Back to back microwave oven transformers for isolation? - Page 1
Forum section: electronics / beginners

author: R Lamparter
author_rank: Contributor
timestamp: January 16, 2018, 02:59:49 am
content: I have five microwave oven transformers (MOTs) none of which are identical. I was thinking of connecting a pair of them together to create an isolation transformer. Initially I was thinking of connecting the high voltage sides to each other, so step up then step down to 120V, but it might be safer to power the high voltage side from mains and connect the low voltage primary sides to each other. Has anyone used MOTs for this? Which side did you use to connect to each other? Did it work? Aside from the fact that 1000V to 2000V secondaries can be dangerous, am I missing anything?

author: neo
author_rank: Super Contributor
timestamp: January 16, 2018, 03:44:54 am
content: Let me first preface this by saying, i am not qualified to be the definitive end all of answers. I am only a hobbyist who happens to like playing with things that can kill me. Safety is your responsibility.You could connect the mains to the secondary as opposed to the primary and it works great, tried it and done it. But it usually only gives out a couple volts of AC, at least it is isolated AC. Then I also tried taking that 2 volts and putting it back through which also worked, gave me 120 volts AC back out.Now here are the problems, sorry nothing is perfect, there may be issues with the ability to supply current and that is realy try as you go method. But also one end of the high volt side is connected to the transformer chassis, so then you would have 120V AC on a massive hunk of metal. You can work around this, drill out the rivet or just cover it but it does pose a safety risk.Ultimately, will it work for your purpose? Up to you to do your own testing and see, just please keep safety in mind. These are not child's toys even without the 2000 VAC.

author: oldway
author_rank: Super Contributor
timestamp: January 16, 2018, 06:59:37 am
content: These transformers are extremely dangerous because the secondary voltage is very high, one side of the secondary is connected to the metal casing of the transformer and the available current is very high ... In short, a perfect killer, probably more efficient than the electric chair .In addition to the extreme danger, these transformers are totally inadequate for use as an isolation transformer because they have a very high short-circuit impedance (between 25 and 30%) .... If you put two back to back , the short circuit impedance will be double, so of the order of 60% ...Your separation transformer will be unusable because the voltage will drop sharply if put any load on the output.These MOT transformers can only be used for two things: either recovering copper and steel, or removing the secondary high voltage and rewinding another winding of lower voltage

author: danadak
author_rank: Super Contributor
timestamp: January 16, 2018, 10:43:33 am
content: If you do rewind here is a table of wire gauge vs R. I would measureprimary R, and its diameter, so you can compute the length of wireto wind on secondary to develop a 1:1 isolation transformer.https://www.google.com/imgres?imgurl=http://www.interfacebus.com/awg-ohms-per-1000-feet.jpg&imgrefurl=http://www.interfacebus.com/AWG-table-of-different-wire-gauge-resistance.html&h=606&w=550&tbnid=Dcd2FPJdkr4aMM:&tbnh=160&tbnw=145&usg=__9sftL1Faq32XKYLEptDn_S30qLU%3D&vet=10ahUKEwjH6JTxpdzYAhVwcd8KHTXIAFAQ9QEILjAA..i&docid=KCZv7yFZkHf35M&sa=X&ved=0ahUKEwjH6JTxpdzYAhVwcd8KHTXIAFAQ9QEILjAARegards, Dana.

author: R Lamparter
author_rank: Contributor
timestamp: January 17, 2018, 05:14:53 pm
content: Thanks for all of the advice. I was hoping to avoid making my own coil, but I suppose I could do it. Has anybody successfully rewound a coil to make a 1:1 transformer out of a MOT or can we assume that since the self made MOT step down transformers used to weld things, any other ratio of winding would also work. ....and I"m a newbie to this sort of thing too.

author: Zero999
author_rank: Super Contributor
timestamp: January 17, 2018, 06:05:09 pm
content: Yes, that's a possibility. There are plenty of tutor microwave oven transformer rewinding tutorials on the Internet.https://www.google.com/search?q=microwave+oven+transformer+rewinding&sourceid=chrome&ie=UTF-8After skimming through a few, here's one of the better ones:http://www.chlorates.exrockets.com/winding.htmlPersonally, I wouldn't bother. I'd buy an isolation transformer. If I bought one today, I'd consider one with two 120V secondary windings, so it can be configured for either 120V or 240V.

