Isolation transformer wanted for oscilloscope - Page 1
Forum section: electronics / beginners

author: windmill john
author_rank: Contributor
timestamp: December 24, 2022, 03:19:28 pm
content: Hi,I’m trying to learn more about electronics just for myself.The more I read, the more…… my head fills!Okay, this sounds like a simple question, but after much Googling, I’m struggling.I’m about to get an old oscilloscope, probably too early yet, but have read about isolating from ground etc.I’m in the UK and don’t want to break the bank or build my own yet, due to knowledge. But… is it possible to buy an isolating transformer without spending three figures?At this moment, I just need one socket for one device and just want to protect myself. It appears I’m asking for the world.Thanks.John

author: nightfire
author_rank: Frequent Contributor
timestamp: December 24, 2022, 03:32:43 pm
content: Actually, budget isolation transformers begin at approx 200 € here in germany where you can assume that they got quality control that would deserve its name. I would assume that same is valid for UK with similar costs.I got mine off ebay in a slightly broken condition- but from the 70s, it had a massive build with a metal case. After cleaning, replacing the fuse holder and "ON" light it was still good to go- in this case you have to do some isolation tests to really make sure that the transformer itself is ok.(=no simple multimeter resistance test)I assume you want to float the DUT? Putting a scope behing an isolation transformer is a bad idea, discussed also in several threads.

author: themadhippy
author_rank: Super Contributor
timestamp: December 24, 2022, 03:36:15 pm
content: Quotehttps://www.tlc-direct.co.uk/Products/RDP17.html£9.45 its still 3 figures,and only good for 20VA but its an isolation transformer.For a more sensible answer what sort of power rating are you after?

author: tunk
author_rank: Super Contributor
timestamp: December 24, 2022, 03:43:28 pm
content: You could also look at some of the handheld scopes like Owon HDS242 or Hantek 2C42.They run on batteries so shouldn't need an isolation transformer (don't think theyshould be used when charging). And they will be cheaper than an old CRO plus isolationtransformer.

author: nightfire
author_rank: Frequent Contributor
timestamp: December 24, 2022, 04:12:36 pm
content: Quote from: tunk on December 24, 2022, 03:43:28 pmI also looked into those as I was questioning myself if i need a differential probe. Turned out that those cheap handhelds are NOT isolated against the USB port they have and usually have a common Ground if they have multiple inputs.Good and expensive portable scopes like Fluke have those isolations and no common reference ground.So an affordable isolation transformer or a differential probe is needed if the OP wants to work on gear that can get dangerous.

author: RoGeorge
author_rank: Super Contributor
timestamp: December 24, 2022, 04:42:16 pm
content: Maybe you don't need an isolating transformer at all. It is preferably to float the circuit under test instead. That would be cheaper, and quite easy to do by powering your circuits from a battery, from a brick wall adapter, or from a floating lab power supply.If you have USB powered project from a PC/laptop than just take care and never clip the probes alligator anywhere else but at the GND. This should be the norm anyway.If you want to probe the voltage on some random component, say a resistor that is not tied directly to GND, then use two probes. Their alligator clip always stays connected to GND, and the tip of each probe connected to each terminal of the resistor. Then set the oscilloscope to display the difference between the two channels (all multi-channel oscilloscopes have such setting). The difference between Ch1 and Ch2 will be the voltage on that R.Always connect the alligator (GND) of any probe only to ground (GND) of your circuit. Never connect the ground of the oscilloscope somewhere in the middle of a circuit. That's would be a very bad idea, and will damage the circuit under test, or the oscilloscope, or both.Beware that usual oscilloscopes have all the GND from all the probes tied together, so always always clip any GND alligator of the probes to the GND of the circuit.Alternatively instead of an isolation transformer, there are some handheld models of oscilloscopes with isolated channel inputs. Instead of burning those 200-300 money on an isolation transformer, you can use them to buy a handheld oscilloscope with 2 isolated channel, floated by design and also isolated from each other, not to say it's portable and battery powered.

