Yet Another Power Over Ethernet

A friend of mine aquired a pair of DLink DWL-900APs which he wanted for a 2.5km point to point link. He wanted to replace the 486's currently in his roof cavity containing ye olde ISA Wavelan cards with something that would be less of a potential fire hazard. Understandable. And he wanted to go above 2Mbit. Even more understandble. So I undertook to make him a Power Over Ethernet kit. I found NYCWireless's POE which I used as a guide. As the DWL-900AP needs 5V to operate, I have included regulation so that the AP will get consistent power over a variety of CAT5 cable lengths.

Note that this POE IS NOT compatible with the IEEE 802.3af POE standard. See the bottom of this page for details on 802.3af.

Ingredients

From the Jaycar store in the Melbourne CBD I bought (prices in Australian dollars):

2.1mm DC Line Socket $2.45
2.1mm DC Line Plug $1.55
uA7805C TO-220 package 5V Linear Regulator $2.40
12V 1.6A Switchmode PSU with 2mm Plug $20.00

From Lawrence and Hanson Electrical Wholesalers in South Melbourne I bought:

2 x 4DIGST42 Eight Wire Modular RJ-45 Jack $3.50ea

From bits and pieces I had lying about:

2 x 1m segments of CAT5 with one connector crimped on one end of each cable
2 x red LEDs (optional)
330R and 1300R resistors(optional)
2 x 1uF surface mount tantalum caps (check your particular 7805 data sheet)
Heatshrink
Heatsink
Screws and nuts for heatsinks
Heat conductive paste (like the stuff you put between your Athlon/P4 and heatsink)

Making It

Upon opening the the RJ-45 modular jack you will see that the wires from the built in RJ-45 socket break out into eight screw terminations. This makes it very easy to connect, you don't need a Krone tool like in the NYC-POE.

For each power cable I used some lengths of twisted pair phone cable and used each pair for the voltage and ground connections. Solder the 2.1mm plug onto one power cable and the 2.1mm socket onto the other. Don't forget the centre pin is the positive rail and the outer pin in the ground rail. On the socket end I placed a 1N5404 diode to ensure that a reverse voltage on the connector didn't toast anything - this is a step for the paranoid (like me) and isn't neccessary if you're meticulous. I had originally intended for the socket to mount in the box but it didn't fit. I used a strip of heatshrink to cover the exposed soldered pins (and diode).

Take each cable and strip about 5cm of sheath off exposing all the pairs. Open up the modular boxes and wire them up as shown in the diagrams below.

POE Top End

Fig 1: Top end wiring.

POE Bottom End

Fig 2: Bottom end wiring.

I used tantalum caps (TANT-A footprint) which fit snugly onto the 7805 pins as shown in the first diagram above. You can use leaded caps, just beware of the polarity! I over compensated by putting 1uF caps on. Check the data sheet for the 7805 you're using for appropriate values.

I mounted the 7805 on top of the box to allow for a heatsink. I chose a small TO-220 heatsink initially but it got very hot. This little tacker couldn't dissipate a few watts fast enough. I ended up bending a strip of aluminium to attach two larger heatsinks, as can be seen below. Frankly, try and make it as big as you can within reason - reason being that when in use at room temperature the heatsink is warm to the touch. Roof cavity's can get very hot so the heatsink won't be as efficient in summer. Don't forget to use the paste either.

You can optionally mount the LEDs. I placed the 330ohm resistor in series with one LED across the 5V rail and ground on the 7805 and the 1300ohm resistor across the 12V rail. The resistors should give an arm waving 10mA of current to the LEDs.

Testing It

Once it's all assembled start off by plugging in the 12V PSU to the bottom end. The LED should come on. Hook the two POE boxes together with regular CAT5. It must not be a crossover cable. If you are unsure about the correct CAT5 wiring, see here. With the two POEs hooked up the top LED should come on. Using a multimeter, place the ground probe on the centre pin or the casing of the 7805. With the red probe test the left pin (pin 1) - it should show approximately 11-12V on it. Test the right pin (pin 3) and it should show 4.9 to 5V.

With the multimeter ground probe still on the 7805 ground, check each CAT5 pin on the top and bottom connectors to ensure there has been no miswiring. You should read no voltage whatsoever on the CAT5 connectors. The power should only appear on the segment between the POE boxes.

With the power disconnected test the continuity between the bottom and top CAT 5 connectors. Pins 1 (orange white), 2 (orange), 3 (green-white) and 6 (green) should run straight through unimpeded between both connectors. These four wires are the two pairs ethernet uses. On the POE in between the boxes pins 4 & 5 (blue) and 7 & 8 (brown) is used to carry the power.

Finally, with the multimeter and the power on test the top end power connector to see that 5V is appearing on the centre pin.

Once you are confident of your wiring, plug in the AP and the bottom end to your hub/switch. If you want to hook the bottom end directly to an ethernet card, simply swap the orange and green pairs on the connector when you crimp it, or when you wire up the bottom end box. Either "ping" the AP or use the SNMP tools to see you you have a working connection to it.

CAT5 Length Between POE Boxes

The 7805 is designed to take anything from 7-25V and regulate it down to 5V. You have to ensure then that whatever length of CAT5 you use in between, that the voltage appearing on the left pin of the 7805 is at least 7-7.5V, else it will not be able to derive 5V properly. Anything above 5V is dissipated as heat through the heat sink - which is why it's good to keep the voltage that's at the top end of the POE as close to 8V as possible. So stricly speaking you could use a 10V PSU at the bottom end, so long as it can supply the current your Access Point needs. In the case of the DWL-900AP it's rated on the label at 1A.

CAT5 wire is rated to be no more than 9.8ohms per 100m. My test length of CAT5 was 15.6m and I measured 1.3ohms resistance on one strand - this translates to 8.33ohm/100m and is well within spec.

Using two strands halves the resistance, but two strands for the positive rail and two strands for the ground rail add up to a total of 1.3ohms. The diode I used has a Vf of 0.7V. With the access point plugged in at the top end I measured 10.7V on the input of the 7805. This leaves 0.6V being dropped by the cable.

Using the relation I=V/R we get 0.6V/1.3ohms = 0.462 amps being drawn by the AP.

Using the relation P=V*I then 0.6V * 0.462A = 0.277 watts being dissipated by the cable.

Finally, of the 10.7V on the input to the 7805, 5.7V have to be dropped by the regulator to get to 5V. At 0.462A the amount of heat being dissipated by the heatsink is: (10.7-5)V * 0.462A = 2.6334W. The 0.462A I measured drawn by the DWL-900AP coincides with I the 0.4A reported by a MelbWireless member. So, the 1.6 amps available from the 12V PSU is more than ample. Don't forget to ensure the PSU you use can handle the current required by your AP.

Using the same sorts of sums above, you can calculate the metrics on a span of CAT5 you wish to use for POE.

If your AP runs on 12V you could used a 7812 regulator instead, so long as you use about a 16V PSU at the bottom.