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.

[mightyohm]

Ohms law will tell you the voltage drop to expect given a certain current through the cable and the resistance of the cable. You can measure the resistance using an ohmmeter or calculate it given a table (if you can figure out the wire gauge). You will actually have to multiply the resistance by 2 because you are running both power and ground thru the cable (the electricity makes a round trip). I just did a quick google search, and it turns out that if the wire is 22 gauge, you will have about 3 ohms total resistance for 70 feet of cable. If the AP draws about 1 amp, this will give you 3 volts of drop, which sounds like what you are seeing (approximately).

There are a few ways to solve this. One way (which might work but it is crude) is to connect a wall wart will an output voltage of approximately 8V to the cable. After a 3V drop this will give you the 5V you need.