Making transformers, order of winding? - Page 1
Forum section: electronics / projects

author: sean87
author_rank: Regular Contributor
timestamp: April 12, 2013, 09:54:21 am
content: HI all,I have to make some transformers for my project.I have calculated that the primary should have 77 turns and secondary should be 40 turns.I bought this stuff (in the picture). My question is, is it important the order of wiring? I mean should I wind first the PRIMARY, then insulate, then wind Secondary? does it matter?Also these stuff have 2 leads on one side and 3 leads on other side. in my PCB there is 2 whole and 3 whole as well...is it an indication for primary or secondary side?Please let me know if my question isnt clear!

author: nctnico
author_rank: Super Contributor
timestamp: April 12, 2013, 10:20:42 am
content: The order does matter (IIRC for the leakage inductance) so sometimes the windings are interleaved. What kind of frequencies / currents do you expect these transformers to handle?

author: CarlG
author_rank: Regular Contributor
timestamp: April 12, 2013, 09:40:29 pm
content: Yep, it does matter. If you want to do it simple, start with the primary. If you want to be more advanced, you can compare difference of interleaved wiring (half primary first, then the secondary, last the second half of the primary). The latter case will probably lower the leakage inductance, but raise the P-S capacitance. It's a trade-off, depending on the application. Wind some different ways and find outAlso experiment with different number of wires while keeping the Cu-area constant (for SMPS the skin effect may be important, but not for line transformers).

author: codeboy2k
author_rank: Super Contributor
timestamp: April 12, 2013, 11:49:18 pm
content: No one has answered your question about the 2-pins vs 3-pins side. There is no real standard, but 99% of the time the 2-pin side is the primary and the 3-pin side is the secondary, because you often have a secondary with a center tap. There is no rule, and it's all dependent on the circuit design, as you can certainly have a tapped primary, and a non-tapped secondary. So you will have to consider your circuit design, to determine whether your primary is 2-pins or 3-pins.As for the windings, others have given you ideas how to reduce leakage inductance. Here's another: if the voltages are low, you can also wind bi-filar, which means winding both primary and secondary together in one pass, side-by-side. This will give the best magnetic coupling (which implies the lowest leakage inductance) but the highest inter-winding capacitance.Note that any attempts to lower leakage inductance will result in higher inter-winding capacitance, since the common ways to reduce leakage inductance is to wind P-S together, or interleaved, or folded over, or spread out over a longer core (which means fewer layers and thus the P-S windings are closer together again).So it's a tradeoff between how much leakage inductance you can tolerate vs. how much capacitance you can tolerate.Leakage inductance will reduce the performance of your transformer and cause a voltage drop under load.Inter-winding capacitance will couple noise across the windings.TI has some good resources:Magnetics Design for Switching Power Supplies Lloyd H. Dixon Section 1Magnetics Design 2 - Magnetic Core CharacteristicsMagnetics Design 3 - WindingsMagnetics Design 4 - Power Transformer DesignMagnetics Design 5 - Inductor and Flyback Transformer DesignMagnetics Design RDS - Magnetic Core Properties (Rev. B)The following are not part of the same series, and not all from TI:Magnetic Amplifier Control for Simple, Low-Cost, Secondary RegulationTransformer and Inductor Design for Optimum Circuit PerformanceMagnetics Design - Specification, Performance and Economics

author: sean87
author_rank: Regular Contributor
timestamp: April 13, 2013, 10:50:05 am
content: Thanks for answers guys. The frequency of my application is around 20KHz, so I guess its a low frequency thing.Ok I already winded the primary side on the 2 lead side, Now I am going to wind the secondary on the 3rd lead side.I winded the primary clockwise, Should I wire the secondary also clockwise or counterclockwise? does this matter?Also should I use a round of insulation tape between P and S ? I'v seen some transformers have this single turn of black tape bethween P-S...considering the wires are coated themselves...is it necessary ?Thanks.

author: Neilm
author_rank: Super Contributor
timestamp: April 13, 2013, 07:51:57 pm
content: It might matter, it depends on the layout of the PCB.Enameled wire is usually good for one hundred volts, I haven't seen any better than five hundred volts. You haven't said what the application is for. If it is low volts then you probably don't need any insulation - if you are winding a transformer for a mains application then you most definitely do.The questions asked show you are not used so winding transformers, so if this is a mains application, I would recommend you don't go any further. Your life will be dependent on who well you wind the transformer as it will be the barrier between the low voltage side and the mains. Any damage to the wire, would leave the potential for the barrier to be broken and the low voltage side be connected to the mains.Neil

author: sean87
author_rank: Regular Contributor
timestamp: April 13, 2013, 10:53:25 pm
content: Haha thanks for pointing out the mains thing, luckly this is not for mains power...I am trying to make some sort of radio, and the voltage level is not that high. maybe 20 30 volts

author: peter.mitchell
author_rank: Super Contributor
timestamp: April 13, 2013, 11:01:32 pm
content: Oh, nice, no death then, just burns. IM JOKING

author: nctnico
author_rank: Super Contributor
timestamp: April 14, 2013, 02:05:49 am
content: Quote from: Neilm on April 13, 2013, 07:51:57 pmThis is exactly why they have bobbins with two (or more seperate sections). Isolation is a piece of cake with those.