What should I know before making an isolation transformer?  - Page 1
Forum section: electronics / beginners

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: January 03, 2023, 06:10:38 pm
content: Thinking of using an old large microwave transformer for use as an isolation transformer. I've already cut the core in half and removed the 2000v secondary and the electrical shunts. Should I make a new primary or pull a primary out of a second MOT transformer?If I'm to rewind the transformers primary + secondary, would it be better to create primaries/secondaries that have more turn (ex. 100 turns of 18AWG wire copper) or fewer turns (ex. 10 turns of 18AWG wire copper)? Does it matter whether the copper is solid core vs stranded? What would improve efficiencies?Thanks!

author: james_s
author_rank: Super Contributor
timestamp: January 03, 2023, 06:44:59 pm
content: MOTs are usually wound with the bare minimum number of turns and will saturate at light load since they are designed to always run at full load. You will probably want to add additional turns to the primary and then wind a secondary with about the same number of turns. Use solid enamel coated magnet wire, there is no reason to use anything else for an iron power transformer. You cut the core in half? As in sawed it? How are you going to put it back together?

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: January 03, 2023, 07:05:27 pm
content: Quote from: james_s on January 03, 2023, 06:44:59 pmTechnically no; I cut it at the weld lines. I thought I could epoxy it back together and that welding wasn't needed to maintain the magnetics?Quote from: james_s on January 03, 2023, 06:44:59 pmWhy would I need to add more turns? What would happens if I used primaries and secondaries with less turns?

author: Gyro
author_rank: Super Contributor
timestamp: January 03, 2023, 07:38:15 pm
content: I'm sorry to discourage you, but if you are planning an isolation transformer with any ambition of safety, an old MOT is absolutely the last place to start. Seriously, isolation transformers are designed and tested to achieve high levels of primary - secondary isolation. The only thing MOTs are useful for is spot welders, where you thread a couple of turns of insulated heavy cable through the space originally occupied by the secondary winding (removed by cutting through the windings and pressing them out rather than cutting the core).Once you have sawn through the magnetic core, you are never going to glue it back together without creating an 'air' gap (in magnetic terms)*. This will seriously reduce the primary inductance and cause it to overheat when you connect it to the mains.If you want to build a reasonably safe, rather than buy, an isolation transformer, then take two identical reasonably high wattage step down transformers and connect them back to back (secondary to secondary).Edit:*Well I suppose if you filed and accurately lapped the sawn ends to be completely parallel and then tightly clamped them you might...

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: January 03, 2023, 07:45:15 pm
content: Quote from: Gyro on January 03, 2023, 07:38:15 pmThis sort of makes sense to me if I were to think of it in terms of wireless charging of mobile phones, in which more power can be given to a phone (secondary coil) that is closer to the wireless charging pad (primary coil).Quote from: james_s on January 03, 2023, 06:44:59 pmCould you answer this part of the question? Why would I need to add more turns? What would happens if I used primaries and secondaries with less turns?Further, could I not increase the safety of the transformer by using thicker insulated wires or even coating the transformer core with a non-conductive coating?Edit: Even if I don't go along with this idea, it would be good to learn from this idea and why it would/wouldn't work, as well as how it could be done safely.

author: Gyro
author_rank: Super Contributor
timestamp: January 03, 2023, 08:12:32 pm
content: More turns increase the inductance of the winding, drawing less current and making it less likely that the core will saturate. MOT cores are undersized for it's power rating and rely to an extent on the magnetic shunts to increase primary inductance (and physical volume core material), providing output current limiting in the process.Yes wireless charging works with the coils in close proximity but relies on high frequencies to work. As the frequency drops, the amount of magnetic material and how 'closed' the magnetic circuit is increases massively.As far as improving the insulation, you probably need split bobbin construction with highly insulating materals (once upon a time, impregnated fibreous materials were used, but these days high temperature plastics are far more predictable) to achieve a large creepage distance between the two windings and windings to core. Approximately 4kV RMS of isolation is requires irrc. Even if you can construct something adequately, you still have the problem of testing it, unless you have the appropriate Hipot tester. Not impossible, but you have to consider whether life is too short, considering the other available alternatives [Edit: and personal safety implications if you get it wrong].