author: windmill john
author_rank: Contributor
timestamp: December 24, 2022, 05:06:52 pm
content: I think this is the issue being new, some of the answers are still above me to be blunt.I did read about the issue with running something USB from a PC, which can still pull from mains ground.I wasn’t 100% sure which scope to go for, small USB, battery etc. but was able to pick up an old 2 channel CRT oscilloscope.As I’m new, I just want to ensure that I don’t stick the ground probe on something I shouldn’t have.I’m aware about not testing a live circuit, but just wanted to protect the scope…… and me!£9.43, that’ll dowhat power do I need?…… I just want to protect the scope when I plug into 240V.

author: wasedadoc
author_rank: Super Contributor
timestamp: December 24, 2022, 05:18:45 pm
content: Quote from: windmill john on December 24, 2022, 05:06:52 pmTo be equally blunt you are clearly not yet knowledgeable enough to be doing anything which requires an isolation transformer.If that offends you please know that I much prefer you to be living and offended rather than dead and not offended.

author: madires
author_rank: Super Contributor
timestamp: December 24, 2022, 05:27:07 pm
content: Usable differential probes start around 200 bucks, e.g. Micsig. In case you want to work on mains voltage stuff like SMPSUs it might be a good idea to get differential probes to prevent grounding of the DUT (via the scope), and also to buy an isolation transformer to float the DUT. As bonus add some rubber gloves. One hand in the pocket! Don't rely on just a single safety measure!

author: Stray Electron
author_rank: Super Contributor
timestamp: December 24, 2022, 05:35:05 pm
content: Go look some place that sells surplus industrial electronics. A lot of electronics machinery and even machine tools use good size, high quality isolation transformers to drive the control circuits. In the US those transformers typically have dual 220VAC primaries and dual 110VAC secondaries. With those windings they can run the machine on 440, 220 or 110 VAC by using various combinations of windings. You can use such a transformer as a 4 to 1 or a 2 to 1 setup transformer, a 4 to 1 or 2 to 1 step down transformer or a 1 to 1 isolation transformer. I have one of these transformers that is conservatively rated at 1500 VA and it's large enough to run anything equipment or TE that I have. I got mine for free out of piece of machinery that that we scrapped out about 40 years ago. It weighs about 40 pounds but I don't have to worry about overloading it!I have an large old Miller transformer based welder and although the welder itself runs strictly on 220 V, it has a 220 to 110V transformer inside that is used to operate the control circuits, water pump, etc.FYI, Despite what they'll try to tell you, large transformers like this are worth next to nothing to a surplus or scrap dealer. They may contain large amounts of copper but it is too tightly wound to get off and is almost always potted with epoxy. And mixed metal scrap is practically worthless.

author: windmill john
author_rank: Contributor
timestamp: December 24, 2022, 06:28:08 pm
content: wasedadoc, where do you live!I’m old enough not to take offence, know I have a lot to learn and will always ask questions.I’m finding the jump from no knowledge to the next step incredibly frustrating. Not that you need to know, but I’m one of those motorcyclists who has worked on my bikes for years, but always viewed electrics as a black art. I wanted to get away from that, so made sure I understood voltage drop etc when it pertains to vehicles. This led me to watching lots of videos, getting interested by many electronic videos and wanting to move forward.This is incredibly basic to you lot, but I bought an old 130 circuits beginner kit and enjoyed looking at the circuits, changing resistors etc to change the output.But… stepping up from this is like walking into a darkened room. I want the isolation transformer so that if I stick my scope probes on something that has a main’s transformer, switch mode power supply etc and stick the probes into the wrong place, I don’t want to bugger anything.If I’m too much of a noob, tell me and I’ll step away.

author: tggzzz
author_rank: Super Contributor
timestamp: December 24, 2022, 06:39:48 pm
content: Quote from: windmill john on December 24, 2022, 03:19:28 pmYou almost certainly do not need an isolation transformer, and if you have one it probably wouldn't protect you.Now that's a strong statement, given that we don't know what you are planning on doing, but it is a good bet.What most people need is the right class of probe, and it these cases it probably implies a high voltage differential probe. Don't take that on blind trust, but do work it out for yourself using the references inhttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/Whatever you do, do not disconnect the scope's earth from the mains earth; that is potentially lethal, literally.

author: coppercone2
author_rank: Super Contributor
timestamp: December 24, 2022, 06:42:31 pm
content: get a scope meter.just keep in mind some scope meters have bootleg isolation between channels. It takes so much hassle out of measuring something if you have a scope meter with proper probes

author: windmill john
author_rank: Contributor
timestamp: December 24, 2022, 07:30:00 pm
content: Thanks all.tggzzz, I’ll take that article as some bedtime reading thanks.

