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.

[bondo]

QUOTE (Tom Perso @ May 3 2005, 09:29 AM)

Couldn't you just double up the wires? Ethernet only needs two pair, you've got two left. So, one pair for the hot, the other for the ground? Cisco has a "Power injector" that does the same thing, worked up to 100m. Later, Tom

Using 1 pair for positive and one pair for negative would negate the effect of having twisted pairs. To use both pairs you would need to put positive on 1 wire of each pair and negative on the other. It will cut the voltage drop in half. Cisco does it the "right way".. at 48v the current (and induced magnetic fields) would be much lower, and the drop in power over distance would be less. (That's why power transmission lines are in the kilovolts) To do it that way you'd need a 48v power supply, and a 48v to 5v DC-DC converter to put at the router end.

In reality using a 9v wall wart is probably fine. Make sure it can handle the current, then try it. If you still don't get enough to run the router, try it with both pairs.