[Server-devel] 12 Volt power system for School Servers.

James Cameron quozl at laptop.org
Thu May 30 01:16:46 EDT 2013

Tony makes an interesting point that may be new to some:

On Thu, May 30, 2013 at 06:47:26AM +0200, Tony Anderson wrote:
> This discussion is relevant because 12vdc does not travel well over
> long distances. It may be necessary in a school with multiple
> classrooms to use an inverter to provide 110 or 220vac to the
> routers via PoE.

As the distance between the battery bank and the access point
increases, so too does the required _diameter_ of copper cable.

Copper cable is quite valuable, and easily traded.  It can be a good
security practice to avoid installing too much of it.

If the cable is too thin, the power demand by an access point causes
too much of a voltage drop.  For instance, one might put 12V DC in one
end, and get 10V DC out the other end.  The access point may still
work fine.  But some of the power will be lost as heat in the cable,
lowering efficiency.

At some point, depending on cost, reliability, and availability of
equipment, it is worth operating at a higher voltage.

The options would seem to be Power over Ethernet (PoE), which will
yield 12.95W (IEEE 802.3af-2003) or 25.5W (IEEE 802.3at-2009), or an
inverter with a separate power circuit.

My best guess is that PoE will be cheaper, because a power circuit
installed to meet electrical regulations would tend to be designed for
the minimum domestic power standard of the country.  For example, in
Australia the minimum is 10A at 240V, or 2400W.  The mass of copper
is far more than what PoE would require.

Then there's the cost of a 12V input PoE power sourcing equipment

James Cameron

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