author: BlackICE
author_rank: Regular Contributor
timestamp: December 24, 2022, 11:53:47 pm
content: Instead of an isolation transformer you can use a standalone UPS. However the same caution about disconnecting the ground are warranted. I have used this technique before. My methodology is to never touch any thing while any dangerous voltages are connected. I just view the screen. If any changes need to be made, I disconnect the power, make the changes and then reconnect.

author: Benta
author_rank: Super Contributor
timestamp: December 25, 2022, 12:05:12 am
content: I'm waiting for this to turn up in the "Darwin Awards" list...

author: tggzzz
author_rank: Super Contributor
timestamp: December 25, 2022, 12:26:55 am
content: Quote from: windmill john on December 24, 2022, 07:30:00 pmYou're welcome. Best to learn from experts rather than random suggestions from random people on the net (myself included!).One of the problems with this topic is that any one person can dream up some of the mechanisms which are dangerous, but unless they are experienced in this topic, they almost certainly will miss many mechanisms.Be wary of anyone that implies there is a simple easy non-standard way of avoiding problems. If there was, it would be universal good practice!You should consider an RCD/ELCB/GFCI. They can considerably reduce the chance of damaging yourself. But, and it is a big but, the combination of RCD plus isolation transformer can negate their benefits!

author: nightfire
author_rank: Frequent Contributor
timestamp: December 25, 2022, 12:53:39 am
content: https://download.tek.com/document/3AW_19134_2_MR_Letter.pdfThis is quite a concise manual of why certain things are dangerous.On my bench the setup is as follows:I have a power strip that i equipped with a PRCD (30mA) due to my wall outlets too old for having dedicated RCD in the fuse box.On that power strip i have my scope directly attached, so it is grounded/earthed, and in parallel I have the isolation transformer when i want to power a critical DUT.So when things go south, some current should also go to earth via the probe on the scope and cause the PRCD to act and cut off the whole power strip with everything attached.In the near future I want to buy an affordable differential probe, but the model I would like to buy (micsig DP10007) is actually hard to get.

author: tggzzz
author_rank: Super Contributor
timestamp: December 25, 2022, 08:12:31 am
content: Quote from: nightfire on December 25, 2022, 12:53:39 amThat is a useful reference. It, and a couple of others are noted inhttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/I too use an RCD in that way, even though nominally have an RCD by the fuse box. I don't have an isolation transformer.I have a couple of HV isolated probes. One was on special offer, one was at a hamfest (and in excellent condition!)

author: 2N2222
author_rank: Newbie
timestamp: December 25, 2022, 10:41:43 am
content: windmill john talked about an "old oscilloscope" that he wants to power through the isolation transformer.My 60MHz oscilloscope from the 1980s, still with a CRT, draws about 36VA (the label at the mains input says "40VA MAX").I guess this is fairly typical for scopes of that era, so 20VA won't do it.

author: 2N2222
author_rank: Newbie
timestamp: December 25, 2022, 11:27:51 am
content: I have a similar requirement. I found these 1.5kVA isolation transformers, but I do not want such a bulky and heavy monster on my desk, especially because I do not need such high powers.So, I bought this one:https://www.amazon.de/Ringkerntrafo-2x115V-1x230V-Sedlbauer-RSO-826043/dp/B00MWQD6U6It is rated for 200VA and tested for 4.4kV isolation voltage.I just need to put it in a suited housing with proper sockets and a switch. Maybe I will even add a 115V/230V selector.One of my next projects for a shop-made tool.But in the end, with all the other parts involved, it may just break the three figures limit.

author: windmill john
author_rank: Contributor
timestamp: December 25, 2022, 12:29:35 pm
content: Thanks all again.Benta, you have to learn somewhere, but I think I’m not stupid enough to get one of those awards……. Hopefully…….I’ve just watched this:https://youtu.be/5SjVm-rF39AMy sort of level, he explains it very clearly. Yes, I’m new. Yes, my knowledge is sadly lacking, but as I’ve said earlier, getting getting from nowhere to somewhere in this field seems sadly lacking. All I needed to know was how not to screw up my scope and how not to kill myself. It seems that to get that far, I need to cross a few minefields anyway.

