Homemade Transformer - Page 1
Forum section: electronics / beginners

author: mishimaBeef
author_rank: Contributor
timestamp: July 04, 2011, 01:56:23 am
content: Hi, I was thinking of making my own power supply and I was wondering how low level I can go with it in terms of designing individual components and then putting them together to form the circuit.So, I'm looking at like a 120V to 15V transformer then through a rectifier to get 15V DC?I'm sure it's more involved than that but ideally that is all you need right? Perhaps some practical knowledge is being missed and if anyone can point that out great.Now to the point, is it feasible to design and build my own transformer using like some iron and wiring the primary and secondary myself?Thanks,New PractitionerElectrical Engineering Undergrad Student

author: Armin_Balija
author_rank: Regular Contributor
timestamp: July 04, 2011, 03:29:00 am
content: If you're looking to build your own power supply there are some great threads floating around on EEVBlog, just have a look around with the search bar.Make sure you BUY a transformer, building one is extremely complicated in reality. You really have to know what you're doing.Anyway, You'll need a center-tap transformer, make sure you get one with good headroom. For example, if your load is consuming 20 W of power, make sure you get a transformer with something like 45VA.Secondly, use a bridge rectifier to give you a positive and negative output that you can use. Make sure your center-tap is ground. Use some capacitors to smooth out the lines as much as possible before connecting them to a voltage regulator with some capacitors on the input, ground, and output for a regulated output.For 15 V I think you can use a LM317 to get the voltage you need. Beware as it starts at 1.25V and not 0.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 04, 2011, 03:47:13 am
content: Re: building your own transformer.It certainly can be done,& has been done,way back in the early days,but I feel you should buy a ready-made transformer.A bought one must meet Electrical standards or they can't sell it,but you have only your own opinion that your homemade one is compliant,soif you make a mistake in the primary side & blow up anything in your house wiring,your Insurance Company may wash their hands of it,& you may be in trouble with the Electrical licencing authority,&,as an Electrical Engineer,you don't want that!Quite a few people rewind secondaries,but very few mess with the primary side.The voltages you are talking about are fairly standard,so you can buy a transformer off the shelf.If you want to run electronic equipment from your supply,there is a lot more to it than just a transformer & rectifier.I suggest you Google under "power supply.",or have a look at some books.VK6ZGO

author: mishimaBeef
author_rank: Contributor
timestamp: July 04, 2011, 09:55:46 am
content: Thanks for the replies so far!Just to further discuss the point about transformer design:So yeah, I was thinking it won't be a piece of cake but thought it would be an interesting design challenge and give insight into transformers and transformer design.Why is it that it's so hard and you can blow stuff up? Won't fusing the primary side protect your household wiring?Does anyone know of any good resources for this stuff? Also, I was thinking to maybe start with a 15V primary and ensure that my design works for stepping this down before I attempt mains 120V primary. Would this be viable or are there some phenomenom that exist at 120V that don't at 15V?Any insight into this stuff would be further appreciated. Thanks!Will also check out the links and info you guys discussed on homemade power supplies.

author: saturation
author_rank: Super Contributor
timestamp: July 04, 2011, 10:54:03 am
content: Making a transformer is fairly easy and straightforward, I had to make one in a college lab exercise, but making it safe and efficient is at least, laborious. These days the cost of copper wire in a retail store may far outstrip the cost of DIY.It works! But is it safe?http://nathan-lee.com/blog/2009/10/15/welding-africa-style/

author: ejeffrey
author_rank: Super Contributor
timestamp: July 04, 2011, 05:53:12 pm
content: Designing and winding your own mains transformer is tedious and not particularly enlightening IMHO. Also, because so much of the cost is copper and iron, it may end up costing more than a commercial unit due to over-design. I wouldn't wind my own mains transformer unless there was really no other option.On the other hand winding your own SMPS transformers is easy and usually necessary. To a lesser extent the same is true of signal transformers -- there are quite some options available but an almost infinite variety of what you might want.

