[MLB-WIRELESS] Power over Ethernet

[Darren Fulton]

Most commcercial PoE boxes use 48V. Firstly because it's the standard used
by telcos but also because with an effiicient DC-DC converter on the mast
the current though the cable is reduced (along with the loses).

The DC-DC converter allows you to tradeoff voltage for current, so if the AP
requires 10W power @ say 10V it will draw 1A current from the converter. The
converter in turn will also draw 10W ( @ 100% efficiency) from the cable,
but since the cable is supplying 48V the current will only be 10W/48V =
208mA. A real DC-DC converter with an efficiency of around 80% will draw
more current from the cable.

Darren
----- Original Message -----
From: "Donovan Baarda" <abo at minkirri.apana.org.au>


> On Tue, Nov 12, 2002 at 08:57:49PM +1100, Tom Parker wrote:
> > Ian,
> >
> > I typed a good reply but the cat ate it, so I'll try again.
> >
> > You need to get an accurate picture of the current consumption using a
meter
> > with the Eden working quite hard (if you're running linux perhaps
repeated
> > filesystem operations of some kind would be good).  Once we know this
figure
> > we can work from here.  I think there are a few people interested in
this
> > board so this would be good to work out.
> >
> > We also need someone to work out how much current is safe/legal over
CAT5.
> > The Australian Wiring Handbook should give it.  I suspect it will be
around
> > 3A.  This will go down with the distance you cover.
>
> Hmm... I dont think any current limits vary with distance. Wires are
> typicaly "current rated" to a particular current value, irrespective of
> their length. However, the longer the distance, the more the voltage drop
at
> the other end...
>
> > Once we have the actual consumption then you can add a bit and put a
fuse in
> > at that rating (provided it is less than the maximum).  Vaskos has done
the
> > right thing putting LOW VOLTAGE AC (say 15V) over the wires and
regulating
> > it at the masthead, the current loss will be less with AC.  You might
like
> > to consider running some figure 8 speaker cable with your CAT5 if the
> > current is more than a few amps.
>
> I thought AC was more likely to screw with the ethernet than DC. I also
> thought that the current loss was about the same AC or DC. I thought the
> main advantage/reason why mains uses AC is it happens to come that way out
> of the generators, and it makes step-down transformers much simpler so you
> can use mega-high voltages for the long hauls (high voltage is what
> minimises the losses).
> However, I was always a TTL man when I was into electronics, so take my
> AC/DC with a grain of salt :-)
>


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