author: wasedadoc
author_rank: Super Contributor
timestamp: December 25, 2022, 03:54:03 pm
content: Quote from: windmill john on December 25, 2022, 12:29:35 pmIf you have got the message "Do not mains isolate the scope, mains isolate the device you are testing" you have made vital and significant progress.

author: andy3055
author_rank: Super Contributor
timestamp: December 25, 2022, 05:15:12 pm
content: To OP:Don't get disheartened by useless comments. Instead, figure out how to filter out them.If you have not already done so, watch Dave's YouTube video on "How not to blo up your scope." It explains what happens in detail. I think it is a "must watch."

author: Benta
author_rank: Super Contributor
timestamp: December 25, 2022, 08:50:40 pm
content: Quote from: wasedadoc on December 25, 2022, 03:54:03 pmThis is the best comment yet and 100% to the point.THAT's thesafeway to do it. Second choice is a differential high-voltage probe (costly).

author: james_s
author_rank: Super Contributor
timestamp: December 25, 2022, 09:14:33 pm
content: IMHO if you are a beginner you should not be attempting to scope anything that requires an isolation transformer, doing so without knowing exactly what you're doing can be very dangerous, both to you and to the DUT and your scope.If you want a cheap isolation transformer you can make one by connecting two similar power transformers back to back, or you can look for a place that sells arcade game parts, most of the older monitors required an isolation transformer so they were mass produced for that application.

author: DrGeoff
author_rank: Frequent Contributor
timestamp: December 25, 2022, 09:26:07 pm
content: When servicing TV's back in the 1980's we would make our own isolation transformers.Just get two identical transformers of the appropriate rating (VA) and connect them pri->sec->sec and use the pri of the second transformer as your isolated mains.

author: BrianHG
author_rank: Super Contributor
timestamp: December 26, 2022, 08:06:46 am
content: Here, a differential high voltage probe:https://www.eevblog.com/product/hvp70/Around 300$ USD.You can always search on ebay for a used one under 100$, example:Tektronix HV differential probe.(Make sure the seller has the power supply for it. The EEVBlog unit runs on batteries instead)Manual:https://download.tek.com/manual/070901806web.pdf

author: MrAl
author_rank: Super Contributor
timestamp: December 27, 2022, 10:43:47 am
content: Hello,I've used regular CRT scopes a million times to scope out circuits that are line connected with no problems at all. The thing is, you have to know your scope. You have to know how the ground is connected. For the scopes i used they were all floating, apparently, and when you get one that has a three conductor plug with ground (USA) you cut the ground prong off or you use a 3 to 2 prong adapter and do not connect the ground of the adapter to anything.Things may have changed since then though, that is with some rather old scopes made probably in the 1970's.One of them is a BK 10MHz CRT scope with the ground prong cut off. I've scoped line voltages with no problem with that thing.With other CRT scopes i've scoped out digital circuits, converter circuits, etc., never a problem. But again you have to know how the scope power system works or else do some testing.In all the years i have used scopes i only had one problem. That was with a scope that was faulty. The high voltage winding of the transformer has some kind of short in it so a low current but high voltage potential would sometimes be injected into the circuit under test. One time i was checking a nice calculator circuit with that scope (CRT) and an arc jumped from the probe to the circuit, and instantly rendered the calculator useless and not repairable.Such a shame too it was a nice calculator.So that's another thing to consider, if the scope is working properly.On the lighter side, i have used the OWON HDS242 to measure line voltages recently and it has no problems as long as the USB charger is not plugged into the scope. I like to unplug it right from the scope. I saw some unusual voltage levels but nothing went wrong because the scope was completely isolated at that point.If you are new to electronics you could easily run into some problems doing this. Someone i know blew out their scope and two USB ports on their computer using a USB type scope to try to scope the line voltage. There's also the possibility of a bad shock. Here in the US we have 120vac but over in the UK and related they have 230vac so the shock could be much worse.You should probably work on some lower voltage circuits first until you get some more hands on experience.

