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Would it be possible to use a multi-core cable to have power and data in the same wire?

I'm using WS2815 LEDs. The total current needed is 9.6A. For a cable of 8 meters, a 2.5mm wire is needed. Because of the distance between the microcontroller and the first LED I am planning to use a MAX485.

Could I use a four core 2.5mm2 cable to transport the needed power (12V and ground) and data (data A and data B,) or is there a high chance of interference on the data lines because of the 12V at 9,6A (almost 120W) in the same cable.

JRE
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Mark
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  • Power supply is constant current, so it should not interference with data lines. – Piotr Apr 30 '22 at 20:09
  • Using a 2.5mm wire to send data is a waste of copper but no reason it wouldn't work. – user1850479 Apr 30 '22 at 21:16
  • If you can possibly feed the power separately it would be good to power the strip from both ends and / or from the middle. Long strips of LEDs tend to be dimmer the further you get from the power injection point. – Peter Jennings Apr 30 '22 at 22:06
  • @Peter also the internal wire traces on the PCB are not rated for 9.6 amps. – Harper - Reinstate Monica May 01 '22 at 02:16
  • @Piotr it's not constant current it's constant voltage AND the rapid current change of color changing leds means the current isn't steady. – Passerby May 01 '22 at 02:48
  • @Passerby The WS2815 diodes are driven constant current, and PWM is implemented by switching the current between a resistive load and the diode, so the current through the line is almost constant. Exception is when diodes are turned off entirely on all three channels, then the regulator is turned off and current drops. – user1850479 May 01 '22 at 04:15
  • @user1850479 yes the individual ic drives the diodes as constant current. But the overall system is not constant current. The supply will be constant voltage and the current will vary as led 1 will be 40 mA led 2 will be 30mA led 5 will be 10 mA etc. And no when it is using pwm it does not shunt the excess current into a resistor. Where did you read that. – Passerby May 01 '22 at 06:52
  • @Passerby You are probably thinking of a different type of LED as the WS2815 can feed 0 or ~15mA and no other other values. See this question if you want to know how they work: https://electronics.stackexchange.com/questions/427463/5v-vs-12v-addressable-led-efficiency/447816#447816 – user1850479 May 01 '22 at 12:58
  • @user1850479 if that's the case, what's the current draw at RGB 4F0000 instead of FF0000 or FFFFFF? – Passerby May 01 '22 at 13:01
  • @Passerby All nonzero values are ~15mA. Off will be close to zero. There's a circuit diagram in that linked question if you're curious why. – user1850479 May 01 '22 at 13:16
  • @user1850479 that seems contrary to common application of pwm led control but you seem to know more than I do about the ic so ok. – Passerby May 01 '22 at 13:20

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No, there won't be any crosstalk like that. You can use appropriate wires for each task. So the signal line can be any modest size of data wire, e.g. 0.2 mm2.

The power wires, however, are a whole different kettle of fish. You can't just use "the default" wire size. You actually have to do a voltage drop calculation and not a guess. The quick pass I did on your 9.6A, 8m, 12V wire is that considerably larger than 2.5mm2 will be required to have an acceptable voltage drop. Unless you consider 13% to be acceptable.

Harper - Reinstate Monica
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  • Yes, 10% would be okay. [See calculator](https://www.omnicalculator.com/physics/wire-size?v=phaseFactor:1,conductorResistivity:0.0000000168,temp:75!C,sourceVoltage:12!volt,current:9.6!ampere,distance:8!m,allowableVoltageDrop:10.5!perc) Would it be considerable smarter to use 24v PSU + 5-meter cables and then a 24v>12v converter for the last 3 meter? – Mark May 01 '22 at 10:12
  • @Mark absolutely, that should do it. Doubling voltage halves current giving 1/4 the losses over the same wire. Also think about voltage drop down the LED string. It can help to feed it in multiple places. – Harper - Reinstate Monica May 01 '22 at 17:45