Arduino365 part 2: Power Cable Length

13th July 2019

So you are thinking of placing your arduino uno in a garden / warehouse to collect some data... because you can or you really need that information for your university project. There is no power outlet down there obviously. You already figured out that Passive Power Over Ethernet will be a perfect match. The only remaining thing is the power supply (you already solved the casing and other issues). Normally a 9v unit is used and everything is smooth sailing - however in this case the voltage drop becomes an issue. A typical ethernet cable has a limit of 100 meters before things tend to go south. In my little experiment I am putting few cables together that have a total length of 85m (1x5m, 1x10m, 1x20m, 1x50m), but I will also be testing other length combinations of the available cables as well. This is not a super scientific test that you could quote in your dissertation - this is more of an observation to give you an idea what to be prepared to deal with - as usual, your mileage may vary.

My brick is 9v and I have increased it to 14v via a cheap voltage regulator. Couplers have been used to connect the RJ45 cables together. At the end of the cable I have attached a step down converter which has an arduino uno with an ethernet shield attached to it (to see if it powers up - not that I am expecting any functionality at 5.6v). The converter has a little LED screen that displays the voltage coming in. So with all of that in place I did the simple measurements and here is the table with results:

Distance Output Voltage Couplers Used
5m 13.4 0
10m 13.2 0
15m 12.4 1
20m 11.8 0
25m 10.0 1
30m 9.2 1
35m 8.3 2
50m 10.8 0
55m 10.3 1
60m 9.9 1
65m 6.7 2
70m 7.7 1
75m 7.1 2
80m 7.0 2
85m 5.6 3

Some of the results seem be a bit odd. The cables are not the same gauge hence that could be the reason for it (or other factors). I have tested my other 50m cable which is a 26AWG and it had a 10.1v at the input of the step down converter screen. That is 0.7v less compared to the 24AWG 50m cable used in my initial testing. Regardless of all that - the main pattern here is that it probably is not a good idea to use a 9v psu when your uno is located around 20 meters away and more... I would consider using a 12v power supply with longer runs (or use a booster to get the voltage even higher). The other suggestion is to buy a 24AWG cable if you are planning to place your uno far away. There is also current.... If you want to have a robot arm at the end of your 60 meter run => it will probably not work (as in: it will not work for sure). Those things (stepper motors and servos) need some serious current to operate. There is also a different problem: I would highly NOT recommend pushing high current through a CAT5 cable...

To wrap this up - this method of delivering power is only really feasible if the arduino at the end of it is not overloaded with current hungry stuff. Things like few sensors will be just fine! And also give your project a bit of an extra buffer just in case things do not go your way 🙂

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