For variuous reasons I need to add an extra transformer between external ethernet connection and an ethernet device that has magnetics built in. The reason being, among others, to elimnate possible high (up to 1500 V) common mode voltages and provide ESD protection. Such high voltages are unacceptable for my application.

The question here is what would be the downsides of this solution?

  • EMC/EMI problems
  • Termination problems
  • IEEE 802.3 standard violations?

I did try to find some answers for the question in the standard. My conclusion is that as long as the attenuation of additional transformer is small enough, and 2 V differential voltage is quaranteed it should not be a problem?

The other aspect is that common layout reccommendation is that distance between PHY and magnetics should be kept around 5 cm (2 in). I will noe have 0.5 m, is it because of Bob smith termination?

Would it make sense to omit the Bob Smith termination of my transformer (on the right), because the device already has one?

enter image description here

  • 4
    \$\begingroup\$ Magnetics inside ethernet products are usually rated at above this voltage anyway so, why add the extra transformer? Hint --> why do you think ethernet products use transformers. \$\endgroup\$
    – Andy aka
    Commented Oct 23, 2022 at 16:08
  • 3
    \$\begingroup\$ ”Such high voltages are unacceptable for my application.” If your product can’t tolerate it, it won’t be Ethernet compliant. \$\endgroup\$
    – winny
    Commented Oct 23, 2022 at 16:34
  • \$\begingroup\$ It is the electronics located in close proximity of my 0.5m cable that cant handle 1.5kv. Yes the ethernet device itself would be fine. I just want to keep all CM voltages and spikes away from the inerior of my device (its a complicated problem, discussing it would be unpractical here) \$\endgroup\$
    – tarmogr
    Commented Oct 23, 2022 at 17:30

2 Answers 2


It is possible to add extra magnetics because that is how external PoE injectors and extractors work.

But Ethernet is already a tranformer coupled isolated interface which by specifcation must withstand at least 1500V.

Sounds like you don't need to add anything.

  • \$\begingroup\$ Ofcourse, PoE injectors and extractors. The working principle is exactly the same and theres even a lot of reference designs ou there i.e: ti.com/lit/ug/slvub75a/slvub75a.pdf \$\endgroup\$
    – tarmogr
    Commented Oct 23, 2022 at 18:02

You are likely to get a small amount of signal degradation and violate the spec if you add two transformers (each transformer has a small amount of loss in the bandpass region). It's not likely that you would get more protection with two transformers as high voltage spikes would be attenuated.

If you need a higher insulation rating then get a better transformer (most start at 1.5kV and some go up to 5 or 6kV).

TVS diodes can be used to short out large ESD spikes, probably best to see if the PHY you are using has a recommended circuit or app note on the topic

enter image description here

Source: Connecting TVS before or behind Ethernet transformer?

  • \$\begingroup\$ As you can see from the image attached, a common mode TVS can only be installed after the magnetics (left one) in order to comply with 802.3 . This is exactly the reason I need to place an external transformer. To remove spikes near the external connector of my device. What kind of other specs would be violated in your opinion? \$\endgroup\$
    – tarmogr
    Commented Oct 23, 2022 at 17:37

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