Rather than using an Ethernet magnetics component, is it possible / reasonable to simply use individual isolating transformers?

Ethernet Magnetics inside

Inside the magnetics component, it's clearly made up of individual transformers. Is there some reason that people always seem to get these as a single package, rather than using them discretely?

What I'd like to do is to get rid of the large plastic casing, and try to save a little precious space on my stupidly tight PCB.

Tiny EtherCAT PCB


After a lot of searching, I think I have found a suitable transformer: Wideband Transformer with 2 Windings and Tap.

  • Turns ratio 1:1:1
  • 3dB Band Limits (MHz) : 4.5 - 1000
  • Loss at 20 MHz (dB) max : 0.7

SMT Transformer SMT Transformer 2 Frequency response


Just discovered that you can buy those from Mouser, and they are very expensive!

up vote 4 down vote accepted

You don't need to use a integrated transformer package, but these are convenient and small for most designs. They are also at the right impedance and are usually 1:1 but different ratios can be found for different voltage systems on the board side.

These transformers often also have baluns integrated on the network side of the two differential pairs. Those aren't necessary according to the ethernet standard, but you may have EMI problems without them.

AT the minimum, you have to provide the two pulse transformers with the right frequency and impedance characteristics somehow.

  • Thanks for that. Actually, after a lot of searching, I think I may have found a candidate transformer. Do you think this would be suitable. Please see my updated question. – Rocketmagnet Apr 19 '12 at 16:40
  • @Rocket: It looks reasonable at first glance, but I don't know the ethernet specification at that level. I have so far always used transformers specifically designed for ethernet use so never had to get into this detail. Like I said, it looks reasonable, but such things sometimes have unobvious gotchas. I don't know enough to answer the question definatively. – Olin Lathrop Apr 19 '12 at 19:49
  • Thanks, that's very helpful. I think what I'll do is make a little test board to sneak into the next batch of PCBs we make, so I can see if it works. – Rocketmagnet Apr 19 '12 at 20:13

You could use a magjack. An ethernet socket with the magnetics included.

  • I would normally use a MagJack, but these don't seem to be available with the following spec: SMT, Vertical, less than 15.5mm wide. – Rocketmagnet Apr 19 '12 at 15:55

Your Answer


By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.