author: Doctorandus_P
author_rank: Super Contributor
timestamp: January 03, 2023, 09:15:24 pm
content: On itself this is a quite nice idea, however, if you write things like:Quote from: LooseJunkHater on January 03, 2023, 07:05:27 pmThen it shows you are a beginner (nothing wrong with that in itself) but you do not have the experience level to bring a project like this to a safe and reliable end.My recommendation is to either buy an isolation transformer, or get two transformers that are the same, and then connect the secondary windings of those transformers to each other. (for example 230Vac to 12Vac, and then in the other transformer from 12Vac to 230Vac again).The combination of a beginner with electronics and tinkering with mains voltages is simply a bad combination.

author: james_s
author_rank: Super Contributor
timestamp: January 03, 2023, 11:35:18 pm
content: I don't think this is completely beyond a beginner, the OP may have plenty of experience with electricity/electronics and just not a lot with magnetics and that's ok, magnetics is a whole specialized field in itself. Being able to assemble a safe and effective transformer does not necessarily require understanding in detail the nuances of how a transformer works. Safety is obviously of the utmost importance, especially in 240V land, but it isn't rocket science.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: January 04, 2023, 12:43:07 am
content: Quote from: Gyro on January 03, 2023, 08:12:32 pmThis part makes a lot of sense to me, especially considering that I've heard/read from others that the MOT's would overheat if they weren't actively cooled. Since I'm aiming for a passively cooled design (as well as having the MOT + safety electronics inside of a metal box), I'd need to draw less current than originally intended. Planning on having input + output fuses, grounding, and a circuit breaker for the additional safety.Quote from: Gyro on January 03, 2023, 08:12:32 pmI don't really think it's realistic to expect an amateur to perform a bunch of safety testing. I know a decent amount of information in terms of electronics repair, DC vs AC, isolation/earthed. I have no intention on working on anything above 240v.Even if I were to buy an off-the-shelf isolation transformer, it really would not provide many additional safety benefits for me.Quote from: Doctorandus_P on January 03, 2023, 09:15:24 pmHow do beginners learn things? They need to start somewhere. See above, but I do plan on adding safety mechanisms to this as well as have a decent amount of knowledge on mains AC.Quote from: james_s on January 03, 2023, 11:35:18 pmThank you for this. I know super little about the world of magnetics and I do want to learn more about it. Things like inductances, core materials, and frequency effecting the inductance are all things I'm super unfamiliar with and am slowly learning more about. I do however understand the basic principles of how an inductor/transformer works and electromagnetism.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: January 04, 2023, 12:47:09 am
content: I have a sort-of followup question. I know in the past when I've shorted windings of transformers, it didn't immediately lead to the melting of the windings (but instead lead to a very significant voltage drop on the output). Is this because there were actually a lot of windings (ie. higher resistant from a longer length of copper wire)? I guess the voltage drop may also be in part because of the core not being capable to deliver enough "power" (I think this is the wrong word?) to the output? Is this also why it's suggested to add additional windings to the MOT to reduce the risk of melting the windings?

author: wilfred
author_rank: Super Contributor
timestamp: January 04, 2023, 01:06:55 am
content: Would transformers from a pair of uninterruptable power supplied be better suited to this project? I have a couple and had a similar thought.

author: EPAIII
author_rank: Super Contributor
timestamp: January 04, 2023, 08:05:20 am
content: What do you need to know?Both the theory and good practices of transformer design.I have worked in electronics for over 55 years, both as a profession and as a hobby and never once used an isolation transformer. So, before you go wild in making a make-shift one, answer just one question. Just why do you think you need one?Good luck!

