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

[Steve Murphy]

On Wed, 2006-11-22 at 12:01 -0700,
asterisk-users-request at lists.digium.com wrote:
> On 11/22/06, Zeeshan Zakaria <zishanov at gmail.com> wrote:
>         Hi,
>         
>         Why Aastra phones use more electricity, i.e. 48VDC whereas
>         other phones use much less, e.g. Grandstream and Linksys both
>         use only 5VDC. I first thought it was because of PoE, but the
>         ones with 5VDC also run fine on PoE. What is the difference in
>         power consumption then? 
> 48V is also a sort of "standard" for telco devices.... if I remember
> it correctly...
> 

IIRC, It's not just that 48 is a popular source. Most POE taps will
regulate the voltage
down to whatever they need, which often is just 5V, or 12V. But we are
talking DC voltage here, and there are significant voltage drops due to
the [small, but not zero] resistance of copper. The longer the cable
from the injector to the tap, the bigger the resistance, and the more
the voltage drop. The amount of current figures in there, also. So, 48V
is a safer voltage in general to inject, as long lines will usually
still see hopefully more than 5V at the tap end.

If you are going to design networks with POE, you'd best pull out your
calculator, multimeter, and V=IR equations, and see if you'll get the
required voltage at the other
end of the wire, given the current the devices will use.

murf

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