JppsHomeBrewPoe

Just a few notes about my PoE equipment. For good directions on how to build one, see NycWirelesss PoE page http://www.nycwireless.net/poe/ Also see http://www.gweep.net/%7Esfoskett/tech/poecalc.html which has a javascript calculator to do important PoE computations.

I basically built two modules that were almost the same. One has a female power conector, the other a male. I used the female near the Wap11, and the male near the wallwart. I used a common conector that can be found at RadioShack. Its number is 274-222. At 99 cents each for a package of one male and one female, it is reasonably priced. It fit very nicly into the wall mount two rj45 box that I bought (at the HomeDepot near the MeetingSpace). You will need two rj45 boxes, and two pairs of power conectors. All told I spent about 40 bucks building my PoE stuff.

I cut the wallwart's cord, and spliced in a mateing male-femle pair of connectors so that I could plug them back together. After testing it three times, I pluged my Wap11 in, and it worked. The cord was wart -> female -> male -> original plug end.

I also built a long (100 foot) cable by crimping ends on some cat5 which was labled "outdoor" rated. It is intended for short non-flexing runs outside. (Like from the telephone company's network interface into the house.) I am runing it outside that spec by using a long run and shoving up to two amps of power down it.

About those amps... I measuered the resistance of my 100 foot setup. I measured from the + input to the - input on one PoE box, through the cable, and through a jumper temporarily connecting between the + and - power out pins. I measured 2.75 ohms. What does this mean in terms of the power at the other end? Here is a table to show your quickly what I learned slowly:

Notice that as the current goes up, the watts wasted goes up by the square. From 250mA to 2.5A is ten times increase. But the watts burned up in the cat5 cable goes up 100 times! At some point you can make wires glow (like in light bulbs) but soon after that they burn up. Perhaps burning up you AP or house. My references say you can get an amazing 3 amps down 24 AWG wire before it starts to burn up (actually before it reached boiling temprature when stored in an ice box!!) If I were you I would avoid running much more than one amp per pair of wires you use in your PoE. I.e. 2.0 amps is about the most you should ever run in a PoE setup. If you are running power only on one pair, try to keep it around one amp. I think Rg1000's are safe to run over 1 pair PoE. But its your house that will burn down. I would test it carefully first if it was my house!!

So when planing your PoE system keep two rules of thumb in your pocket: Shorter is better, higher voltage is better (since it will require fewer amps for the same power, thus waste much less power in the cable). Naturaly going over the insulation rating on the cat5 (say about 50v) is asking for the cable to burn up for a different reason. Your cable's voltage max rating is likely different than mine. RTFM or test carefully. If you can pop through at 50 volts, don't run more than 25 or so long term (insulation ages and gets worse with time.)

For a Wap11 (which runs on 5 volts) a run of 25 feet or so is about the max you can get away with. That would result in about 1.1Volt drop over the cable. That would make the Wap11 run on 3.9 volts or so. My Wap11 was acting funny at 3.6 volts. Even at 3.9 volts it still acts up from time to time. Feel free to burn yours up trying to find the lowest voltage it will run happily at.

A better AP to run PoE is one which will take a higher voltage. I have heard rumors that Rg1000's will run at from 12 to 15 volts. So with 3 volts you can afford to drop, and 1.1 amps draw, you need to limit the resistance in your PoE system to under 2.72 Ohms. I bet you could run a two pair PoE to about 90 feet. And a one pair PoE to about 45 feet. Assuming that you get a 15 volt wall wart and that the rumor I heard was true. YMMVV and it will be your Rg1000 that might burn up.

So my tested system is two two conector boxes, 25 feet of outdoor rated cat5, and a Wap11 with standard wallwart.

Well, to tell the truth, I did not dare to, but imagine you put the power block on the other side, and let the 110 (or 220) volts runs along the cat5. As the current is divided by 22 (or 44), the loss in the cable become negligible (divided by ~500 (or ~2000)). Of course, it seems dangerous and is probably out of cat5's specs. But the ISDN running in telephone cables is already 90 volts. I am not sure it is less dangerous to cook the wire with high current than it is to let 110 volts run in it. But, of course, if you set fire to your house (or burn your WAP), I am not responsible... --A. Nonymous

Like I mentioned above, you need to find out the maximum voltage rating of your cat5 cable. If ISDN is really running 90 V you might be able to find some outdoor rated cat5 that you could get away with running 110. But there is another failure mode to worry about. Imagine what happens if the insulation between the power lines and the data lines wears out due to age and weather. I think puting 12 or 15V on to the data lines of the cat5 will be kinder to your AP than 110. But it is a tough call. Too little voltage in the power conector of your AP will kill it just as dead as too much voltage in the net connector. -- JayPrimePositive

I tried the idea of running 120VAC over Cat5e cable.

On the remote end, I mounted a surface mounted 120VAC socket to a Ethernet box, then hard-wired it a locally mounted Ethernet box, where I mounted a fuse holder, and a cord/plug exits the box and plugs into the wall 120VAC outlet. Plugged in the power supply brick to the PoE outlet and plugged in the power to the AP, plugged in the network cables on both ends, and the AP works without problems! No effect on speed or reliability. I could not detect any extra voltage induced onto the data pairs.

I did run into one (self-created) problem the next day, when I could not contact the AP, and it would not respond to pings. The AP blew the 0.3 Amp fuse that I had installed. The AP power supply said it draws 0.5 Amp @ 120VAC, and as most devices almost never actually draw their rated consumption, I was being very conservative when I selected the fuse value of 0.3 Amp. It turns out, this AP does draw something close to it's actual rating (probably when I upped the power output of the AP,) so I have now replaced the fuse with a 1 Amp version, and I have had no further issues. The 1 Amp fuse is still *way* within what Cat5 will carry.

Anyway, even though Cat5 is not certified for 120VAC, it works for me. As mentioned elsewhere, ISDN is already running at 90 Volts. -- fasttimes(AT)mochamail.com

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