author: Neilm
author_rank: Super Contributor
timestamp: July 04, 2011, 07:54:55 pm
content: The main problem with building your own mains transformer is ensuring that you have sufficient clearance and creepage from the primary side to the secondary to ensure safety.Neil

author: vk6zgo
author_rank: Super Contributor
timestamp: July 05, 2011, 09:12:29 am
content: Quote from: mishimaBeef on July 04, 2011, 09:55:46 amYes,you are right,fusing would protect the wiring,& is the Engineering answer to the problem.I brought it up because:(1)Although many posters on these forums are very capable,some are very much at a beginner level,& I had no way of knowing which was your level.(2) The standards for transformers are based on Engineering specifications,but are usually enshrined in law,which is enforced by people whose expertiseis in the legal rather than the Engineering area,so they are likely to take the view that if your transformer does not have a compliance marking,it is the cause of any problems which may occur.If you are making the transformer from scratch,are you going to cut laminations out by hand,which is how it was done by people on very low budgets in the old days,or are you going to tear down an existing transformer to use its laminations?You should be able to find information on transformer construction in University libraries,probably in the older books.VK6ZGO

author: mishimaBeef
author_rank: Contributor
timestamp: July 05, 2011, 10:15:11 am
content: Are the eddy current losses substantial so that I need to use laminated slabs instead of one solid chunk of iron?My philosophy with this design is not to be an ultimate solution for the bench power supply but more for experimentation, testing and analysis of what makes a good transformer and what kind of quality you can expect from a good/bad transformer. At the end of the day I want to compare how my design measures up with store bought and to be able to answer why.

author: Armin_Balija
author_rank: Regular Contributor
timestamp: July 05, 2011, 03:53:14 pm
content: Quote from: mishimaBeef on July 05, 2011, 10:15:11 amTo be completely honest, just buy the dang transformer. You're willing to risk a house fire to build a transformer?It's just not a good idea IMO. If you're going to build your own transformer..build your own resistors..capacitors..etc., It's just pointless. Spend more time tinkering with the circuit design instead of designing everything down to component level.

author: Zero999
author_rank: Super Contributor
timestamp: July 05, 2011, 04:15:18 pm
content: The only reason to build your own transformer is when you need custom output voltages which you can't get from a ready made unit. You can buy transformer kits but they work out more expensive than just buying a transformer.It's also possible to rewind some transformers. I've rewound cheap wall wart transformers which have the iron laminations weakly bonded together. I prise apart the laminations, rewind the secondary and glue the laminations back with super glue.

author: Armin_Balija
author_rank: Regular Contributor
timestamp: July 05, 2011, 04:21:52 pm
content: Quote from: Hero999 on July 05, 2011, 04:15:18 pmI tend to stay away from rewinding transformers just because I've been told that they're made to work at certain frequencies.

author: Zero999
author_rank: Super Contributor
timestamp: July 05, 2011, 05:13:16 pm
content: You're generally safe as long as you only rewind the secondary coil which is easy on most mains transformers, as it's normally on a separate bobbin.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 05, 2011, 09:07:30 pm
content: Quote from: Armin_Balija on July 05, 2011, 04:21:52 pmAs far as operating frequency goes, that is mostly set by the core. You can rewind a transformer with new secondaries to generate different voltages with no problem. For RF transformers, skin effect in your copper is important, but for line frequency (and most audio applications where the current is low), you just need to wind the right number of turns with a gauge that has low enough ohmic losses for the power you want to draw (not exceeding the original power specification).One thing that is often useful to do is to add windings to a transformer. For instance, you can add a low-power 15 volt winding to an amplifier's power transformer to power a preamp or other small electronics.

