On Wed, 2006-11-22 at 12:01 -0700, [EMAIL PROTECTED] wrote: > On 11/22/06, Zeeshan Zakaria <[EMAIL PROTECTED]> 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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