Basics on winding your own transformers - Page 1
Forum section: electronics / beginners

author: Dan123456
author_rank: Regular Contributor
timestamp: October 02, 2023, 09:52:58 am
content: Hey allHopefully a quick one.I am looking to make a simple single phase step up transformer that turns 12VAC, 0.3A to 90VAC (I don’t really care about output amps too much but my understanding is this would be 0.04A) to power a neon bulb.Why?… well simply just to play with them cos neon look cool!I was thinking of doing this with 1.25mm copper wire on the primary and either 0.8mm or 0.3mm on the secondary with the primary and secondary wrapped around opposite sides of a 35mm ferrite “donut”. I know this wire gauges are way thicker than what I theoretically need but that is simply just to make it easy for me to build.I have 2 questions though,1. Does the wire gauge make a difference to anything other than the just max current it can take? I was thinking that 50 turns of 0.8mm wire would have a lot more surface area than 50 turns of 0.3mm so surely that must effect something yeah?2. Following on from that logic, is there a rule of thumb / way to work out how many turns of the primary you should do? I understand I would need a turn ratio of 7.5 in this situation but can I just do any old number of turns on the primary then 7.5 times that on the secondary? Again, 1 turn would have less surface area compared to 10 etc. so I assume this would effect something right?Thanks so much in advance

author: srb1954
author_rank: Super Contributor
timestamp: October 02, 2023, 12:03:54 pm
content: Quote from: Dan123456 on October 02, 2023, 09:52:58 amYou need to have sufficient turns in each transformer winding to ensure that the peak flux density doesn't get to high. Having too few turns will saturate the core material, lowering the magnetising inductance of the transformer and causing excessive current to be drawn from your power source.The governing equation is:N = Vrms/(4.44 x f x B x A)where Vrmsis the winding voltage, f is the supply frequency, A is the effective cross-sectional area of the core and B is the maximum allowable flux density, typically 1.2 Teslas for laminated iron cores and 0.3T for ferrite cores.

author: MrAl
author_rank: Super Contributor
timestamp: October 02, 2023, 12:36:17 pm
content: Quote from: Dan123456 on October 02, 2023, 09:52:58 amHello there,Well, there are a number of things to think about.The first has already been mentioned in the previous post. It's sometime called the "transformer equation" but there are a number of these equations that's just one of them. It really relates the primary inductance to the turns, frequency, applied voltage, core area, so that the primary current does not go too high can cause the metal core to become ineffective.The second would be the type of core material. For line operated transformers the usual choice is silicon steel laminations, often in the form of an "E" and an "I".You can find this kind of construction in old microwave oven transformers. In fact, many people have cut off the secondary and provided their own secondary, although i also recommend adding some primary turns too because the mic oven transformers are not made as well as a 24/7 run regular line transformer.It's a bit hard to get the scondary off, but you can do it.The third is insulation. You have to have good insulation between the primary and secondary or else you subject the end user (yourself) to a dangerous shock risk. In fact, higher class transformers do not put the secondary right on top of the primary, they put the windings side by side with a spacer between. That helps to eliminate the crushing problem where the transformer is accidentally crushed and the secondary gets pushed into the primary and thus the secondary ends up having the line potential on it as well as the primary. You want to keep the secondary isolated as well as possible.The fourth is wire sizes. The current is inversely proportional to the voltage, so you can size the wire for the secondary based on the wire size of the primary. The key here is not the surface area of the wire as much as the cross sectional area. Basically if the input voltage was 10v and the output was 100v and the input current was 1 amp, then the output current would be 0.1 amp. This 10 fold ratio means you can use wire with one-tenth the area of the primary for the secondary.There is also the loading factor that kicks in here too though. You have to consider how much the secondary voltage will sag under full load. That means you may have to go slightly higher for the secondary voltage and use a slightly larger area for this wire. For example, instead of going to one-tenth the area for the secondary, maybe just one-eighth or something like that. You figure how the loading based on the total resistance of the primary and total resistance of the secondary, along with the turns ratio. The primary resistance reflects to the secondary as the square of the turns ratio. That means once you reflect this resistance to the secondary, you add that to the secondary resistance to get the full loading series resistance on the secondary.There are a few other things to consider but these are the most important for line operated transformers.For example, you have to include some sort of fusing, probably on the primary and on the secondary too for the most safe way to do it.

author: Zero999
author_rank: Super Contributor
timestamp: October 02, 2023, 12:49:48 pm
content: Don't bother making your own. Use a mains transformer, in reverse.You need a turns ratio of 1:7.5A mains transformer with a 120V primary and 15V secondary, run in reverse should give just over 90V out, but in reality it'll be slightly less, as the 15V winding will have a slightly highe voltage, to compensate for loading.Note, your neon lamp will need a resistor to limit the current, as it has a sharply negative voltage/current curve.