author: mishimaBeef
author_rank: Contributor
timestamp: July 06, 2011, 12:48:05 am
content: I don't understand what all the fuss is about. Fusing the primary is a good precaution isn't it? Also, isn't the household wiring protected by a 15 A circuit breaker and then fuses downstream from that?Why is wiring the secondary so much less trouble than wiring the primary exactly?Part of the reason I'm doing this project is to gain insight into the transformer as a component and all the physical relationships that play a role in its operation.To the comment about making my own resistor and capacitor, I do intend to at some point for the same reasons as this project. Am I going to build every capacitor and resistor I ever use? No. But I do wish to gain deep insight into the electromagnetic principles at play in our everyday devices.

author: gregariz
author_rank: Frequent Contributor
timestamp: July 06, 2011, 01:11:41 am
content: I have no problem with you experimenting with components. You have had plenty of warnings so you should know the risks. I wound a transformer years ago, although it was a high voltage secondary transformer that I used in a Valve/Tube amplifier. I did this because at the time it was extremely expensive to buy a 350v CT transformerSo what I did was this;I bought an existing transformer because you will not be able to get the silicon steel E-core laminations you need. Yes eddy losses are significant and the laminations have a lacquered or plastic coating to insulate them from each other. You may contact a transformer rewind business in your local area and ask them but I never did that. I bought the following transformerhttp://dicksmith.com.au/product/M1991/general-purpose-ac-ac-transformer-12-30v-2-amps.jsp?bmForm=selImgPathSubFrom&bmFormID=j2YzadP&bmUID=j2YzadQ&bmIsForm=true&bmPrevTemplate=%2Fproduct%2FM1991%2Fgeneral-purpose-ac-ac-transformer-12-30v-2-amps.jsp&bmEditable=selImgPathHdn&bmHidden=selImgPathHdn&selImgPathHdn=%2Fmedia%2FDSAU%2FProduct%2F300x300%2FM1991_0_med_v1_m56577569835038708.JPG&bmSubmit=selImgPathSub&selImgPathSub=Submit+Query&bmHidden=PRODUCT%3C%3Eprd_id&PRODUCT%3C%3Eprd_id=845524441781404&bmHidden=FOLDER%3C%3Efolder_id&FOLDER%3C%3Efolder_id=2534374302025589&bmFields=bmFormID%2CbmHidden%2CbmIsForm%2CbmUID%2CbmForm%2CbmEditable%2CbmPrevTemplate%2CbmSubmit&bmHash=2267c7187747727594f6a7e1cc5d32018fba73cdYou will notice that the secondary and primary windings are seperated in the center by a plastic insulator. The windings are not wound on top of each other but rather side by side. This means you need not touch the primary winding.I then removed the steel shroud and carefully tapped the first lamination out. Doing this a few times will loosen the laminations enoug h to get to the bobbin. I did not touch the primary winding as I said. I didnt touch it because they use non flamable insulating material that also is not readily available - but maybe you can find it. I then unwound the secondary and counted the number of turns. Using the specified secondary voltage and knowing the secondary turns I then calculated the turns ratio or turns per volts. Then knowing your desired secondary voltage I calculated the new number of secondary turns and rewound the secondary with these turns. I taped it up and then carfully tapped the laminations back in being careful not to scratch the laminations and reinstalled the shroud. BingoIIRC if you dont know what you are doing then you should be careful or buy a book (jaycar sells a small coil transformer design book) or go talk you a motor/transformer rewinder in your city (probably best)

author: mishimaBeef
author_rank: Contributor
timestamp: July 06, 2011, 01:52:25 am
content: Thanks for the info, do you have the link for the book? I searched through twelve pages of results after searching "books" on the websitehttp://www.jaycar.com.au/index.aspand found nothing about transformer design.Also is there a reason they are using non-flammable insulation for the primary? I guess to prevent fires from developing due to arcs in the transformer? If that's the case, what is the cause of the arc? Insulator breakdown?Seems like including a large safety factor for the insulation system would probably increase safety (although suffer from larger flux leakage losses). Does that sound like a reasonable precaution for safer implementation?