author: Psi
author_rank: Super Contributor
timestamp: January 04, 2023, 08:11:22 am
content: Dead computer UPS's are sometimes good for making isolation transforms.Some of them already have a transformer with two mains windings, all ready to go.The UPS i built an isolating transformer from had a core with 3 windings.220V input winding24V input windingA tapped output winding at 210V 220V 230V 240V etc..I guess it uses some sort of switching to keep the output voltage steady as the mains input changes or the battery voltage fell

author: inse
author_rank: Super Contributor
timestamp: January 04, 2023, 08:43:10 am
content: A back-to-back arrangement of similar transformers will make a isolation transformer but with limitations.As so many windings are included, the output voltage would be relatively soft (load dependent).Depending on your power demand, the required transformers might be hard to source (think of 12V 300W).Also the unit gets heavy.An alternative might be transformers salvaged from audio amplifiers with maybe something like 90V secondary voltage and decent wattage.

author: beatman
author_rank: Regular Contributor
timestamp: January 04, 2023, 01:25:12 pm
content: https://www.giannakopoulos.com.gr/%CE%B1%CF%80%CE%BF%CE%BC%CF%8C%CE%BD%CF%89%CF%83%CE%B7%CF%82/5176-MTR1-2000.htmlI by two of these nice masive transport friendly.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 04, 2023, 06:05:12 pm
content: My advcie is don't try to convert an old MOT.Its not just a matter of adding windings to get the right ratio. The contruction of the transformer will affect its effectiveness as an isolation transformer.High interwinding capacitance will lead to some shocking results. MOTs are not designed without much concern for leakage currents.This too will limit its value as an isolation transformer.Lab isolation XFMRs are pricey for a reason.

author: CaptDon
author_rank: Super Contributor
timestamp: January 04, 2023, 07:28:12 pm
content: EPA III NEVER USED AN ISOLATION TRANSFORMER?? So how did you work on 'hot chassis' televisions and radios? How did you examine switching waveforms on the 'hot' side of SMPS's? How did you work on the high power / high current +/- 85vdc to +/- 125vdc supplies in 1KW+ audio amplifiers that were basically SMPS power converters? How did you work on Variable Frequency Drives where 90% of what is inside the box was line referenced and often 3 phase in and out? Do you own the worlds largest collection of differential probes for your grounded chassis oscilloscope? I do Carver PM2.0-T and Peavey DECA 1200 repairs where the power supplies are line powered SMPS's and my isolation transformer is in nearly daily use and none of my test equipment or customer equipment was ever ruined do to an un-isolated 'oopps'. Every good service bench needs a variac and an isolation transformer or the all-in-one combo. I have measured isolation transformers with under 100pf cross-coupling but I have found some at over 1000pf!! My daily is 150pf and 1000vac indefinite withstand isolation. I trust my life to it.

author: Psi
author_rank: Super Contributor
timestamp: January 04, 2023, 11:12:01 pm
content: The quickest way to hack together an isolating transformer is probably to find two 220/110 conversion transformers and link them back to back.Since the intermediate voltage is still quite high the current is still quite low.And 220/110 transformers are pretty common and easy to find 2ndhand for cheap

author: Sherlock Holmes
author_rank: Frequent Contributor
timestamp: January 04, 2023, 11:30:37 pm
content: Quote from: LooseJunkHater on January 03, 2023, 06:10:38 pmPlease, I mean, please...

author: james_s
author_rank: Super Contributor
timestamp: January 05, 2023, 01:09:49 am
content: He said in his original post that he already removed the HV winding, so this warning is irrelevant. A MOT without the HV winding is no different than any other iron transformer.

author: Psi
author_rank: Super Contributor
timestamp: January 05, 2023, 07:49:50 am
content: Another option if you live in a 110V area is to find a transformer that's been designed for 110V 220V compatibility.They have two 110V windings which you are supposed to connect series for 220V countries or parallel for 110V countries.But, you could switch it up and have two separate 110V windings for an isolation transformer.

author: wasedadoc
author_rank: Super Contributor
timestamp: January 05, 2023, 09:35:07 am
content: Quote from: Psi on January 05, 2023, 07:49:50 amI would not trust the isolation between the two windings.

