diy isolation transformer - Page 1
Forum section: electronics / beginners

author: p.larner
author_rank: Frequent Contributor
timestamp: October 23, 2023, 12:48:09 pm
content: can old microwave oven transfos be wired back to back to make one,or best to rewind both secondarys for say 24v then connect back to back?.

author: Ian.M
author_rank: Super Contributor
timestamp: October 23, 2023, 12:59:54 pm
content: Too much work for a crappy result. Microwave oven transformers run on the ragged edge of saturation, relying on the limited duty cycle and fan cooling to prevent overheating, and are designed to current limit (which can be alleviated to some extent by knocking out the magnetic shunts) so two back to back will run hot and the output has very poor regulation i.e. will droop severely on load. Rewinding their secondaries removes the HV hazard but doesn't do anything for the overheating or regulation.

author: jonpaul
author_rank: Super Contributor
timestamp: October 23, 2023, 04:26:03 pm
content: NO DO NOT DO IT:Microwave oven trsf designed ONLY for short duty, will overheat in long term use > 5..10 min2000 V 1A secondary is LETHAL.Any normal LV/filament 2x back to back like 120V:24V etc.ORGet a normal 1:1 120/240 V isolation like BelFuse /Signal DU seriesJon

author: johansen
author_rank: Super Contributor
timestamp: October 23, 2023, 05:08:02 pm
content: You can do it.Find 2 identical transformers and put both primary coils on one.Then add 20 extra turns to both coils.

author: p.larner
author_rank: Frequent Contributor
timestamp: October 23, 2023, 06:36:44 pm
content: Quote from: johansen on October 23, 2023, 05:08:02 pmcan you explaine that better or draw a schematic?.

author: Zero999
author_rank: Super Contributor
timestamp: October 23, 2023, 07:02:57 pm
content: Quote from: p.larner on October 23, 2023, 06:36:44 pmAn isolation transformer typically has extra turns on the secondary, to compensate for the voltage dropping under load.I wouldn't bother making one. It's much cheaper to buy one.Quote from: johansen on October 23, 2023, 05:08:02 pmThe extra turns should only be on the secondary. The number will depend on the voltage drop under load. I would wind it 1:1, test it under full load, then add enough turns to get the required voltage under full load.

author: Ian.M
author_rank: Super Contributor
timestamp: October 23, 2023, 07:20:58 pm
content: If you are starting with a MOT, you'd need typically 10% extra turns on the primary to stop it saturating, and maybe 20% extra on the secondary to bring the on-load output voltage up. Unfortunately there is no good safe way to wind an extra coil to go in series with the primary, as MOTs are typically welded core, so you cant wind it on a bobbin and put it on the core. If you cut the welds, you are very unlikely to be able to get the core back together with a small enough gap.

author: alligatorblues
author_rank: Regular Contributor
timestamp: October 23, 2023, 07:30:39 pm
content: If the idea of an isolation transformer is safety, microwave oven tsfr would be my last choice. They are made as cheaply as possible, specifically for use in a microwave, and nothing more.

author: p.larner
author_rank: Frequent Contributor
timestamp: October 23, 2023, 07:36:33 pm
content: i will find another way then,is putting 2 diodes back to back in equipment ground leads a good idea to help with hf rfi?.

author: Thunderer
author_rank: Regular Contributor
timestamp: October 23, 2023, 11:17:49 pm
content: Nope, what you will get is a ground lifter (missing a resistor and a cap though). Its main purpose is to cut ground loops, thus the humm in audio power amplifiers. So low-freq, not HF. See below the schematic.What is the real problem you try to solve with isolation transformers?

