[c-nsp] Galvanic isolation for Ethernet?

[Michael Loftis]

On Wed, Aug 20, 2014 at 1:10 AM, Victor Sudakov <vas at mpeks.tomsk.su> wrote:
> Michael Loftis wrote:
>>
>> Many RJ45 ports have integrated magnetics now (transformer) - external
>> options are made by pulse, bel, halo, and many others.  As for
>> external magnetics they're small - they don't need to be very large
>> because they're not designed to be carrying current.
>> http://www.pulseelectronics.com/products/lan has a dozen or more
>> examples.
>
> At first glance, these look exactly what I was asking for in the first
> post, a transformer, non-powered, right?

This is what is already inside every ethernet device.  It's part of
the specification.  It's part of how they work.  And part of why
you're having trouble finding an external device that isn't a media
converter, hub, or switch - I doubt that even 10mbit ethernet would
withstand an extra round of transformers in the path.  The 100M signal
is only +/- 1V.

>
> There are so many models however, it's not clear if there are such
> ones could be placed between a regular UTP/FTP cable and regular
> switchport.

Those aren't complete products, those are components.  They're for
mounting to a PCB as part of a product design.

>
>>
>> >
>> >> If you've enough common mode
>> >> voltage to fault those out you may still be well short of "surge"
>> >> voltages that are protected by most equipment.  You might improve the
>> >> issue by re-terminating each end only leaving the "spare" copper pairs
>> >> connected at one end of the RJ-45 (the switch end
>> >> generally)...
>> >
>> > Sounds like magic but could you elaborate? I did not quite catch the
>> > picture.
>>
>> You remove the extra (non 100mbit/10mbit) pins from the far end, don't
>> terminate them through, this sometimes is the path your stray ground
>> currents take.
>
> What to you mean by terminating/not terminating them?

If you're not using PoE 10M/100M ethernet only uses four wires, two
pairs.  You do not connect them to your field equipment (some
equipment will connect them to ground - which they shouldn't be doing,
by not connecting the extra wire pairs you avoid that...)  however
anythign using PoE shouldn't be causing a ground loop in the first
place either.  And like I mentioned, you could still be getting
inductive non ground related current instead by having cable runs by
high power and/or high voltage.

>
> And thank you a lot for your input.
>