[asterisk-users] Why Aastra uses 48V whereas other IP Phones usemuch less, i.e. 5-12V

[Brad Templeton]

On Wed, Nov 22, 2006 at 11:29:01PM -0500, Michelle Dupuis wrote:
> 48VDC is a long time telco standard - and has become the Power over Ethernet
> standard.
>  
> Keep in mind that 'electricity' isn't the measure - it's power.  Power is
> not synonymous with voltage.

More to the point, there is a tradeoff.   For a given power, the higher
the voltage, the lower the current.  The lower the current, the thinner
the wire you can get away with.   Power over ethernet uses very thin
wire, so you want high voltage and low current.

Power transmission lines use very high voltage because they need (comparatively)
low current through the wires.  The higher the voltage, the more power you
can put through the same wire.

To a point.  As voltage gets higher, it also gets more dangerous, and
needs a bit more insulation.   It's very hard to hurt somebody with 12
volts.   And 48 volts, while not quite as safe, is still pretty safe.  It's
been chosen as a voltage that mixes the right combination of safety and
power.   The higher the voltage, the more heat you can generate if you have
the current behind it.  (If you are current limited or fuse/breaker protected
you are just as safe from fire if things are calibrated right.)

In the past, we often drove things with batteries, or wanted to sometimes.
Getting 48v with batteries takes a lot of cells with most technologies.
Phone central offices had big banks of batteries -- no problem.

Today, with advanced switched-mode power supply technology, we can turn
just about any voltage into any voltage.  So we don't care as much
about being able to run on batteries as low voltage, though it's still
nice in portable tech.   And of course the chips all run on very low
voltages today (TTL was 5 volts and it's getting rarer) and they want to
be low power.    Most of the PoE phones that take 48 volts are converting
it down to lower voltages to use.   But 48 is a good voltage to be
sending on the wires.


The USA uses 120v for house current.  That's enough to hurt you and can
kill you if you touch it wrong, though I've touched it a few times.

A lot of the world uses 220.  This causes enough of a spark that they
require all receptacles to have a switch on them so you don't plug things
in live.  On the other hand, 220 can deliver twice the power in the same
current.  Kettles in the 220 world are _really_ fast.  Your dryer and oven
run on 220 even in the 110 world, only way to get enough power.  Same with
electric car chargers.