author: Psi
author_rank: Super Contributor
timestamp: October 23, 2023, 11:29:03 pm
content: Some types of older UPS's can be a good way to get a transformer for DIY'ing an isolation transformer.Usually when UPS's fail the semiconductors explode but the transformer is totally fine.Depending on type, some of them actually have dual mains voltage windings already !My one had a winding tapped at 220,230,240VAC and another one for just 230VAC.They are used so the UPS can, with a few relays, step up or step down the mains voltage if it detects it's a bit high or a bit low. Not all UPSs have this feature though.Consumer UPS transformers may not be intended to run 24/7. So you can't assume a 1500W UPS transformer will make a 1500W isolating transformer. But 1000W is likely fine. Just check the temp and see.If you can't find a UPS with dual mains windings you can use two UPS transformers and wire them back to back on their lower voltage battery winding to make a step down and then step up. But this isn't very efficient and you want to find a UPS that runs from at least 36V (3 battery) so the intermediate voltage is higher and current lower.It's still a bit janky, best to find one that has dual mains windings already.Keep in mind any DIY isolation transformer doesn't have the safety factor of a professional isolating transformer. If you DIY it the input and output windings might be stacked right on top of each other and if one burns up it may short to the other. Where as a professional isolating transformer has enough physical isolation so if one winding burns up it wont short to the other side.DIY isolating transformers are fine for circuit testing, but not to for safety when using power tools outside.

author: johansen
author_rank: Super Contributor
timestamp: October 24, 2023, 05:15:08 am
content: Quote from: p.larner on October 23, 2023, 06:36:44 pmIf you need a schematic to wind 20 extra turns on both primary and secondary of a transformer then you should not be messing around with 120vac.SorryAnd to the people saying 20 extra turns only on the secondary, you have clearly never used a mot as a tx. Without 20% more turns on the primary you will have about 100 watts of parasitic losses.And no additional extra beyond the same 20% more are needed on the secondary unless you like extra voltage at no load. Whats the point?

author: Zero999
author_rank: Super Contributor
timestamp: October 24, 2023, 11:25:32 am
content: Quote from: johansen on October 24, 2023, 05:15:08 amPoint taken about requiring more turns on the primary to avoid saturation, but I was right about requiring more turns on the secondary to account for voltage drop under full load. In other words, the secondary will require even more than 20% more turns to provide the rated voltage, under full load.

author: TheFloyd
author_rank: Contributor
timestamp: August 11, 2025, 05:15:26 am
content: I find this topic both interesting and frustrating. There are so many posts online about it, and once you sift through all of the examples where people are just hooking two transformers together, and others that are titled "build your own iso transformer", then turn out to be a person hooking up a transformer that has a convenient secondary winding at 1:1, you end with forums where people get so far into the weeds on the fine details, that it is difficult to get some clear advice. This thread was useful and i loved the fact that someone suggested the use of two primary windings off identical salvaged transformers.I just have done that very thing, however i have only just finished it and tested with low voltage AC so far. I have no intention of using it for long periods or for high current scenarios, it will be purely for testing and hopefully preventing unpleasant events while restoring some old valve gear etc. I have seen many posts that say not to use Microwave transformers, due to their quality, the danger of their secondary voltage, and their inefficiency and apparent current limiting (which i admit i didnt really understand...). I salvaged two identical microwaves, and carefully cut the welds and removed the windings. I pushed both of the primary windings into a single transformer, used epoxy and a bench vice to glue the base back on the transformer core and then a blow torch and solder to electrically connect each side where the welds were.The insulation for both windings remained intact and all of my testing so far shows absolutely no electrical connectivity between either primary or between the the windings and the core.My questions that i would love some guidance on are :under the assumption that this is for low-current, brief testing purposes, and it is all in a safe enclusure with the chassis and the core of the transformer earthed to mains, and fuses on both the primary and secondary windings:1 - Is there anything really wrong with doing this, that is material-enough for it not to assist with testing on my work-bench?2 - other than the wire gauge of the primary and the number of turns, being different to the old secondary, they are essentially no different, right ?3 - someone in a thread mentioned that this wouldn't stop electrocution, because there would be too much "capacitive coupling" ? errrm... thoughts on this comment ?4 - This thread mentioned that microwave oven transformers use 'magnetic shunts'? can you please clarify what they look like for me? i did notice that between the primary and the secondary, there was a small insulated stack of laminated metal. I just assumed that they were to physically separate the two windings, so i put them back in between the two primaries to prevent any air-gap and rattling etc. was this a bad idea ?Thanks in advance for any thoughts or guidance on this topic!Cheers,Floyd.

