Thursday, October 9, 2014

DIY (Do It Yourself) PoE (Power over Ethernet)

I've used this so far to supply power to remote devices like VisionNet modems re-purposed into Wireless Access Points (WAPs), DSL modems, and IP cameras. A basic voltmeter is helpful, as it allows you to troubleshoot and calculate voltage drop across the Ethernet links you set up. Of course I'm going to say the standard disclaimer, what you do is at your own risk, if you aren't careful you will release the "magic smoke" out of your device.

I like working with these parts because it is non-standard PoE voltages (12V DC instead of 48 volts), and has conventional connectors that are easy to source. In fact, I am in the process of setting up a 12 volt "rail" at my house (more on that in a later article) for powering all the Internet access equipment and my security cameras and DVR. The 12 volt supply is battery-backed (no, not a car battery, though it could be) so it operates for a time even if the AC power is out at my residence.

The first picture is the components for a single PoE cable to power one device. "Barrel" connectors used for device power are standard 5.5mm OD (outside diameter) / 2.1mm ID (inside diameter). Many devices, including the BEC DSL modems, VisionNet M505, and M505N modems use that connection for power, however you may need an adapter if your equipment uses something different.

Your power adapter is plugged into the connection on the lower left of the picture, the Ethernet connection above it plugs into the Ethernet switch or modem that is the data source. On the right of that lower cable assembly you have an Ethernet cable plugged in that runs to your device. Keep that cable as short as possible (even though many power supplies actually provide more voltage than needed) so that the voltage drop isn't too significant (I've used lengths of 25 to 50 feet which can easily carry 12 volts).

The upper cable assembly in the photo is at the device, your Ethernet cable plugs into the end on the left-hand side. Connections on the right plug into the device. You may want to rig up a cable lead (at the device) that is able to check the voltage level when the equipment is powered up when first installed. You will probably need one of the upper cable assemblies (they are sold as a set, and I haven't found a source that sells the ends individually) for the next component I will describe: PoE powering up to four devices of the same voltage.

The second picture shows that component, although it isn't marked there for the connections (the far side Ethernet ports are to your LAN / switch, towards the front are to the devices being powered by PoE). On the board itself it wasn't marked for the blue connection for optional power (I use it for equipment local to the board that are also 12VDC), the most inboard terminal is typically positive (most "barrel" power connections have the center pin as positive, and as shown there is a 5.5mm/2.1mm barrel connection on the board for that easier use), and the outboard terminal is your power supply ground.

Make sure you have a power supply for the summed current of all connected devices (each modem or device powered by 12VDC is usually an amp of current, and it will be marked on a label on the device). In the office I have an 8 amp supply, and my 12 volt rail at home will be able to supply at least that as well. As I said, you will need one half of the cable assemblies at the other end to break out the power to the device.

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