Balun Tranformers

Thread Starter

Yvanohe

Joined Feb 14, 2011
6
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.
 

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Wendy

Joined Mar 24, 2008
23,415
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.
 

Thread Starter

Yvanohe

Joined Feb 14, 2011
6
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...
 

Teri

Joined Apr 3, 2009
12
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.
 

tgotwalt1158

Joined Feb 28, 2011
110
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.
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.
 

Commander#1

Joined Jan 22, 2011
14
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.
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 :)
 

Wendy

Joined Mar 24, 2008
23,415
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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