author: Cliff Matthews
author_rank: Unknown
timestamp: January 18, 2018, 01:26:24 am
content: Quote from: R Lamparter on January 17, 2018, 05:14:53 pmMOT's are made cheap and generally supposed to run in saturation (at high currents) for short period's with fan-forced cooling. If you have so many, why not knock out the shunts and HV secondaries in just two and wire-up the primaries in series? With all that open space, I found that winding the secondary (for an experiment) was easy and I got enough flux to test at 990watts from 3 in series at 240v. Although I never tested loads for long periods I did measure core temp at 58C after 3-hours under no-load. For a bunch of thoughts on this, (not that mine are all that profound..) see this thread:https://www.eevblog.com/forum/projects/diy-1kva-isolation-transformer/With a little luck, you may be able to get 400VA (at 60v across each primary) if you are careful and knowledgeably confident. *YMMV*

author: james_s
author_rank: Super Contributor
timestamp: January 18, 2018, 02:12:07 am
content: You can knock out the magnetic shunts and greatly reduce the effective source impedance, note that this makes the transformer even more dangerous but it can be useful for certain purposes. The problem with using the HV secondary as a 120V winding is that it's made with thin wire that will greatly limit the available current you have.You can buy inexpensive isolation transformers from places that sell parts for arcade games as most earlier CRT arcade monitors require one. You can also make one out of some more sensible transformers with lower voltage secondaries.

author: danadak
author_rank: Super Contributor
timestamp: January 18, 2018, 02:14:44 am
content: Google this "microwave oven transformer" on youtube, quite a fewexamples of people rewinding for various purposes, like welder,induction heaters,.....Regards, Dana.

author: fable
author_rank: Regular Contributor
timestamp: January 18, 2018, 02:18:06 am
content: First of all sorry for my bad EnglishYou can cut open two of them and remove secondary winding from one and instead of it put a primary winding from other one. Weld it and you are done

author: Cliff Matthews
author_rank: Unknown
timestamp: January 18, 2018, 02:44:45 am
content: Your English is fine! Actually on the thread I referred to, that was mentioned. If I would have had access to a welder, I might have tried it using using both primaries in series (one at each end, surrounding a correctly sized nylon bobbin).

author: R Lamparter
author_rank: Contributor
timestamp: January 18, 2018, 03:15:08 am
content: I'll use the MOTs for some other project. I ran across an inexpensive isolation transformer here:http://www.adamsstore.top/thordarson-23v366-isolation-transformer-p-6872.htmlWith the current Canadian to US exchange rate it should be around $25 with free shipping.250Watts should be plenty for my purposes.Where are the shops with isolation transformers for old video games? I'm curious now to compare prices and specs to what I just bought.

author: james_s
author_rank: Super Contributor
timestamp: January 18, 2018, 03:26:21 am
content: The places I used to buy parts are mostly gone now but if you search for arcade parts there are others around. Arcadeshop is one place I've gotten stuff from, and I think another company may have bought Happ Controls. Ebay is another place worth looking but $25 sounds pretty reasonable.

author: R Lamparter
author_rank: Contributor
timestamp: January 18, 2018, 03:12:07 pm
content: Arcadeshop is there and has an isolation transformer for about $25, but for others who are looking, using the search term "arcade isolation transformer" brings up other listings and the price seems to be $20 to $25 without shipping.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 18, 2018, 04:15:27 pm
content: A safer option would be to remove the HV winding and wire the filament windings back-to-back. Probably won't be very efficient but should do the job.

author: Zero999
author_rank: Super Contributor
timestamp: January 18, 2018, 04:29:37 pm
content: Quote from: mikeselectricstuff on January 18, 2018, 04:15:27 pmThe trouble with that is the filament windings are only rated for 30W.

