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Ethernet transformers, example schematics, and PICTURES !

Posted by AbeCollins (A) on August 1, 2016 at 08:26:41

In Reply to: Yes. I am looking especially for galvanic isolation posted by beppe61 on August 1, 2016 at 03:12:28:

fmak hates it when I post pictures!

The original intent:
Ethernet can support long runs such that equipment on different ends can be powered from distant branches of the AC power distribution system within a building or across different buildings.

This means there can be significant ground offset between Ethernet devices. The Ethernet coupling transformers mitigate these ground offsets and provides galvanic isolation.

The transformers also provide for common mode noise rejection as well as relatively high voltage isolation between the primary and secondary sides of the transformer.

Simplified Schematic Example

The small transformers can be separate devices on the PCB or built into the RJ45 Ethernet jack itself.

If your low-cost USB / Ethernet extender device does not use some sort of transformer (magnetic coupling) or optical coupling you will not achieve the benefit of galvanic isolation. One way to tell w/o breaking into the device is to use a DMM to measure for DC continuity from the input to output end of the device. Transformers are meant to pass AC. You do NOT want DC continuity between each end.

Example Ethernet transformers

Example RJ45 jacks with built-in transformers

Edits: 08/01/16

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