author: vk6zgo
author_rank: Super Contributor
timestamp: July 06, 2011, 01:59:23 am
content: Quote from: mishimaBeef on July 06, 2011, 12:48:05 amI am more than a little worried about your University if you are in the 3rd year & they still haven't addressed the reasons for using laminations in transformers.Basically,a solid iron core looks like a shorted turn,& will have massive currents induced in it.Due to transformer action,the primary current will also be very high,& will vapourise your fuse in an instant!The previous posters said why they suggest you stick to rewinding of the secondary.-------Special insulation,wire type,etc.I suggest you consult your lecturer before you go any further .He/she will be able to show you where to look for the required information.VK6ZGO

author: gregariz
author_rank: Frequent Contributor
timestamp: July 06, 2011, 02:04:03 am
content: Quote from: mishimaBeef on July 06, 2011, 01:52:25 amHeat is due to copper and core losses. You should have noticed when you use a Wallwart/plug pack that they get hot. Since you have listed yourself as a student, and I am not an expert, you perhaps really should talk to one of your power engineering lecturers about this. You will find that students kill themselves every now and then because they do not know what they are doing. When I went through one of our graduates killed himself within a few months of graduation because he didn't have the commonsense as to where not to stick his fingers. Mains will kill you that is why you've had so many warnings. Almost everything connected to the mains is significantly overdesigned to cope with varying usage conditions.

author: mishimaBeef
author_rank: Contributor
timestamp: July 06, 2011, 02:10:26 am
content: Thanks. Yeah I am about to take electromagnetics II and power devices this year so I will probably get some more insight into the subject. It's not the best university around but I won't make excuses as I feel learning and understanding is the student's responsibility, not the university's. It's actually amazing how many students I see managing their way through with so little knowledge. I guess people learn to play the test-taking game. But I'm getting off topic here.I was thinking about the eddy current losses and I guess I need to investigate further why the losses are so much higher for a solid core. It seems to me that the eddy currents are set up to oppose the flux that is forced through the core's cross section. But isn't the magnitude the same in any case? In other words, don't the eddy currents need to generate the same amount of opposing flux?Yes you make a good point about the inrush current blowing the fuse instantly. I have not considered this as of yet. Do you know how transformer designs protect against this?---Also I should probably mention that I am in no rush to jump into connecting to the mains but that is something I would like to try at some point (after much design and analysis of the physics involved).

author: gregariz
author_rank: Frequent Contributor
timestamp: July 06, 2011, 02:18:30 am
content: By laminating the core you have significantly halted the flow of eddy currents an hence the source of heating/losses.

author: mishimaBeef
author_rank: Contributor
timestamp: July 06, 2011, 02:24:36 am
content: Okay thanks everyone for all the replies I really am enjoying this community so far!I guess my ambition seems a little far fetched and possibly misplaced and I realize that it is better to be safe than sorry. Perhaps I will commit my design to paper and it will stay there. I guess there are more interesting and safer things in circuit design than to have to worry about the electromagnetics of transformer design at such a dangerous electricity level. But thanks for the discussion, you have all given me something to think about.By the way, how much power can be drawn from a function generator typically?

author: saturation
author_rank: Super Contributor
timestamp: July 06, 2011, 10:03:07 am
content: Much less than < 1W. Most are current limited to 40-60mA in 50 ohms, and maximum tRMS voltage of about 4VAC, at 60 Hz sine, that's ~ < 0.24W.Quote from: mishimaBeef on July 06, 2011, 02:24:36 am

author: Zero999
author_rank: Super Contributor
timestamp: July 06, 2011, 04:49:06 pm
content: Quote from: mishimaBeef on July 06, 2011, 12:48:05 amWhat if you don't use adequate insulation between the primary and secondary? Someone could be electrocuted when they touch something connected to the secondary?The 15A breaker offers no protection against the transformer catching fire. If the DC resistance of the primary coil is 20 Ohms and you didn't use a laminated iron core, effectively creating a short circuit turn on the secondary, assuming the mains is 230V, the current would only be 230/20 = 12.5A but dissipation in the transformer will be 230*12.5 = 2875W which will cause it to glow red hot and catch fire.A decent quality transformer bought from an electronics shop will have a laminated core, the correct number of turns on the primary, sufficient insulation between the primary and secondary and a thermal fuse or bimetal strip for protection against fire.

