Off The Shelf PoE Adaptor Brings Success to our Power Problems
Some times it’s best to let someone else work things out! Making our own PoE and voltage regulator was perhaps a step too far when this off the shelf PoE adaptor is so inexpensive and easy to use. It was fun to try and make our own. Except for the time I clamped the device in my crocodile clips to test the voltage which of course resulted in a puff of smoke because I shorted the whole thing! I learnt a little more about the importance of heat sinks and neat soldering. But perhaps our time would have been better spent on more important elements of the project.
These TP-Link devices come with a 48v supply and converts down to 12V, 9V, or 5V DC over a 100 meters of Ethernet. I had to make my own Micro USB to 5V adaptor to power the Pi – only because that’s it’s a simple bit of bodging and I have loads of USB cables I could hack into.
The unit we’re using in the CDC Weather Station, the TP-Link POE150S, isn’t available any more. The replacement is the TP-POE10R – which costs around £10.00 – worth every penny!
Power Over Ethernet (PoE) Introduces It’s Own Problems.
To run data and power down a network cable you need to use a PoE injector. Often implemented when you need to power wireless routers throughout a building but don’t want the cost of putting a 13amp socket next to each one as well. At the CDC Weather Station we hit a problem with distance. Running a DC current down a 90 meter cable using a PoE injector will get some voltage to the end, but the drop off due to resistants in the cable is significant. Just like watering the garden; the longer the hose the more the water trickles out at the other end.
Using the voltage drop calculator below we determined we’d need to send 24 volts down the cable to guarantee a decent output at the other end. We were potentially facing a 50% drop in voltage. The Raspberry Pi only needs 5v consistently, no computer likes a varying current. As other factors such as kinks in the cable and change in temperature could play a part we’ve given ourselves some headroom so 24v should get us to around 12v, which in reality is probably going to be more like 7v or 8v.