author: MariuszD
author_rank: Regular Contributor
timestamp: August 11, 2025, 06:04:58 am
content: Quote from: TheFloyd on August 11, 2025, 05:15:26 amThe capacitance between the windings will not exceed 100pF. The current flowing thru here will be less than in completely safe factory produced power supplies in Class II insulation, which have a capacitor of 1nF or more between the mains and the output.Quote from: TheFloyd on August 11, 2025, 05:15:26 ama stack of laminated sheets is a magnetic shunt. This element limits the output current of the transformer. Part of the magnetic flux from one winding bypasses the other, resulting in high output resistance. You should remove it or you can insert it between the winding and the core. Inserted in this place, it will eliminate the gap and will improve the magnetic coupling instead of worsening it.

author: Ian.M
author_rank: Super Contributor
timestamp: August 11, 2025, 09:32:53 am
content: Gluing the core back together is very very unlikely to have got the gap anywhere near as small as it was with the original welded construction, unless your bench vice could (and did) apply hundreds of pounds of pressure to the joint, and the glue used was close to water-thin. Increasing the gap will have reduced the permeability and inductance, significantly increasing the magnetizing current, and thus idle losses.Electrically connecting the sides where the welds were is absolutely pointless. The welds are only on the outside because the OEM doesn't want them to act as a shorted turn so even if access permitted, they CANNOT be welded on the inside (core aperture). As the flux through the core prefers a shorter path (cutting the corners) and the weld depth is carefully controlled the weld area (in the plane of the mating faces) does not cause a significant eddy current.Its a pity you didn't measure the no-load primary current (and preferably also power with a plug-in wattmeter or a DMM capable of measuring power and power factor) before you dismantled the transformers, as then you'd be able to remeasure and know how much worse gluing it back together has made it.TLDR: splitting a crappy welded core transformer makes it even crappier unless you can re-weld it or clamp it back together with nothing extra between the mating core faces.To prevent the modified transformer overheating due to saturation causing excessive idle current, you will need to reduce the supply voltage between 10% - 20%. If you've got a Variac, use it! If not, you'll need a buck transformer - ask if you need help with that.Ideally, to avoid the need to reduce the voltage, you'd add primary turns to keep it out of saturation, as discussed above, and probably secondary turns as well to bring the output voltage closer to nominal at the desired load current. Modifying the existing windings isn't a good option so you'd need to add the extra turns as separate windings occupying the part of the core aperture formerly occupied by the magnetic shunts. You'd need to construct a split bobbin to hold the extra windings, and adequately isolate them from the core, and each other.On the safety side of things, as long as you follow the usual precautions for working on live mains circuits, put the transformer on an insulating fire resistant surface, and work in an area with good ventilation, it shouldn't get too exciting. Expect to pop some fuses or trip some breakers.

author: johansen
author_rank: Super Contributor
timestamp: August 13, 2025, 05:03:21 am
content: Quote from: TheFloyd on August 11, 2025, 05:15:26 amYes those are magnetic shunts between the two coils, they help make the secondary more "constant current" -ish when used as a 4000Vdc voltage doubler.I would like to know what the voltage drop under the load is before and after you remove those shunts. I would get data from at least 3 loads, 1, 5, 15 amps. (a hair dryer is 15 amps, space heaters max out at 1500 watts or 12 amps) (just punch the shunts out with a hammer).No there is nothing wrong with what you did.Repeatedly (and really only on this forum) I've seen a handful of people keep insisting that isolation transformers have turns added. This is not the case in my experience, and is definitely not so with the older bidirectional isolation transformers that could step down 120 to 240 or back again.A typical microwave oven with its 120 turns of 16-15 or even 14 gauge aluminum wire is on the order of 1 ohm. At best such a transformer might be rated for 500 watts continuous, at which you would have a 10 volt drop under "full load." This is completely acceptable for any task that requires an isolation transformer. If you cant live with that and need your full voltage at full load (but why then could you tolerate too much voltage at low loads?) then you need to hire more competent circuit designers. What's going to happen to your circuitry when there is a brown out? or a 160vac voltage surge that barely tickles the MOV breakdown threshold?Getting back to gluing the core back together. Yes that's fine. A gap of 1 part out of 2000 will not make a significant effect on the magnetizing current (the core is already saturated unless you added at least 10% more turns)I am more impressed with how you got it hot enough to solder the core back together. (if you actually did).I've used microwave oven cores for inductors at 3000hz and you can feel the weld bead warm up faster than the rest of the core does. but at 60hz, its negligible. if you really want to go overboard, you can file the weld down on both sides with a new sharp file to avoid smearing the metal between the laminations.For example, from memory (and i may still have this transformer), the no load current at 120vac was 4.6 amps, 115vac was 2.3 amps and 110vac was 1.2 amps.-that's how quick it drops.

