metal Toaster (tm) . N may have a lot of annoying spikes and surges. I really don't want to touch N while a 30 year old purely mechanical washer starts the tumbling.And even if it´s not allowed, in reality N is switched at some places. Then there is a very good chance to have L on N and then you get electroboomed.Just don't touch N. If you really have to, spawn up your private TNCS island using an isolation transformer. There you may have unlimited trust in its design...

author: Analog Kid
author_rank: Super Contributor
timestamp: October 09, 2024, 06:55:27 am
content: ... except that here (in the US), "neutral" and "ground" are actually bonded together at the service entrance panel. (Or are supposed to be.) So there shouldn't be much/any potential difference between them, right?

author: Phil1977
author_rank: Frequent Contributor
timestamp: October 09, 2024, 07:02:00 am
content: This "should be" exactly is the problem.Most parts of the grid are designed in a way that a single point error does not harm you.From this point of view touching N is already one point of failure. A second deviation somewhere else may zapp you.

author: Ice-Tea
author_rank: Super Contributor
timestamp: October 09, 2024, 07:03:04 am
content: There shouldn't untill there is. It's a really bad habit to learn. What if there's wiring fault? Trafo fault? Cheap chinese extention cable? Uncle Harold that spliced his own extention cord? What happens if you travel abroad and still think N is safe?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 09, 2024, 12:22:31 pm
content: RCD and GFI is pretty much the same thing regardless of how you call it.Mains voltages have been going up (but not really) lately here in the EU. It used to be 220Vac 30 years ago, then they changed it (a long while back) to a nominal 230Vac, but they changed the tolerances too. I think 220Vac was plus or minus 10% while for 230Vac it was +5% - 15%. Apparently it has changed again, and now solar installations are required to turn themselves off if the mains voltage goes above 254Vac.As far as I know there is a worldwide effort (goal) to harmonize mains voltages, and I guess that 230Vac (or 240Vac) is becoming more common in the US. I heard some rumors that it's common to have these sockets in the kitchen. But if you look at the speed with which the US adopts the metric system, it may be a while before this gets harmonized. (Just like mains voltage plugs, it's a real mess)Quote from: Analog Kid on October 09, 2024, 06:04:35 amBut please don't use "ground" in this context. It's a worldwide forum, and "ground" just has too many other meanings (I.e: a "randomly" chosen reference) in electronics. PE (Indeed Protective Earth) is unambiguous and avoids confusion, wherever you live.Quote from: Ice-Tea on October 09, 2024, 07:03:04 amWe are not talking general use here, but a dedicated work table set up do to electronics design or repair work. Wiring faults, Uncle Harold and abroad can be pretty much excluded.But I tend to agree with the overall direction. Safety comes in layers, with the intention that there is no single point of failure that leads to hazardous situations. By opening up equipment and poking around in live circuits, you have already removed one safety layer, and getting some of that back with an isolation transformer does help a bit to get some safety (or at least peace at mind) back. It's one less thing to worry about.

author: CaptDon
author_rank: Super Contributor
timestamp: October 09, 2024, 02:24:20 pm
content: In the repair field our most encountered use of isolation transformers is in the servicing of Hot Chassis televisions or the mains side of nearly any line operated SMPS. Our test equipment is at P.E. or Earth Ground and the isolation transformer allows the earth grounded test equipment to be connected to what would usually be considered the Return Rail or Negative Return of the Hot Chassis or SMPS. Most voltage or signal test points are referenced to that rail. If the hot chassis or SMPS were not isolated it would be instant disaster when connecting to the grounded test equipment. And yes, once that chassis is grounded through the grounded test equipment connection the +145vdc or +280vdc (doubler circuit) is potentially lethal if you are stupid enough to touch it. In the hospital environment where I worked as a BioMed Tech the O.R.'s were supplied with a floating 120VAC 5KW isolation transformer. The center tap (60 / 0 / 60) went through a 2.2Meg resistor to earth ground and a device from Square D called a LEM monitored and alarmed if there were any imbalance leakage detected. There was a 'Press to Test' switch and fault detection was tested for every O.R. at the start of each work day. The results were kept in a log book. The input side of the transformers was 208vac, one pair of phases from a three phase 120 / 208 'Wye' supply.

author: Seekonk
author_rank: Super Contributor
timestamp: October 09, 2024, 02:44:48 pm
content: I don't think I can tell you what I do with the AC cord on my scope. Scope is all plastic.I think it was back in the 60's I saw a HP tech article about putting a large bridge rectifier in series with the ground lead inside the scope. That kept ground isolated until potential exceeded 1.3V.

author: dietert1
author_rank: Super Contributor
timestamp: October 09, 2024, 02:55:25 pm
content: It's a tool people use for isolating a non-isolated mains power supply in order to get scope measurements from the device. Working on a live device with high voltages (e.g. 400 V in a PFC supply) is always dangerous. Put your left hand into the pocket and also keep your head at a safe distance.You can kill somebody with a hammer or a screw driver and on the electronics bench the isolation transformer isn't really a safety tool. It solves a certain technical problem. The alternative method is not using an isolation transformer and cutting the scopes PE connection instead. That is stupid and dangerous.As mentioned above isolation transformers can also be used to provide a second isolation barrier as required for medical equipment connected to a human body. That means you have a device with isolated power supply but you want to have a second isolation in case the first one develops a fault.Regards, Dieter

author: SteveThackery
author_rank: Super Contributor
timestamp: October 09, 2024, 03:54:29 pm
content: Quote from: dietert1 on October 09, 2024, 02:55:25 pmI very much agree with this analysis. Thinking of the transformer as a safety aid can imply that it improves safety, whereas in reality it turns one type of safety hazard into another type. It is an indispensable tool for a specific technical problem.

author: johansen
author_rank: Super Contributor
timestamp: October 09, 2024, 04:49:58 pm
content: Quote from: dasein on March 15, 2024, 03:48:40 pmThey exist these days, to argue about how lethal they aren't.I cut the ground pin off my rigol oscope.I also cut the ground pin on my hakko soldering iron, and I installed a neon lamp and resistor, so when I'm soldering on live equipment (accidently i swear) the light will illuminate.

author: jonpaul
author_rank: Super Contributor
timestamp: October 10, 2024, 05:57:23 am
content: Was EE and power supply consultant since 1970s, always used this setup.120 or 240 Mains>>2 KVA Signal isolation 240:240>>10A GenRad Variac>>DUT.All my Scopes are always used with normal grounded IEC 3 wire to mains.Never had any issues.Jon

author: Ice-Tea
author_rank: Super Contributor
timestamp: October 10, 2024, 06:43:04 am
content: Quote from: johansen on October 09, 2024, 04:49:58 pmMight I suggest you be carefull with throwing around remarks like that, even more so in the beginner section?