author: Dan123456
author_rank: Regular Contributor
timestamp: October 02, 2023, 02:30:03 pm
content: Ah! Thank you so much!This is exactly what I was looking forJust wondering, what units should I be using?I tried googleing the formula and it seems like people are using all different units with different variations on the formula (such as Hz vs MHz and cm2 vs m2 vs inches2 or gauss rather than T).Firstly, I worked out the effective cross-sectional area of my rings =“35x21x13” = 7mm walls X 13mm height = 91mm2.The answer to the equation that looked the most “right” to me was N = 12VAC /(4.44 x 60Hz x 0.3T x 0.091m2 = 1.65 turns (this also used the units from a picture of a textbook I found - below).Tried a few other units and got number ranging from virtually 0 to ~1700 turns and, I have no reference for what “right” looks like, I figured I’d just askP.s. actually… is the 12VAC right or should that be the highest voltage out of the two (I.e. 90V or 90Vrms - I didn’t bother with the rms for the calcs at 12v as was negligible but it would make more of a difference at 90V)? I assumed it meant primary coil input voltage but just thought of that and, if that is the case, that gives me roughly 12 turns which sounds much more reasonable!Thanks so much again!

author: pqass
author_rank: Super Contributor
timestamp: October 02, 2023, 03:14:39 pm
content: Quote from: Dan123456 on October 02, 2023, 09:52:58 amIf the goal is just to light "cool" neon bulbs, then you can do that with a simple boost converter. Like this 555-basednixie supply(use Vin=12V). You just need to adjust R38 to about 120K (from 220K) to limit the output to around 110V. A current limiting resistor of about 20K (in series with the neon bulb) should keep the current to about 1mA.

author: Kim Christensen
author_rank: Super Contributor
timestamp: October 02, 2023, 03:52:29 pm
content: Quote from: Dan123456 on October 02, 2023, 02:30:03 pmWhat's the source of the "12VAC"? Because if you're making an inverter, and running it off of 12VDC, then you'll actually end up with around 8.5VAC on the primary.

author: Dan123456
author_rank: Regular Contributor
timestamp: October 02, 2023, 07:49:38 pm
content: Quote from: Kim Christensen on October 02, 2023, 03:52:29 pmOh right. Why would it be 8.5V from an inverter?I see how you got that number by using the peak voltage (pv) but I assumed I should use the average voltage which gives me a Vrms of 13.3V (which is why I didn’t think it was worth taking into consideration in my initial calculations)Edit: sorry, I misread your post! You said if I was running it off 12VDC I would get 8.5VAC! Na, I’ve got a 12VAC power supply I intend to run it off (not DC)That said, is using the average voltage correct for AC and it would be peak for DC (I’m still new to all this so chances of me being wrong are very high)?Also, I still just want to clarify if the voltage used in the transformer equation should be the input voltage or the “boosted” voltage (i.e. 8.5V/12V/13.3V or 63V/90V/~100V depending on which rms calc is correct)?

author: Terry Bites
author_rank: Super Contributor
timestamp: October 02, 2023, 08:28:21 pm
content: Exactly.A 240V to 28V or 30V or 32V is near enough. No tears.If you need custom there are a ton of comercial winders out there.One off's don't cost the earth. Or the live or the neutral.

author: srb1954
author_rank: Super Contributor
timestamp: October 02, 2023, 09:01:42 pm
content: Quote from: Dan123456 on October 02, 2023, 02:30:03 pmUnless otherwise stated stick with the primary SI units e.g. metres, kilograms, seconds, amps, Tesla, Hz for electrical formulas.QuoteYou have got your area conversion wrong so your answer is out by a factor of 1000. 91mm2 is equal to 0.000091m2 so the correct number of turns is 1650.QuoteYou can use the same formula for both sides of the transformer. The number of turns you calculate for the 90V secondary will just work out as the primary turns times the required turns ratio.

author: Dave
author_rank: Super Contributor
timestamp: October 02, 2023, 09:09:48 pm
content: Quote from: Dan123456 on October 02, 2023, 02:30:03 pm91mm^2 is 0.000091m^2.Did you get the 0.3T from the datasheet of the core, or is it just a guesstimate?