author: Psi
author_rank: Super Contributor
timestamp: January 05, 2023, 09:43:50 am
content: It depends why you need one I guess.I wouldn't recommend using it on a job site for safety isolation of power tools.Probably fine on your work desk to isolate a DUT for scoping.

author: james_s
author_rank: Super Contributor
timestamp: January 05, 2023, 06:19:36 pm
content: I have some transformers like that with the two primaries on separate legs of the core, so I see no reason not to trust those. The ones that have them wound together though yeah, not sure I'd trust those.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: January 05, 2023, 10:52:14 pm
content: Quote from: Terry Bites on January 04, 2023, 06:05:12 pmThe thing is, I'm not paying $200+ for a simple transformer... I'm either going to make my own isolation transformer, or CONTINUE to repair things referenced to mains. It's obvious that using ANY sort of isolation transformer is safer compared to what I'm currently doing. If I can use a MOT or UPS transformer or any other readily available transformer, great! But I do also need the transformer to carry +100W at times so using a step down transformer and then another step up transformer really isn't ideal.Quote from: Psi on January 05, 2023, 09:43:50 amVery much for home use. I actually already almost blew up my scope due to probing the wrong line but thankfully the DUT traces blew... Currently I just have my scopes grounding removed but that is obviously not the way to go about this, which is partially why I need an isolation transformer.

author: Psi
author_rank: Super Contributor
timestamp: January 05, 2023, 11:06:11 pm
content: If good safety/isolation is a requirement then the "two transformers back to back" option is probably the best.

author: james_s
author_rank: Super Contributor
timestamp: January 06, 2023, 12:03:01 am
content: Quote from: LooseJunkHater on January 05, 2023, 10:52:14 pmDon't worry about it, just build one and use it, and continue to be careful because an isolation transformer only protects you against certain conditions anyway. Wind the two windings separately and insulate the core well with Nomex or polyamide tape and make sure the two windings are separated from each other and it'll be fine.

author: helius
author_rank: Super Contributor
timestamp: January 06, 2023, 02:18:38 am
content: In addition to the sage advice that an isolation transformer only protects from certain conditions, it reduces safety in other ways. So it's definitely not the case that it's guaranteed to be safer than what you are already doing. In particular, it will prevent any upstream RCD from tripping in case there is a wiring fault in the device under test.You asked a question earlier about shorted turns. Shorted turns in a primary winding reduce the flux coupled into the transformer, just as if the winding had a smaller turn number to begin with. Unless all (or most) of the turns are shorted, the winding still has significant inductance and wouldn't be expected to overheat given the safety factors.

author: bastl_r
author_rank: Regular Contributor
timestamp: January 06, 2023, 02:31:01 am
content: Quote from: james_s on January 05, 2023, 01:09:49 amSo it is.The very fact that he is thinking of building or tinkering with an isolating transformer already shows a certain sense of responsibility.The easiest way to build such a transformer with the help of MOTs would be, in my opinion, to find two identical transformers and to mount their primary windings on a core. Because the problem with winding yourself will be that he will not get the winding to be built without winding body sensibly wound.Alternatively, it is certainly also good if he searches the classified ad markets again and again for an isolating variable transformer.bastl_r

author: james_s
author_rank: Super Contributor
timestamp: January 06, 2023, 03:57:30 am
content: Quote from: helius on January 06, 2023, 02:18:38 amWill it? I thought a shorted turn in any winding would act like making a single turn of wire around the core and shorting the ends which is how a soldering gun or a spot welder works. Does it make a difference if the shorted turn is part of the primary or secondary?