author: ipman
author_rank: Regular Contributor
timestamp: July 07, 2011, 11:03:23 am
content: Building a resistor is as simple as measuring with tester probes along a copper wire, stopping at desired value and cutting the wire. And that's a resistor! For bigger values, wind the wire on something which does not conduct electricity, but beware: this will increase the inductance.A capacitor is not much complicated to produce, but not much.Buutt ... you will be crazy if you connected those at mains voltage! That's why everybody is telling you to get an industrial one.

author: Zero999
author_rank: Super Contributor
timestamp: July 07, 2011, 05:24:13 pm
content: Quote from: ipman on July 07, 2011, 11:03:23 amWho said anything about building a resistor?He wants to build a transformer, in which case the inductance needs to be at a maximum and the resistance minimum. I was talking about under short circuit conditions, the resistance being too high to allow enough current to trip the breaker but it will still be high enough to cause a fire.

author: ipman
author_rank: Regular Contributor
timestamp: July 07, 2011, 08:18:11 pm
content: Quote from: mishimaBeef on July 06, 2011, 12:48:05 amLook at the quote above.

author: Frangible
author_rank: Regular Contributor
timestamp: July 07, 2011, 10:41:50 pm
content: You might want to get a microwave oven transformer (colloquially known as a MOT) and remove the high voltage secondary. As stated earlier, these transformers are usually the type with the primary and secondary wound on separate bobbins. This way there is very little chance that you will mess up the primary.You can hack out the secondary using a hacksaw or your favorite rotary tool. At that point you can wind the secondary with different kinds of wire and experiment with it. Winding without removing the laminations (which are usually welded together) will be a pain, but getting a MOT should be cheap if not free.

author: gregariz
author_rank: Frequent Contributor
timestamp: July 07, 2011, 11:28:16 pm
content: Quote from: mishimaBeef on July 06, 2011, 01:52:25 amhttp://www.jaycar.com.au/productView.asp?ID=BM2440&keywords=coils&form=KEYWORD

author: Razrburn
author_rank: Newbie
timestamp: October 24, 2018, 01:55:50 am
content: Sorry to do some grave digging here, but this topic came up in my Google searches, and it seemed to be far from completely answered. I choose to revive it because it's easy to find on Google already and might save some peoples timeI have a vintage outboard that is missing it's lighting coilIt's no longer I production and I've exhausted searching for nos and used all over the web. Short of looking at outboards on every sunken wreck I'm out of options. Outboard sat unused for decades, so I basically have a new vintage outboard, I'm not going to buy a new one 2k USD+, it runs like new. Which leads me to making a lighting coil. I came to this hoping to get information on making a core. I'm not working with anything crazy I'm aiming for no higher than 16vac at 6amps so I can rectify it to DC and filter out any spikes. I might just go with a car alternators regulator for simplicityI know the best material is laminated electrical steel with the grain going the same way. The part I need is a bit like a curved bar. I know I'd have a difficult time finding the right size laminations to salvage from.I don't seem to have any options then making my own coil, and laminations cause I doubt a company will make me about ten laminations to my specs.I'm hoping someone has had a similar delema and found something that worked well for them. If I had more room under the engine cover I'd probably make a generator off the flyweel, but even if I did I'd loose efficiency to additional bearings spinning, and a little more effort to start.I'll close this off by saying I have a decent amount of electrical experience, I've built circuit boards inside of ospreys and f35 joint strike fighters. I also am not short on tools, I have two oscilloscopes. My favorite being a NASA surplus unit used for the start of the apollo program development. I could waste my time trying to make a homemade core. I'm hoping someone might save me the grief and give me some hints on what might work decent as a substitute, or potential sources, that dont want a kidney.