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Topic: Long distance power over Cat5 twisted pair (Read 137 times) previous topic - next topic


I've got a project for which I plan to use cat5 cable for full duplex data transmission between an arduino and a laptop over RS422 over tens of metres (up to 100m/328ft). I figure that since I've got a twisted pair to spare I could possibly power the microcontroller, an IMU and a pair of UART-RS422 breakouts. The arduino will be gathering sensor readings, doing some filtering and streaming data back to the laptop. I could hook it up to a laptop usb jack or a power bank that outputs 5V DC at 2.1 A.

My idea is to use cut open a usb cable and splice the power and ground to a cat5 twisted pair to send power to the arduino. It works out over a short cable, but my main concern is how much power I'll lose over long distance and if this will be enough to power the board. I'm thinking of using a Pro Mini 5V but the 3.3V variant should be fine also. Also, if I'm only using the cable for RS422 communication and power, will the interference from the power cable be significant enough to affect data transmission?


Instead of sending 5v, send something like 12v and use a voltage regulator on the remote end.
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I have used the same cable idea at 40+ feet.
I sent 12VDC on the twisted pair to a SMPS at the Arduino end to get the +5v.


The way you have it in your schematic isn't the same as how you have it wired up!


Jun 13, 2015, 03:40 am Last Edit: Jun 13, 2015, 03:41 am by MarkT
Power over ethernet uses 48V because that is the highest voltage that can be used without
legal considerations of high voltage equipment.  For long distance runs you want to send
power at a high voltage and low current otherwise wiring losses will rob you of your power.

The cable resistance goes up linearly with distance, so want to increase voltage by the
square root of distance - ie a cable good for 5m at 5V/2A would want 50V/200mA for 500m.
(ten times the voltage for 100 times the distance).

Think of the supply as a resistance - 5V at 2A is 2.5 ohms.  Your cable wants to be a lot
smaller than that to keep losses down to a few percent, perhaps 0.1 ohms or less.  100 times
the cable length would then be 10 ohms or so, competely hopeless for 5V/2A. Cable of 10
times the diameter and 100 times the length would work, but thats 10,000 times as much
copper, which isn't economic.

So basically you want DC-DC converters at each end, preferably 5V->24V or more, and back
down at the other end.
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