author: MrAl
author_rank: Super Contributor
timestamp: January 06, 2023, 07:59:56 am
content: Quote from: LooseJunkHater on January 03, 2023, 06:10:38 pmHello,There are a number of factors that go into making a transformer for line voltage use. There are more factors for making a transformer that has to be safe enough to prevent shocks from the secondary usually to ground, which includes isolation transformers.The first then is to think about isolation.For a certain class of wall warts for example, the primary and secondary coils have to be separated by a physical distance. That is, side by side, not one on top of the other. There is also an insulator on the core that goes in between the two windings that helps to prevent them from touching in the even there is some damage.In industry there are a number of transformers that are wound with the secondary on top of the primary, but there is sufficient insulation between windings, for example mylar. Mylar tape can be used for the layers of the windings too.I dont know if you will be able to find an actual bobbin so you may end up making your own. You can use fish paper and mylar tape. The primary has to be insulated from the core metal of course and it has to be enough insulation.I would be good to hi pot the design once it is done also.As to the pure electrical part of it, the number of turns of the primary is chosen based on a transformer equation:B=(Vrms*10^8)/(4.44*F*A*N)whereVrms is the AC rms voltage,F is the frequency in Hertz,A is the area of the core in square centimeters (the center section cross sectional area in a regular EI type core),N is the number of turns,B is the flux density in Gausses.You will know everything except B and N at first, but you can find B from the type of core material. For many line operated transformers the Bmax is about 15 kilogausses so you cant go over that. If you go over Bmax you start to draw too much current in the primary and end up with higher core losses as well.The size of the wire should be such that it can handle the required currents. The tradeoff is the window area can only hold so many turns of wire when there is a primary and secondary as well as insulating materials taking up some of the window. This means part of the choice of wire will be limited by the window area, unless you want to use thinner wire than what would be optimum for that size core. To get the optimum is takes a little bit of calculating the area the turns take up when they are layered. You might want to play around with this idea a little. You can get a rough idea what this is about by looking at the attachment. That's for a toroid though and does not show any tape or other insulation.One unusual thing about your project is that you are starting with a core that has no good way to join the two parts that were cut apart. Normally a transformer is bolted or clamped in some way which makes sure the joining faces are very tight against each other. Im not sure how you could get this to happen unless you make up some kind of special clamp or something. One of the problems is if they are not tight you loose inductance and also get a lot of audio noise from the transformer. Since the two parts will be butt joined, you could lose as much as 10 percent of the ability to handle the line voltage on the primary, which means you need more turns. In fact, 10 percent more turns, which may or may not be a lot depending on the center part cross sectional area of the core. But that figure is based on machined parts that fit together fairly well. If you sawed the thing apart you may have a very rough surface that does not fit snug and so you could loose 20 percent of the effectiveness of the primary turns to handle the line voltage level. Mismatched surfaces represent a gap which means loss of inductance so the above formula has to be modified. For a 10 percent estimated loss you would have to place a stacking factor of 0.9 in the denominator, and for 20 percent loss 0.8 for example, which means that in order to handle the line voltage and not go over the Bmax limit you need that many more turns.As i was saying, Bmax for many line operated transformers is around 15kG, but you may want to shoot for 10kG max.To recap:1. You have to observe Bmax for the required number of turns.2. You have to keep the two windings apart as much as possible with tape and fish paper.3. You have to choose the wire size based on the required currents and window area.4. You have to test the design both for leakage and saturation.This last one reminds me that you may not be able to find any specs for your core, so you may have to do some tests. If you make a make shift primary winding you can run it up with a variac and test for input current. The input current can not go too high, so once you get to a certain point you can use that to figure out the best number of turns. For example, if you use 60 turns and you apply 60 volts AC rms and you see the current go to maybe 1 amp, then you know you need 120 turns to handle 120 volts AC rms unless you can live with a higher excitation current (more core loss, etc.).Let us know how you make out and if you run into any problems.Oh BTW, a shorted turn acts like a short for the entire transformer in most cases. The shorted turn makes the transformer look like a step down transformer with very large ratio which means it can pump a huge current though that shorted turn. Imagine having an autotransformer with just one or two windings so that the input is 120vac and the output is 1vac. If you put 1 amp in, in theory you could get 120amps out. Now short that 1vac output and expect to get 120 amps through that short. Of course that takes a perfect short. If the resistance of the short is 1 Ohm then it only draws 1 amp, but if it is 0.1 Ohms then it draws 10 amps, etc.