author: tggzzz
author_rank: Super Contributor
timestamp: December 27, 2022, 11:25:19 am
content: Quote from: MrAl on December 27, 2022, 10:43:47 amI've run into the road a million times without looking. You just have to know how the road is connected to other roads.Fortunately the OP appears to be sensible enough to have a clue about which random people on the net to trust, and which not to.#QuoteYou are lucky, others haven't been.Others have killed themselves by floating a scope that was in perfect working order. Read the references I gave earlier.

author: BlackICE
author_rank: Regular Contributor
timestamp: December 28, 2022, 03:37:47 am
content: People have killed themselves inside of homes with out of date or faulty wiring. When I was <10 years old I lived in a home with a lamp with a metal base and shade. If I ran my fingers over the metal surface I felt a small tingle in my fingers. At the time I thought it was an interesting effect and wonder why. Now older and hopefully wiser I'm guessing that the lamp having a 2 prong non-polarized plug was connected so that the hot lead was touching the outer metal case, or the that 2 prong outlet was wired backwards. It's a good thing the lamp was over the bed sitting on a wooden floor. I was insulated from ground by the mattress and flooring. Around 1990s a friend bought an older home built around the 1930s is a guess with only ungrounded 2 prong outlets with knob and tube wiring the kitchen refrigerated metal handle had the same hot lead connected to chassis. A much more dangerous situation due to being in the kitchen with many grounds, like water pipes and facets. These potentially fatal problems are quite common in older homes in the USA.I wouldn't recommend disconnecting the ground from an oscilloscope or any other device. However a more general safety rule would be never get any of you body parts close to anything with high voltage. Even high current low voltage devices can be dangerous, like holding metal tools near a 12v car battery terminals. If you follow these rules and still disconnect oscilloscope's ground you may fry the scope or DUT but you won't get killed. The story of the engineer killed using the scope didn't follow the don't touch rules. He had access to the front and rear of the scope and probably the DUT. He must have touched one of them when the power was connected.I was amazed at a common practice of electricians working on home wiring. They touch live wiring with their bare hands to install twist nuts on switches and fixture all the time. Something I will always avoid doing, even if I wore insulating gloves. I turn the power off! Along the same lines I see workers using angle grinders with the guards removed to quicken or ease the work.

author: tggzzz
author_rank: Super Contributor
timestamp: December 28, 2022, 10:55:43 am
content: Quote from: BlackICE on December 28, 2022, 03:37:47 am<good points snipped> Let's hope the OP understands themI came across something similar with a eurocard card cage we were building in the lab: a slight tingle when the doorflap was touched. The relevant person both denied it and refused to touch itEventully we realised that the door was completely isolated from the crate by plastic hinges and catch. It was probably forming a capacitive divider w.r.t. the mains and elsewhere, or perhaps some inductive transformer effect was involved. Either way the issue was solved by connecting the doorflap to the rest of the frame.

author: MrAl
author_rank: Super Contributor
timestamp: December 28, 2022, 11:10:03 am
content: Quote from: tggzzz on December 27, 2022, 11:25:19 amHi,Actually i have read posts from many others on the web about this and most of them dont like using scopes like i discribed, yet countless people in the past i have known have done it that way without a single incident. The thing is, you cant depend on 'luck' you either know what you are doing or you dont, period. Probably the most important thing is knowing what you can touch and what you cant not touch. Touch the wrong thing and you get shocked, dont touch the wrong thing and you never get shocked, and it required constant vigilance. With the scope though as long as you dont touch anything metal you are ok just let the scope do the work not your fingers (ha ha).But for someone just getting into electronics i fear it's not a good idea to work on line voltage stuff or even higher voltage stuff. Many many years ago i got shocked by a 300vdc power source in a tube amplifier. It was a nasty shock and i remember it to this day even though i was under 15 years old at the time. It's mainly because i didnt realize what i was touching and that my feet were partially grounded.The 'old' rule is to keep one hand in your pocket when you do a voltage measurement, but also make sure your feet are not grounded. Standing on the ground outdoors or standing in the basement on concrete could mean your feet are grounded depending on what kind of shoes you wear.Either that or just go out and by a casket so you're ready (ha ha).

