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.

[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.

[lapuwali]

I was just about to type in almost exactly what jkeyzer said. The router almost certainly has a 5V voltage regulator in it, so using a very commonly available 9V wall wart should work fine and not smoke anything.

[Loser_Cruiser]

If you truly need a power over CAT 5 cable, it would appear your power source is not large enough to source your load. V= RI, as you increase your resistance of the cable by increasing the length of wire, the voltage will drops with a constant current. It gets worst as you max out the current capability of the wire gauge. Therefore as you increase length of wire you are increasing resistance in the wire and need to increase current (Larger Power supply in Amps, 5V 1amp, or 2 amps 3amps). 5Vdc=1.8Amps X 3 Ohms (70 ft of wire). It looks like a 5Vdc @ 2 amps would work for that application.

[bondo]

What you need is a power supply with remote sensing.. two wires will power your device.. two other wires will carry no current but measure the voltage at the device. The power supply will use the voltage at the device to regulate its ouput. Thus if you need 9v to get 5v, it will do that. Unfotunately such a supply will cost more, and you'll need 4 wires.

Just do what jkeyzer and lapuwali said. (IMG:http://www.914world.com/bbs2/html/emoticons/smile.gif)

[bd1308]

the AC adaptor is OEM.....5VDC....regulator would be intresting....now THEY do sell PoE things for the router, but they are expensive....

[bd1308]

ethernet does have 2 unused pairs....cool stuff. how much do these things cost?

[bondo]

QUOTE (bd1308 @ May 2 2005, 10:36 PM)

ethernet does have 2 unused pairs....cool stuff. how much do these things cost?

Cheapest one I could find

[bd1308]

okay....just thought of something....why couldn't I just get a 12VDC ac adaptor, and get a voltage regulator to lower the voltage to 5VDC....and put a HUGE heatsink with fan on there.????

[lapuwali]

QUOTE (bd1308 @ May 2 2005, 09:52 PM)

okay....just thought of something....why couldn't I just get a 12VDC ac adaptor, and get a voltage regulator to lower the voltage to 5VDC....and put a HUGE heatsink with fan on there.????

Again, the router very probably has a 5V voltage regulator inside. No need to regulate at the router, as long as you don't give it way too much voltage. A 9V wall wart with a 3V drop across your cable is 6V at the router, which an onboard 5V regulator will handle just fine.

[mightyohm]

Make sure the 9V adapter is capable of supplying the current that the router requires. It's probably best to have some margin too. So if the router needs 1A of current (you can measure this or look on the bottom of the router) try to find an adapter that supplies 9VDC at 1 to 1.5A.

[Dr. Roger]

won't the EMF surrounding these power wires totally f-up the data signal coming thru the other 6 strands? or am i just trippin'?

i've had cat 5 cable near power sources and it had a substantial effect.

[mightyohm]

Well regulated DC will not have a significant effect. AC might cause problems. Some additional filtering across the supply lines at the point where the DC feeds into the CAT5 (a 0.1uF cap in parallel with a 1uF or 10uF) probably wouldn't hurt.

[davep]

DC can cause problems also if the current draw is not constant. As the current changes so does the magnetic field, and in turn the induced EMF on any adjacent wires.

[Tom Perso]

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

[bd1308]

jkeyzer: AC power wouldnt work...the bleedthrough from the 60 Hz would kill my 10/100 connection.

tom: would the doubling of the pairs be enough to lower the effect of voltage drop? cisco also uses 48VDC too...

[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.

[bd1308]

yeah...i'm just not so sure about this whole built-in power regulator.......I just don't wanna see fire or sparks, ya know? Plus that burning electrical smell is soemthing I also don't like to experience. That happened to an old 8088 once, smelled up the basement for days.....I had to come up with a REAL GOOD excuse for that....

[914GT]

Lynksys makes a power over ethernet adapter kit. I've no idea what the thing costs.