Balun Tranformers

Discussion in 'General Electronics Chat' started by Yvanohe, Mar 4, 2011.

  1. Yvanohe

    Thread Starter New Member

    Feb 14, 2011
    6
    0
    Hello,
    I'd like to know why Balun tranformers have an input and an output. A transformer is, in theory, symetrical.


    See p7 and p8 of enclosed .pdf.
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Balun's generally match impedances. Transformers generally are not symetrical, in the case of a power transformer one side is the primary, the other is the secondary. In the case of Balun's they usually have impedance mentioned.

    Case in point, antennas. The 300Ω:75Ω is a classic use of a Balun.
     
  3. Yvanohe

    Thread Starter New Member

    Feb 14, 2011
    6
    0
    I agree for the typical application of impedance matching but for my application (Ethernet interface) the impedance explanation does not seem to be the correct one. If you check the pdf I attached, the Balun is switched following the direction of the signal: Tx or Rx. I can understand why...
     
  4. Teri

    Member

    Apr 3, 2009
    12
    0
    The device labeld as T2 in Figure 5 is not a balun/transformer -- it is a common-mode choke. Fig 5 is in error. T2 should be as shown in Fig 6.

    Baluns are bi-directional in fact, but if they are used in a uni-directional circuit as shown, the terminals may be named "input" and "output" by convention.

    In eithernet systems that use cat-5 cable, the Tx and Rx are transmitted over two different uni-directional twisted pairs, so they must be processed by two different uni-directional systems, each with its own isolation transformer / balun.
     
  5. tgotwalt1158

    Member

    Feb 28, 2011
    111
    18
    I agree that transformer is symmetrical i.e. input to output ratio is equal but as soon as it is put to some use, it acquires input and output as per of requirement or objective of the situation. Baluns are no exception, they are signal level transformers.
     
  6. Commander#1

    New Member

    Jan 22, 2011
    14
    0
    I agree - the labels in fig.'s 5 & 6 are a little confusing, but, T2 in fig. 5 is a classic example of a choke. For a good
    explanation of a BALUN - go to www.wikipedia/balun. Balun's are built for high frequency RF work.
    Hope this helps a little.
    .
    :) Phil Potter :)
     
  7. Wendy

    Moderator

    Mar 24, 2008
    20,764
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    Speaking for myself, I was not looking at the choke, there is a transformer in every schematic. Ethernet starts at 10Mb/sec, and goes up (way up there). The OP did specify pages 7 and 8, and there is a transformer on every one.

    The above link is not valid, try this one...

    http://en.wikipedia.org/wiki/Balun
     
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