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Providing Power

Obviously, DC power can be provided by simply poking a hole in your enclosure and running a wire. If your enclosure is large enough (say, an outdoor electrical box) you could even wire an AC outlet inside the box. But manufacturers are increasingly supporting a very handy feature that eliminates the need for an additional hole in the box: Power over Ethernet (POE).

The 802.3af standard defines a method for supplying power to devices using the unused pairs in a standard Ethernet cable. Nearly 13 Watts of power can be provided safely on a CAT5 cable without interfering with data transmissions on the same wire. Newer 802.3af compliant Ethernet switches (called end span injectors) supply power directly to connected devices. End span switches can supply power on the same wires that are used for data (pairs 12 and 3-6) or on the unused wires (pairs 4-5 and 7-8). Other equipment, called mid span injectors, are inserted between Ethernet switches and the device to be powered. These injectors supply power on the unused pairs.

If your wireless router or CPE includes support for 802.3af, you could in theory simply connect it to an injector. Unfortunately, some manufacturers (notably Cisco) disagree on power polarity, and connecting mismatching gear can damage the injector and the equipment to be powered. Read the fine print and be sure that your injector and wireless equipment agree on which pins and polarity should be used for power.

If your wireless equipment doesn't support power over Ethernet, you can still use the unused pairs in a CAT5 cable to carry power. You can either use a passive POE injector, or simply build one yourself. These devices manually connect DC power to the unused wires on one end of the cable, and connect the other end directly to a barrel connector inserted in the device's power receptacle. A pair of passive POE devices can typically be purchased for under $20.

To make your own, you will need to find out how much power the device requires to operate, and provide at least that much current and voltage, plus enough to account for loss in the Ethernet run. You don't want to supply too much power, as the resistance of the small cable can present a fire hazard. Here is an online calculator that will help you calculate the voltage drop for a given run of CAT5 : www.gweep.net/~sfoskett/tech/poecalc.html

Once you know the proper power and electrical polarity needed to power your wireless gear, crimp a CAT5 cable only using the data wires (pairs 1-2 and 3-6). Then simply connect the transformer to pairs 4-5 (usually blue / blue-white) and 7-8 (brown / brown-white) on one end, and a matching barrel connector on the other. For a complete guide to building your own POE injector from scratch, see this terrific guide from NYCwireless: nycwireless.net/poe/

Last Update: 2007-01-13