author: Dan123456
author_rank: Regular Contributor
timestamp: October 02, 2023, 09:28:07 pm
content: Quote from: Dave on October 02, 2023, 09:09:48 pmAhhhh pooThanks for picking that up! That is a reminder to me to not do maths at 2amWith that corrected it is giving me the ~1700 turns answer again (12/(4.44 x 60 x 0.3 x 0.000091) = 1650 turns)!0.3 was the “typical” value that srb1954 gave for ferrite. I did check the “data sheet” after and I think it says it is 2500 gauss ( or “20Oe”? whole thing looks pretty sus - below) which works out at 0.25T so close enough for the initial back of the envelope calculationsIf the answer is in fact something close to 1700 turns on the primary, I will definitely just go with a commercial option (no way in hell am I winding 10,000 odd turns on the secondary!) but would still like to understand this stuff regardless

author: EPAIII
author_rank: Super Contributor
timestamp: October 03, 2023, 02:10:00 am
content: From 12 VAC!WHY? Why from 12 VAC?Are you a novice and afraid of 230 VAC (or 115 VAC) as supplied by the power company in your country? And under the assumption that turning that mains Voltage to a safer 12 VAC first will provide protection against the danger that the higher Voltage (230 VAC) presents? So you are only going up to the 90 VAC level because you believe it is safer?I am not saying that you should not do this. Do have fun. But, let me say that 90 VAC can still give you a nasty shock and even be lethal in some cases. 90 VAC is beyond the level that I would call safe. And a beginner is the worst person to work with that Voltage level THINKING it is safe. It is NOT. And you should be just as afraid of it as you would be of 230 or even 115 VAC. And one or two transformers between that 90 VAC and the mains supply you have do NOT make it any safer.You need TWO things. The first one is a real fear of that 90 VAC. In a career spanning over 45 years as a professional and at least another 20 as an amateur or semi-professional, have worked with Voltages up to 40 or 50 KV and still have a real healthy respect for those as low as 100, 90, 75 VAC, and perhaps even lower ones. I don't feel I am on relatively safe grounds with anything over 48 VAC or VDC. And even at 12 or 5 Volts, high currents can produce burns. So, just 90 VAC can be dangerous and even lethal.The second thing is you need to take precautions against a lethal current in that 90 VAC circuit. We have all been shocked by static charges of tens and even hundreds of thousands of Volts just by walking across a carpet and then touching a door knob or other ground in the winter. That was so bad when I was in college I carried a device with a 10 MOhm resistor to discharge myself in certain rooms there before touching door knobs. But those charges had little current available to them so there was no real danger. A certain amount of current needs to pass through the human body before a shock is dangerous. And if you can limit that current, then the danger is also limited or eliminated. My 10 MOhm device would be of no use with a current source that can sustain that current for a longer period of time. That protection must be built into the circuit.So if I understand this properly, you are thinking of hooking one step up (12 VAC to 90 VAC) transformer to another step down (your mains to 12 VAC) transformer. And you probably think that the 12 VAC at the point between those transformers being only 12 VAC will protect you from danger. Nothing could be more WRONG. This is specially true when that second transformer is being designed and constructed by an amateur who must ask about how to construct a transformer. It is not a danger of the mains Voltage jumping the insulation in both transformers: that most probably will not happen. The problem lies in an understanding of the Voltage and current relationships in such a system. Your mains will probably be limited to 5 or 10 Amps or perhaps a bit more. But due to the nature of transformers, that could become 20, 30, even 40 or more Amps when at the "safer" 12 VAC Voltage level. Then you are stepping the Voltage back up to 90 Volts and the available current would be stepped down, but only to, perhaps 2.6, 4, or 5.3 Amps. using my numbers in the previous sentence. Even if the 12 VAC level has a lower current level of perhaps just 2 Amps, then that is only stepped down to 267 mA at the 90 Volt level.The lethal level of current is hard to pin down to one number, but it is often said to be 100 mA. Much smaller currents across the heart will be lethal, perhaps as small as 10 mA or even less. Don't trust me, do your own internet search.My point is, simply that dropping the Voltage to 12 VAC at a previous point in the circuit, by itselfDOES NOT PROVIDE ANY SAFETY.The primary method of providing safety is with the KNOWLEDGE and CAREFUL ACTIONS of the person working with the circuit. And with a careful and knowledgeable construction of the circuit itself.A home made transformer will not provide any limit to the current beyond that introduced by a bad design. Add to that a primary transformer (mains to 12 VAC) that may or may not provide any current limit and you have a dangerous situation. I urge you to either proceed with great care or to find another way to play with the neon light.I will say this, neon bulbs are often sold with a resistor included with the assembly that allows them to be directly connected to the mains Voltage, just like any incandescent, fluorescent, or LED light bulbs are. You simply purchase an appropriate socket and a piece of lamp cord with a plug for your mains sockets. But if you are thinking about neon tube lights as used in outdoor signs, then a proper transformer and probably a current limiting resistor would be needed. And extra care in hooking them up and displaying them.

