[TriLUG] OT: PoE injector/splitter - recommendations?

[Scott G. Hall]

On Mon, May 12, 2008 at 5:27 PM, Joseph Mack NA3T <jmack at wm7d.net> wrote:

> I need to power about 6 WAPs at the ends of fairly long
> ethernet cables, and don't want to use 120V at the ends. I
> assume I'll have a set of injectors where the ethernet
> cables converge on a switch and I'll have a splitter at each
> WAP. There are switches that inject the PoE, but these are
> quite expensive.

Of course being an electronics person, you could always roll your own.
 From Wikipedia:
| Clause 33 of IEEE 802.3-2005 (commonly referred to as IEEE 802.3af)
| is how Power over Ethernet is usually implemented. It provides a
| device selectable 36–57 V DC, though usually 48 V, over two of the
| four available pairs on a Cat.3/Cat.5e cable with a selectable
| current of 10–400 mA subject to a maximum load power of 15.40W.
| Only about 12.95W are available after counting cable losses, and
| most switched power supplies will lose another 10–20% of the
| available energy. A "phantom power" technique is used so that the
| powered pairs may also carry data. This permits its use not only
| with 10BASE-T and 100BASE-TX, which use only two of the four pairs
| in the cable, but also with 1000BASE-T (Gigabit Ethernet), which
| uses all four pairs for data transmission. This is possible
| because all versions of Ethernet over twisted pair cable specify
| differential data transmission over each pair with transformer
| coupling; the DC supply and load connections can be made to the
| transformer center-taps at each end. Each pair thus operates in
| "common mode" as one side of the DC supply, so two pairs are
| required to complete the circuit. The polarity of the DC supply
| is unspecified; the powered device must operate with either polarity
| or pair 45+78 or 12+36 with the use of a bridge rectifier.

I have done phantom power for pro-audio microphones, and so I know
that this wouldn't be much of a problem. Just be careful on your
selection of transformers (due to the frequencies involved), or
balancing the outputs of OP-Amp circuits.

-- 
Scott G. Hall
Raleigh, NC, USA
ScottGHall at BellSouth.Net