In my prior post, I talked about troubleshooting power issues. And this past week we had another problem with out system that turned out to be power related again.
At our relay points, one of the difficulties is figuring out where the closest AC power is, and how to get from there to where we need our radios to live. Generally this distance is not very far. But in a couple cases it’s a long ways, like with the relay point in the middle of Tom’s field. In this case we trenched the 120v AC power all the way to the relay point.
In another case over at Jim Nelson’s point, we had to run 200 feet from the closest house, out to a tree on a point, and instead of bringing 120v AC out to the tree, we ran the power over POE the 200′.
At first we only had 2 radios at this relay point, and everything worked really well. Eventually we added a 3rd radio, and most recently we added a webcam. Adding the webcam pushed everything over the edge and suddenly everything at that relay point was rebooting over and over and over. WTF?
Well, there is this thing call line loss, when transmitting power over certain distances. Here is a great little webpage and helps you with those calculations.
So, we are transmissiong 24v DC power, 200′, and we are using Ubiquity Carrier Class ethernet cable, which has 24 AWG wire. And POE uses 2 pairs to carry the power.
The last thing we need to fill in is amps. When we had 2 radios, each using 8 watts, that means about 0.4 amps (8 watts/24 volts) per radio.
Plugging all this into our calculator shows we end up with just short of 20 volts at our radios.
Adding the 3rd radio (and amps up to 1.2), we fall to just shy of 18 volts, and adding the webcam we are under 16 volts, at which point things obviously started failing, probably the touchswitch that all this was plugged into.
So, our solution was to turn off the webcam to get everything running again, and then to order a bunch of 12/2 outdoor landscape lighting cable, and use that to bring the 24v power out to the relay point. Plugging in 12 AWG wire into our calculator across 200′ and 2.5 amps (the max the power supply will put out), gives us 22 volts at the relay point.
So, if you are running more then a couple of radios across a long distance POE link, you really need to do something different for power because that tiny 24 AWG wire just doesn’t cut it for high power needs.