author: CaptDon
author_rank: Super Contributor
timestamp: January 06, 2023, 02:04:36 pm
content: It is a given that 'one shorted turn' will kill any transformer or cause a SMPS to shut down or self destruct. Some of the worst oil filled transformer explosions come from one shorted turn. The amount of energy available to that one turn can be in the 10's of kilowatts to megawatts and flashes the oil to vapor so violently and so quickly that safety measures may be to slow to react and then you have a case rupture and massive fireball.

author: MrAl
author_rank: Super Contributor
timestamp: January 06, 2023, 06:38:56 pm
content: Quote from: MrAl on January 06, 2023, 07:59:56 amI had forgotten to mention that sometimes there is also a copper shield placed between the primary and secondary coils. That would be a layer of copper foil that goes on top of the primary (or secondary if that goes on first) and covers the entire winding, then the next winding goes on top of that after some fish paper and mylar tape. So you end up with a complete layer of copper between the two windings, and an extra lead comes out that is connected to that copper layer that can get grounded for protection.Of course you also have to test for AC leakage between the primary and copper layer, and secondary and copper layer, but now leakage between primary and secondary should be moot due to the copper shield.Note very important, the copper shield is not connected at both ends or else it creates a short. It has to be treated like a separate single turn winding that covers the entire winding beneath it.Also, a lead from the metal core to ground.I still recommend buying one that is known to work well for instruments, and testing it yourself when you get it home.I also think it is hard to beat a battery operated tester. Perhaps also along those lines would be a battery operated inverter that puts out a clean sine wave power. That would put out 120vac 60Hz (or 230vac 50Hz) but run from battery power such as a 12 volt lead acid battery. There are a lot of converters now that put out a pure sine wave so you could use that to power the instrument used for testing. That would ensure that it could never be tied to the mains voltage. Note i have not tried this idea yet, but i dont see why it would not help to isolate the test equipment fully from the mains line voltage. Of course you dont have the charger for the unit plugged in at the time of the testing, only when charging the battery(s).I also forgot to attach the drawing so it's here now. That's a cross sectional view of the windings in the round window of a toroid but yours would have a square window and some insulating materials and the layers would be visible as well as the copper shield. This one just shows the basic view of how the windings fit into a round window. The math behind it is based on how circles join together when several layers are placed directly on top of the lower layers. In a real transformer there would be tape between layers for best protection against shorts and capacitance between layers. Remember that as the level of the next layer increases so does the voltage difference between it and the layer below and the layer above it.

author: wasedadoc
author_rank: Super Contributor
timestamp: January 06, 2023, 08:15:47 pm
content: Quote from: MrAl on January 06, 2023, 06:38:56 pmThat is what I do when working on mains input SMPS but a proper sine wave inverter is not necessary. A £40 "modified sine wave", in reality one that outputs +, 0 and -, UPS works fine with PSUs that have a bridge rectifier directly, except for fuse and common mode choke, on the input. Grounded scope powered normally from mains.

author: MrAl
author_rank: Super Contributor
timestamp: January 07, 2023, 02:37:03 pm
content: Quote from: wasedadoc on January 06, 2023, 08:15:47 pmHi,What do you mean "except for fuse and common mode choke" ?

author: wasedadoc
author_rank: Super Contributor
timestamp: January 07, 2023, 03:15:22 pm
content: Quote from: MrAl on January 07, 2023, 02:37:03 pmI mean that those components (and mechanical on/off switch) if fitted are the only items between the mains input and the bridge rectifier.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 07, 2023, 05:09:03 pm
content: For scope use i would automatically use an isolation x-former ...My 2 cents and saving lots if something goes wrong

author: Jwillis
author_rank: Super Contributor
timestamp: January 09, 2023, 03:12:48 am
content: Here's a video on the workings of isolation transformers. If you don't mind meeting his pet cat. He makes one with two transformers and tests with unfavourable results.He also explains what a true medical grade isolation transformer differs from other isolation transformers. True Medical grade are designed to eliminate line noise for sensitive medical equipment but the secondary is not floating. This is for safety reasons . The ground plug is still connected to earth.For oscilloscope use the ground plug needs to be unconnected so the secondary floats. This isolates the scope completely from earth ground reference.