author: tggzzz
author_rank: Super Contributor
timestamp: December 28, 2022, 11:41:57 am
content: Quote from: MrAl on December 28, 2022, 11:10:03 amSigh. Some people never learn from other peoples' experience...Myemphasisfromhttps://groups.io/g/TekScopes/message/10754QuoteIf you still want to float a scope,startlike this:

author: Kalvin
author_rank: Super Contributor
timestamp: December 28, 2022, 11:58:18 am
content: When you use a proper isolation transformer to power the scope, the scope will be floating. At the very instance when you attach the scope's probe and ground clip to the circuit, the scope won't be floating any longer. Instead the scope's chassis/enclosure will be at the same voltage potential as the voltage exhibited by the circuit where the scope's ground clip is attached. If that voltage potential happens to be like 110V / 240V you are in danger.As a general rule: Never float the scope, instead use a proper isolation transformer to float the device-under-test you are measuring. NOTE: A typical variac transformer is not a proper isolation transformer.

author: BlackICE
author_rank: Regular Contributor
timestamp: December 28, 2022, 12:50:38 pm
content: That's gotta be Photoshopped. Who in their right mind would go to such extremes when better solutions are much easier.

author: nfmax
author_rank: Super Contributor
timestamp: December 28, 2022, 12:59:16 pm
content: Quote from: BlackICE on December 28, 2022, 12:50:38 pmSomeone who has just returned from a colleague’s funeral, perhaps?

author: tggzzz
author_rank: Super Contributor
timestamp: December 28, 2022, 02:39:12 pm
content: Quote from: BlackICE on December 28, 2022, 12:50:38 pmIt was an item being sold on fleabay (byhttps://c2management.com/) a few years ago, so I believe it is real.Clearly it is extreme, and clearly someone knew why they needed to go to that trouble. I have no idea of what the use case was, but I suspect it was a solid requirement without other simpler alternatives.Note that the front-front panel is customised to read "grid heater cathode (?modulation?)", so perhaps a high power radio transmitter or government laboratory.

author: bdunham7
author_rank: Super Contributor
timestamp: December 28, 2022, 02:59:28 pm
content: Quote from: windmill john on December 24, 2022, 03:19:28 pmPlease learn a lot more before you start doing insane things like floating an old scope to measure live circuits.Using an isolation transformer and breaking the ground circuit are two different things! Many isolation transformers pass through the ground by default. Many scopes already have reasonably isolated power supplies and can be 'floated' by simply cutting the ground wire in the power cord. However this can be insanely dangerous as connecting to a live circuit may result in the exposed metal on the oscilloscope, including the BNC shells and the ground leads of the probes and anything they are connected to, all becoming energized with voltages high enough to kill you.There is almost never a real need to float your scope. And when there is, the only sane way to do it is to use an isolated scope that is designed to be floated (these are expensive and uncommon).

author: Old Printer
author_rank: Frequent Contributor
timestamp: December 28, 2022, 03:28:09 pm
content: Disabling a safety device to make something “more safe” is not a good idea. I have disabled countless safety devices to make something easier to use, but occasionally paid the price. Luckily I have all my fingers, but it’s been close a few times. Knowledge is your friend, particularly knowing when you don’t have enough of it.

author: Stray Electron
author_rank: Super Contributor
timestamp: December 28, 2022, 03:34:16 pm
content: Quote from: wasedadoc on December 25, 2022, 03:54:03 pmThis subject has come up before and probably will again. Somebody should boil down the numerous posts on this subject into a couple of short-and-to-the-point paragraphs and post it as a sticky.

author: james_s
author_rank: Super Contributor
timestamp: December 28, 2022, 06:55:01 pm
content: All a person really needs to know is don't float a scope, ever, period.Yes, lots of people have done it and lived to tell the tale. Lots of people have survived driving drunk too and made it home in one piece. That doesn't mean it's ok to do if you know what you're doing. These days differential probes are affordable if you need to take measurements like this.

author: Kim Christensen
author_rank: Super Contributor
timestamp: December 28, 2022, 08:03:28 pm
content: The key is to know exactly what you are measuring with your scope. An isolation transformer, even if it's powering the DUT (device under test), is no guarantee that you won't blow up your scope. This is especially true when you have a dual channel scope. ie: If you put the ground clip of channel A on the +100Vdc supply line and then put the ground clip of channel B on the 0V supply line, I guarantee that sparks will fly no matter where that isolation transformer is placed.