I need to decide about how to connect two custom designed electronic units. One is main (execution) unit, and the second is hand-controller unit. Those units have to be connected by wire. The communication between units require 3 twisted pairs (for data transfer) and power supply lines (VCC and GND). The power lines are needed for hand-controller supply from the main unit. The power supply is 6V with max current consumption of 500mA. The distance between main unit and hand-controller is up to 3 meters. The perfect solution would be to have individually shielded twisted pairs.

At the first, I was thinking about RJ45 connector and CAT cable, but I am not sure about current rating. In that case I would use one twisted pair for power supply and the rest of 3 for data transfer. It seems as not reliable option for powering. There is also combination of CAT cable with two additional power lines, but it would require two connectors, so not pretty elegant solution (prefer to stay within one connector). Additionally, I need robust connection as the hand-controller would not be stationary.

What do you suggest that could be suitable for my application? The connector size is not much important. I just need to be sure that I can find also the proper cable for such connector. It is also not important if it is not exactly the right fit (e.g. have 1-2 twisted pairs more than I need).

Thank you.

  • \$\begingroup\$ PoE sends power over Cat-5 cables. 802.3af can deliver up to 13W at 100m using 2 pairs (either dedicated power pairs or the data pairs), and later variations can deliver even more, so 3W over 3 meters should definitely not be a problem. \$\endgroup\$
    – jcaron
    Commented Mar 18, 2020 at 13:36

1 Answer 1


I'd suggest that RJ-45 and Cat-5 cable is perfectly fine: it's cheap and easy, cables available ready-made in any colour or shape you like without any difficulty. The main downside is risk of connecting to ethernet (wouldn't damage anything, but would it be okay in your environment?), and worth thinking about the power.

At 3 metres and 500 mA you can work around the power issues: Cat 5 is about 0.2Ω/metre, call it 1Ω for your cable Vdrop ~ 0.5 V. I've found it very beneficial to have a small local power module eg Recom R-78E5.0-0.5 and send the supply at 24V. They are very cheap and made all my power problems disappear. In your circumstance it would mean you could change to eg 6 metre cable without any issue at all. I'd certainly much rather have local power than a bigger connector or custom cables.


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