author: calzap
author_rank: Frequent Contributor
timestamp: August 14, 2025, 07:05:11 pm
content: What are you going to use the isolation transformer for? If it’s for testing mains-powered devices, a GFCI (aka RCD) power source should be adequate protection against grounding yourself in most cases. It has some advantages: 1. can get full power allowed by the circuit breaker of power source, 2. you’ll get a (non-lethal) reminder if you touch something you shouldn’t when you’re grounded. Test the GFCI before each work session.Neither an isolation transformer nor GFCI will protect you if you put your body between two parts that have high voltage between them and don’t involve ground. Careful planning, neat work area, rubber gloves, insulated tools, and one-hand rule can help in this case.Mike

author: TheFloyd
author_rank: Contributor
timestamp: August 15, 2025, 10:06:04 pm
content: Quote from: MariuszD on August 11, 2025, 06:04:58 amThanks for this info, especially for your clarification about the practical implication of capacitance. Additionally, for the confirmation about those lamination and their function. I am not new to electronics, and have been making and fixing gear all my life, yet somehow I never learnt (or possibly forgot) about the application of using lamination between the windings to limit output current. your explanation entirely makes sense! Unfortunately, the high temp JB Weld that i used in the clamp-and-glue process will remain until the cooling of the universe, so there is literally no chance of me ever modifying what i have done. but i have absolutely learnt for next time! thanks!

author: TheFloyd
author_rank: Contributor
timestamp: August 15, 2025, 10:23:29 pm
content: Quote from: Ian.M on August 11, 2025, 09:32:53 amGood info, and point taken about the effectiveness of my effort with glue and clamps, and the irrelevance of my attempt to electrically bond the joint where the weld was. I guess the fact is that this sort of experimentation is all part of the process and how we learn, and I expect is something that everyone on forums like this, have in common. It is decades since i studied theory surrounding this stuff at uni, so tinkering, testing and refining is part of the fun. Having a forum like this one where like-minded folks like yourself, who are happy to provide advice without judgement, is fantastic!Over heating and idle current is something I will watch out for. in truth though, i am not sure that i will even use this one, now that i have a better understanding of the elements i got wrong. But it has made me eager to build another one, from hopefully more appropriate transformers and see how they differ.Buying a good bench variac is on my list, but they are so damn expensive for how often i would use it. I have some wonderful old A&R benchtop power supplies that have both AC and DC operation, and are based around a huge variac. This is exactly what i used when i first lit-up the primary of my DIY transformer. sadly, it only took me up to 20v AC, but the utility of this ability for testing AC gear, is very helpful! When i eventually get one, I will use it to light up the primary of my DIY transformer and carefully test its operation.

author: TheFloyd
author_rank: Contributor
timestamp: August 15, 2025, 10:49:36 pm
content: Quote from: calzap on August 14, 2025, 07:05:11 pmThe primary use of the transformer, was to simply have built it, to experiment with it and to learn from it. I understand exactly why mains isolation can be a helpful thing to have, and when it is less helpful. I suspect like most people in this forum, over the decades I have had plenty of belts off electrical gear that i was experimenting on or repairing, especially from before the time of wonderful devices like RCD's. I decided years ago, that my days of shocking myself, need to be over, before they truly end up being so! RCD's are ultimately saving people from the unpleasantness of carelessly touching things that are a bit "spicy" (to use a Big Clive termIn terms of actual use for an isolation transformer. Well, if i had it 20 years ago when i was constantly fiddling with and fixing up old CRT's, and tinkering with old gear in my evenings, I am sure that it would have helped wave me from a few brushes with a hot chassis, but these day's I dont really dabble with many things that have that degree of 'excitement potential'. I do however have a growing collection of lovely old vintage gear, and an increasing desire to explore the world of valves. Years ago, I used to tinker a lot in this space, but it was at a time that i really didnt know what i was doing, and when i look back at the wonderful collection of valves that i discarded from amps, tv's, transmitters, that i had acquired, experimented on and pulled to bits, i shudder with shame! These days, if i find gear like that, i love to reverse engineer it, understand it and plan to restore it at some point. In the spirit of self preservation, this has left me with a goal of adding an isolation transformer, variac and dim-bulb, to my workshop.On a related topic, I have recently salvaged a hand-made valve amplifier, that was thrown out in a curb-side waste collection day. I cant find any information on the manufacturer, and sadly it was missing all of its tubes. The construction of these systems truly is a thing of beauty, and i would really love to get it operational. I have been in the process of reverse engineering it and drawing up a schematic for it. It uses 300B tubes for its output, and possibly 6m1n / 6SN7 tubes for the preamp. unfortunately with no information i am trying to reverse engineer it based on the pinouts of the valves and the circuit topology that is commonly used. If anyone in this forum can guide me to a good thread where i can post some pictures and the info i have been discovering along the way, I would love to hear thoughts on the circuit, and some thoughts about the rectifier tube and pre-amps etc ?

