[MLB-WIRELESS] Power over Ethernet

[Donovan Baarda]

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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