EE help, Voltage Drop over copper Options

[bd1308]

okay, i have a question. I'm trying to do Power-Over-Ethernet using the (unused) blue pair of ethernet wires. 5VDC......i run the wire (~70 or so feet) and plug it in, and not enough voltage to boot my wireless router....seems that i need under 1.4VDC....how the hell can i add 5VDC to 1.4VDC. I tried getting one of those adjustable AC adaptors and hooking them in series and parallel... in series neither worked...got like 1VDC, and in parallel, well....i tested it later (after it was working) and found the voltage to be around 14-15VDC.....what the hell?

AC adaptor->modified 10/100->~70 foot run to attic->blue pair seperated from RJ45 Jack->in-to router.

[914GT]

First of all, your adapter needs to be able to supply enough current to run the router even without the extra length of wire. Maybe it just can't provide the power and the voltage is dropping. If the supply is stiff enough then you'd have to be able to make it high enough to offset the resistive drop in the cable. The trouble with the adjustable adapters is they can't provide as much current at the upper ranges, plus the ripple/noise goes up. Another problem, even if you get 5V at the load with the right supply voltage on the other end, the voltage will be sensitive to how much current the router is pulling. If the router draws more power when it's transferring data its supply voltage will jump around due to the wiring losses.