author: TheFloyd
author_rank: Contributor
timestamp: August 15, 2025, 11:19:08 pm
content: Quote from: johansen on August 13, 2025, 05:03:21 amOnce again, thanks for helpful and practical advice, rather than schooling me for having a go and learning ! This forum is better than most others, because of the people who are in it! Some practical answers about what i did and what i was asking, and the context of what i would do with it, is very helpful and refreshing. Given that it would (at most) be for brief experimentation, it is great to get some good info to refresh my lost knowledge about saturation, flux density and losses, while getting non-academic 'maker' advice like : "Yep that's fine... it wont be great, but it will work... just dont kill yourself while playing with it"."Getting the core hot enough". well, i think you are on the money here... I really doubt that my solder took hold, so it was probably me ticking a box mentally, than having any material effect on electrical coupling! haha... I used a small butane torch, with a very fine flame that could localise the intense heat, but as expected, the huge iron core wicked the heat away very fast. It tool a lot of patience, but after a period of time and a good amount of frustrated swearing, I got the corners of the core to be hot enough for the solder to flow onto. not sure if it made an electrical bond, to simply cooled in-place, held there mechanically. but, i guess that doesnt matter really in hindsightI actually have a welder, but i hate the thing! that said, if i do this again, my beloved JB Weld will stay in the drawer and i will tack the core back together with my welder.Your comment about the weld bead heating up is super interesting! I had not considered that, but it makes sense now that you say it.I wish i had understood more about those magnetic shunts, before i built this thing... given that they were wrapped in insulating card, and then potted in some kind of enamel, i just thought they were low-cost insulators between the old windings, by a manufacturer using the lowest cost solution available to them. the explanation in another comment about them effectively reducing the magnetic flux that crosses the secondary, really was both interesting (and a bit of a duh... moment for me in hindsight!)

author: vk6zgo
author_rank: Super Contributor
timestamp: August 15, 2025, 11:57:51 pm
content: Quote from: TheFloyd on August 15, 2025, 10:49:36 pmWith old Australian equipment, you will very rarely find transformerless designs, with the few AC/DC ones being almost "cottage industry" style as distinct from the plethora of transformerless stuff that was produced in USA, UK, EU in the immediate post war years.The only times I have regularly used an isolation transformer was with solid state stuff from the early 1970s onwards.

author: Analog Kid
author_rank: Super Contributor
timestamp: August 16, 2025, 12:16:24 am
content: Just for reference (and ha-has), here's my small isolation transformer:Two 24-volt transformers back-to-back. Low current only (even says so right on the case!).The case is from an old wall wart that had a convenient clamshell design held together by 3 screws.

author: Grandchuck
author_rank: Super Contributor
timestamp: August 16, 2025, 04:54:10 pm
content: Good for about 15 Watts, I guess

author: Analog Kid
author_rank: Super Contributor
timestamp: August 16, 2025, 07:06:40 pm
content: Quote from: Grandchuck on August 16, 2025, 04:54:10 pmI'd go as high as 20, at least for short times; the transformers are rated at 24V @ 1A.

author: Zero999
author_rank: Super Contributor
timestamp: August 18, 2025, 12:48:00 pm
content: I recommend adding a fuse somewhere. At the very least, 1A between the two transformers.The downside to two transformers connected back-to-back is lower efficiency: higher standby losses and greater voltage drop on-load.I rewound an old 12V out mains transformer for 240V out. It works quite well, but I did need to experiment with the number of turns, since my measurements weren't perfect.