PoE (Power over Ethernet) / What's on Pin #3 of the Magjack?

[Kp]

Hi,

I'm looking to see if it's possible to use/wire a nanode for Power
over Ethernet...

Looking at the schematic there's certainly some spare pairs on the
Magjack that could be used - but I was wondering what the 'TXC'
connection is used for (appears to be wired to Pin #3 of the Magjack)
- as it might be conflicting with one of the wires usually used for
PoE.

The others are pretty obvious (TPIN/TPOUT + and -) - but I cant' work
out what TXC is (yup, I obviously know very little about ethernet at
the physical level :)

Thanks,

-Karl

[Ken Boak]

Karl,

 

TxC is the centre tap of the isolation transformer which TX+ and TX- connect to either end.

 

There is very little opportunity for PoE using this magjack as the internal connections are just not brought out to pins

 

See the magjack data sheet here http://www.coolcomponents.co.uk/catalog/resources/19154.pdf

 

You would have to get inside the jack and get access to the 75 ohm resistors currently connected to J8,J7, J5 and J4

 

If you put 12V across j8, j5 the internal resistors (add up to 150 ohm) would draw 80 milliamps and dissipate half a watt each - this would probably be enough to fry them badly.

 

You could drop to say a 7V supply - but again you would be dissipating 0.3W

 

PoE Magjacks don't come cheap - and this would add another £5 to the cost of the board - a cost we felt that for most peoples applications should be avoided.

 

 

Ken

[Kp]

On Jul 15, 9:40 am, Ken Boak <ken.b...@gmail.com> wrote:

> Karl,
>
> TxC is the centre tap of the isolation transformer which TX+ and TX- connect
> to either end.
>
> There is very little opportunity for PoE using this magjack as the internal
> connections are just not brought out to pins

Thanks for the info - I'm too tired to think straight this morning
(not enough coffee yet either!) - I forgot the Magjack is more than
just a connector - it's got active components in it :)

I've also just thought of Plan 'B' (which I should have done before) -
if I wire my own plug onto the Cat5 cable coming in - I can 'break
out' the PoE wires before they get anywhere near the Magjack, and just
hook them up to Vin / Gnd. The whole things going in an enclosure
anyway - so you'll just see the cable disappearing into the box, via a
grommet.

As it's only 10BaseT I could also just make a short 'dongle' - i.e.
Cat5 plug (to connect to the Magjack) to inline Cat5 socket, which
handles the breakout - I can't see 10BaseT getting too upset by the
extra joins :) Again, should have thought of that before - doh!


Definitely time for more coffee in the morning...

-Karl

[S. Ford]

> As it's only 10BaseT I could also just make a short 'dongle' - i.e.
> Cat5 plug (to connect to the Magjack) to inline Cat5 socket, which
> handles the breakout - I can't see 10BaseT getting too upset by the
> extra joins :) Again, should have thought of that before - doh!
>
>
> Definitely time for more coffee in the morning...
>
> -Karl

Hello Karl,

I too am interested in PoE for the nanode. I had tinkered with a similar setup as your plan B, however have made little progress as my electrical knowledge is limited and I was too scared of frying things.

If you do end up with working Poe injectors and ejectors please could you share the design. I am sure others would appreciate it too.

-Steve

[Gadget]

For the amount of effort can I recommend a POE injector / splitter from linitx.

http://go.je/linitxpoe

No connection, just a very happy customer of Linitx.

Cheers

Stuart

[Andrew Elwell]

> For the amount of effort can I recommend a POE injector / splitter from
> linitx.
>
> http://go.je/linitxpoe

just be aware that for that one you'd need to feed power in for the nanode

I tend to use 802.3af ones as thats what my voip phone uses -- I have
a tp-link 8 port switch on desk (which providees 4 PoE ports, 4
normal) and use their splitters to power non-poe compliant 5 / 12v
stuff

http://www.tp-link.com/products/productlist.asp?class=poe

seemed to be the cheapest I could rind

[Karl Pielorz]

Yeah, that looks like the ticket - thanks! - I had looked into PoE
adapters/injectors, but they all seemed very large, and very expensive
(though they almost certainly had more gubbins in - but those look
like they'll do nicely for what I'm doing :)

-Karl