author: oldway
author_rank: Super Contributor
timestamp: January 18, 2018, 05:03:45 pm
content: An insulation transformer is primarily a safety equipment ....I am surprised by the reactions of some participants in this forum who are fierce defenders of the principle of buying expensive professional products when it comes to multimeters, but who offer the lowest level of DIY when it comes to an isolation transformer.Two weights, two measures .... why?And yet, an insulation fault in an isolation transformer can be much more dangerous than using a low-quality multimeter.An isolation transformer must be built with quality insulation, by competent people and be tested according to standards .... All this is out of reach of an amateur construction.https://www.arisafety.com/wp-content/uploads/pdfs/3-Ways-to-Simplify-Medical-Device-Testing.pdfHere is the medical grade isolation transformer I use.http://www.amplimo.nl/images/downloads/ds%20medical/zn229.pdf

author: chris_leyson
author_rank: Super Contributor
timestamp: January 18, 2018, 05:24:13 pm
content: The only sensible place for a microwave oven transformer is in a microwave oven. Besides the high resistance secondaries the other thing you need to consider is the working flux density. To keep weight and costs down the cores are under sized when compared to other transformers with a similar VA rating and they're meant for intermittent use and not continuous use so they're really not much good for anything apart from microwave ovens.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 18, 2018, 05:36:04 pm
content: Quote from: oldway on January 18, 2018, 05:03:45 pmBecause you use a DMM all the time, but often only have an occasional need for an isolation transformerQuoteBut it's a pretty unlikely occurrance, and in many use cases ( e.g. grounding the -ve rail of a SMPSU) would still trip a breaker or blow a fuse.QuoteLike any other mains transformerQuoteUtter nonsense. It is not hard to wind a safe transformer, or adapt an existing one safely.Quotemedical isolation is a differnet ballgame - designed for situations where people are permanently connected to a device, and may not be able to get themselves away if something goes wrong.

author: Seekonk
author_rank: Super Contributor
timestamp: January 18, 2018, 05:40:46 pm
content: Saw a great article for using these in RE transmitting power long distances at high voltage, search microformer. Short of it, remove shorting bars and add extra turns to primary.

author: Cliff Matthews
author_rank: Unknown
timestamp: January 18, 2018, 06:06:00 pm
content: Quote from: Seekonk on January 18, 2018, 05:40:46 pmIs that the one where they fill metal paint cans with mineral-oil, drop in a MOT and bolt porcilin HV isolators on the lid?(can't find link..)*edit - OK, yeah is was "The Microformer Project" from 2012http://www.microformer.org/make-your-microformer/

author: oldway
author_rank: Super Contributor
timestamp: January 18, 2018, 06:17:05 pm
content: Quote from: mikeselectricstuff on January 18, 2018, 05:36:04 pmFirst, that's not true.....a DMM is not used all the time to measure dangerous voltages, only occasionally like an isolation transformer.....As most of modern equipments have a SMPS and that they fails often, it is not so occasional at all.Second, do you buy a car without safety belts only because you use your car only occasionaly ?QuoteIf you rely on other safety devices, you don't need an isolation transformer.....QuoteNot right because other mains transformers have grounded secondary, isolation transformers not always.QuoteYes, perhaps using cardboard packaging and masking tape as whe have seen in another topic....Quoteonly a high quality isolation transformer, not a different ballgame at all...!!!!

author: james_s
author_rank: Super Contributor
timestamp: January 18, 2018, 06:41:05 pm
content: Quote from: chris_leyson on January 18, 2018, 05:24:13 pmThey work well for powering vacuum tube Tesla coils, obviously a niche application but it's one they are a nice fit for. They're also handy in modified form for building small spot welders.

author: R Lamparter
author_rank: Contributor
timestamp: January 19, 2018, 12:19:20 pm
content: Boy. Didn't realize I might stir up disagreements with my inquiry, but I did get an explanation of why people don't make isolation transformers from them, although I did see one comment about putting two primaries in series that might make it technically possible, although impractical outside of a developing nation situation (or my impoverished student years) to make one this way. I had hoped to do this for free since I had already saved a few MOTs, but it's not worth the effort and risk when you can get a commercial isolation transformer for $25 - $30 if you look using the right search terms, another thing I got out of the topic.

author: neo
author_rank: Super Contributor
timestamp: January 25, 2018, 11:57:51 pm
content: I finally found a good use for a MOT, i'm using one as a counterweight.

author: AllTheGearNoIdea
author_rank: Unknown
timestamp: January 27, 2018, 10:25:06 pm
content: Are you taking the piss?Can I just say no. If you want an isolation transformer that your health or perhaps more importantly the health of some poor innocent test instrument can depend on then. BUY AN APPROVED ISOLATION TRANSFORMER!