author: Dan123456
author_rank: Regular Contributor
timestamp: October 03, 2023, 07:25:57 am
content: Quote from: EPAIII on October 03, 2023, 02:10:00 amYou are spot onThe reason I was planning on doing it with my 12VAC power supply is due to it only having a max current rating of 0.3A (so, as I understand it a max of 0.04A at 90V)I very well could be misunderstanding that whole concept and my logic could be flawed as I am still learning daily, but regardless, I am aware that even “just” 0.04A at 90v definitely can still be lethal!I just feel slightly more comfortable dealing with that than 240VAC 10A mains at this pointEdit: just wanted to say, I do really appreciate the concern thoughLearning how to do things safely is ultra important (especially for us newbies)! Thank you

author: jonpaul
author_rank: Super Contributor
timestamp: October 03, 2023, 08:08:56 am
content: Bonjour cher Dan: BRAVO for your initiative!Welcome to the world of magnetics and transformers.Wound my first one in 18956!See the old classics in 1950s..1960 electronics magazines espRADIO-TV EXPERIMENTER Magazine, #576, 1961, page 153 and onEXCELLENT article on mains transformers, by Harold P. STRAND:https://worldradiohistory.com/Archive-Radio-TV-Experimenter/Radio-TV-Experimenter-1961-Spring.pdfOther refs:https://worldradiohistory.com/hd2/IDX-Consumer/Archive-Radio-TV-Experimenter-IDX/IDX/Radio-TV-Experimenter-1961-Spring-OCR-Page-0155.pdfhttps://www.americanradiohistory.com/Archive-Radio-Electronics/80s/1983/Radio-Electronics-1983-05.pdfSee pages 75, onwards.https://worldradiohistory.com/hd2/IDX-Short-Wave/archive-73-idx/idx/60s/73-magazine-02-february-1967-OCR-Page-0090.pdfResearch, read, learn, make !BON CHANCE!Jon

author: Kleinstein
author_rank: Super Contributor
timestamp: October 03, 2023, 09:23:08 am
content: Main frequency transformers usually use iron cores and these can work with a magnetic field up to some 1 (rather old material from the 1960s) to 1.6 T, not just some 0.3 T as with ferrites. It is OK to not really use the core to the full potential if power / weight is not critical.For the ready made transformers keep in mind that the voltage is given under load and this includes some 10-50% drop for small transformers. So a 15 V to 110 V would not be right. More like 12 to 110 or even 10 V to 110 V. The very small transformers in the < 2 VA range may not work well in reverse, as there the losses and magnetizing current get so large that the primary side needed more volume than the secondary. Similar small cores are usually quite a bit away from an ideal transformer. 91 mm² does look pretty small and the magnetizing current can be quite relevant - so not much reserve to use a larger than actually needed wire.

author: MrAl
author_rank: Super Contributor
timestamp: October 03, 2023, 10:07:06 am
content: Quote from: Dan123456 on October 02, 2023, 02:30:03 pmHi,Yes you are right units are very important in order to get the right result and thus the right number of turns.The more common formula is this:B=Vrms*10^8/(4.44*F*A*N)and notice this is somewhat easy to remember because of the "FAN" mnemonic.The units are:B in gausses,Vrms in Volts RMS (120v RMS is for US line voltages in most places),F in Hertz,A in square centimeters,N is just the number of turns so it's just a number.Since we often work in square inches, converting the area to square inches:square centimeters to square inches: square centimeters = square inches*2.54^2This will allow us to change the formula a little with A in square inches:B=Vrms*10^8/(4.44*F*A*6.4516*N)or more simply put:B=Vrms*10^8/(28.65*F*A*N)again with A in square inches.Solving for N we get:N=Vrms*10^8/(28.65*F*A*B)and rounding the constant in the denominator:N=Vrms*10^8/(29*F*A*B)and setting B=10000 we end up with:N=10000*Vrms/(29*A*F)Now combining constants and setting F=60 Hertz we have:N=5.75*Vrms/Awhich can be approximated as:N=6*Vrms/Aor setting F=50 Hertz we have:N=6.9*Vrms/Awhich can be approximated as:N=7*Vrms/AI don't think it gets any simpler than that.Also note that most EI laminations made for line voltage use (like 50 or 60Hz) have a maximum flux density (B) of 15000 Gausses. You shouldn't design for that high though, but rather around 10000 Gausses. That allows for various variations during actual use like high line which can go up higher than 120v RMS. The maximum high line is supposed to be 132v RMS but I've never seen it that high and i wonder in today's world if the grid would ever allow that due to the problems with energy these days.