Help to understan Toroid/Binocular core transformers

Discussion in 'Homebrew and Kit Projects' started by K2LRV, Dec 27, 2011.

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  1. K2LRV

    K2LRV Ham Member QRZ Page

    I am trying to understand toroid transformers. I know what transformers do, but I can't seem tro grasp how to say for example do do X I need to have Z bifiliar turns on a Y core.

    Like I am lookng at a schematic, and it has a transformer that has 2 bilifilier turns of #30 wire on a BN-43-2402 core. What is the mothod behind designing this, or is there some sore of calculator out there that will give you this information??

    I am looking to do some home brewing, but I need to understand this first. Maybe someone can point me to a good reference book.
  2. W8JI

    W8JI Ham Member QRZ Page

    With a core, almost all of the change requires going through a hole. What happens outside the hole, with regard to core presence, is very minor because most flux is in the air and not in the core.

    A binocular core is almost like a regular E-I lamination core. If the conductor runs through the hole it is, in effect, a half-turn.

    Say I have a 73 material that is about 100 ohms per inch of hole depth on 2 MHz. Once through one hole of a binocular core that depth is 100 ohms. Back through the other hole adds another 100. Around both twice is 4x (turns squared) or 400 ohms.

    It is no different than a pass through a core twice as long, except now the core is folded over on itself.

    The only real advantage is you have less wire outside the core. That can help increase bandwidth and reduce losses, because most conductor length is inside core holes. You are not wasting wire hanging outside where the magnetic properties are not using the core's positive effects.

    Does that help?
  3. K2LRV

    K2LRV Ham Member QRZ Page

    Sort of.

    I guess I need to just keep reading this transformer stuff until it sinks in. I understand the whole impedance transformation, i.e. 4:1 etc). From your post I gather that if your using 73 material, and the primary needs to be say 50 ohms, and the core is a binocular type with a length of 1/4", so each length of the hole will give 25ohms. So in this case 1 turn will give 50 ohms.
  4. W8JI

    W8JI Ham Member QRZ Page

    Sort of.

    The general rule of thumb I observed is a core material has roughly about the same characteristics for a given hole depth, or thickness of core the wire parallels as the wire runs through the hole. What happens outside the hole doesn't matter much at all, except it adds extra wire and flux leakage off into space around the wire.

    It is the stuff inside the closed area of the "hole" that matters. I hope I am not confusing you.

    By the way, there is a lot of flux leakage in lower permeability toroidal cores. They are not as self-shielding as we might think. Squeezing or spreading turns can make a big difference in inductance of a -2 mix core, for example.
  5. AB9LZ

    AB9LZ Ham Member QRZ Page

    Remember, the quoted impedances were at a specific frequency, since these are usually used in broadband applications, you'll need to do a little more figuring, typically folks use a 4x the output impedance at the lowest frequency in the desired range (seems to work for me anyway) i.e. if the "load" is 50 ohms, calc the secondary for say 200 ohms at 3.5 mhz (or whatever your lowest desired frequency will be), and then find a core that provides a reasonable number of turns for both the primary and the secondary.

    These rules of thumbs typically are dangerous, but this usually provides a good starting point.

    73 m/4
  6. KA2QFX

    KA2QFX Ham Member QRZ Page

    I highly recommend this book if you want to get into winding broadband transformers. Transmission Line Transformers by Jerry Sevick W2FMI Third Edition, © 1996, Noble Publishing.
    (ISBN: 1-884932-66-5)
    There may be later editions but this is the one I have. Written for the amateur.
    I also have some simple descriptions of these types of transformers on my web site which focuses on Balun/Un-un applications, but the theory is the same.

    Most of the time, when you see bifilar or coaxial windings on a transformer your looking at a Transmission Line Transformer. They are marvelous devices but work quite a bit differently than conventional magnetic core coupled winding transformers. I use them extensively with great results, but they must be tailored for the job.

    Amidon, and CWS Bytemark as well as other resources on the web should provide you with lots of informative reading.
    Last edited: Mar 7, 2012
  7. KD0CAC

    KD0CAC Ham Member QRZ Page

    QFX ,
    Clicked on the link 3 times and get a n error message / not found each time ?
  8. KA2QFX

    KA2QFX Ham Member QRZ Page

    Fixed link error

    It works now.
    Thank you.
  9. KD0CAC

    KD0CAC Ham Member QRZ Page

    Thanks , looks good .
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