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so I plan to build a house (at some point in future) and I'd like to implement some sort of home automation. Now one of the things I'd like to have is PWMed lighting. Most of the lights would be 12V LED lights which consume around 0.3A each.

I plan to have Arduino (under plasterboard ceiling accessible via a revision door) in each room which would (among other things) control the lights via TLC5940 + TLP250 + MOSFETs. This is just for illustration how it might look like in a room:

Ceiling lights plan

Each red line represents a pair of wires. I estimate that the longest wire could be 8m (26ft) and for that I need to use 20 AWG wire to be under 3% of voltage drop. Each light would be dimmable (provided by TLC5940 at PWM frequency of 1kHz) and powered from a single 12V power source (per room).

This all works fine on a breadboard with short cables and couple of lights but I am not sure how well it will behave with long wires and lots of lights (like 20). I already bought some wire so I can test a few lights on long wire. And it even might seemingly work well but I am trying to design it so I don't have to replace parts every month.

So here are things that I would like to ask:

  1. Will PWM work reasonably well at such a long distance? Such long wires have not negligible capacitance, inductance and resistance so I know that I am not gonna get a square signal at the other end of the cable (far from it) but I am not trying to communicate with the light, just trying to light it up. Will the lack of square cause any ill-effects on the brightness or the LED itself?

  2. Long wires are antennas. Could a lightning storm induct such a high voltage on those wires that would damage either the lights or the MOSFETs? Arduino will be opto-isolated via TLP250 so there should be no harm. Also if there will be any power lines (230V) crossing those wires e.g. for controlling electrical jalousies, will switching the jalousies cause the lights to flicker?

  3. Do I need any protection for the lights or the MOSFETs? I mean like a flyback diode...

  4. 20 lights PWMing at 1kHz on 8m long wire...Will that generate lots of EMI that could distract other devices in home? Btw. there will also be dimmable LED strips (12V, 4-5A) but hopefully not on a very long wire. As I have mentioned I plan to use 20 AWG cable (might use smaller diameter cables for shorter distances) for the lights and there seem to be lack of (or more an absence of) a twisted and shielded 20 AWG cable. On the other hand 20 AWG speaker cable is available everywhere. How much of a difference is there going to be between a speaker cable and twisted and/or shielded cable (if I can get hold of some)?

  5. Anything else that might cause problems of any kind you can think of?

Btw. I did not state it explicitly in the questions but I am not only looking for the answers but also for solutions. If you know how a particular issue might be fixed that is. I welcome and thank you for any comments/feedback.

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Wire capacitance at such current is negligibly small. Don't worry about this at all.

According to wikipedia - typical AWG 20 wire resistance is 33.31ohm/km.

With 8m distance current travels 16m of conductor total (there are 2 lines/cores in wire). 0,016km * 33 ohm = about 0,5ohm At 1A current you will loose U=I*R = 1*0,5 = 0,5V

Maybe use a bit thicker wire to reduce voltage/power losses? Additionally - resistance differences will dim your "far leds" more than "closer leds".

Long unshielded wires with few Amperes PWM signal will emit some EM indeed, and you may hear it in your audio equipment. To aviod that you may put transistors at leds, and use 2 wires - one coaxial for just PWM signal and AWG for power. At the end of power wire and before transistor put electrolytic capacitor to reduce EMC noise emitted by power wire. Like this (thats just idea visualisation):

enter image description here

You have very small inductance there, you don't need flyback diode.

Sorry if I did some language mistakes, my english is not perfect.

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  • \$\begingroup\$ According to this I will loose 1V on 20 AWG wire and 1A. But as I say, the LEDs consume 0.3A so the voltage drop will be around 0.3V. That should hopefully not cause to much of a brightness difference between near and far lights (maybe on very low brightnesses but I can compensate in code). And I can also definitely use a thicker wire if I find the voltage drop to be an issue. \$\endgroup\$ – Marek Apr 7 '14 at 11:01
  • \$\begingroup\$ Coax cable is an interesting idea, you mean like a standard 75 ohm cable? I would then need to buy twice as much cable :) but rather to invest now than have problems later. Would a shielded 20 AWG wire not be sufficient to reduce EMI? \$\endgroup\$ – Marek Apr 7 '14 at 11:14
  • \$\begingroup\$ I meant cheaper, thin cable. 75 ohm is pretty thick and too expensive for this. And yes, you can use shielded 20 AWG. I thought there will be more current. \$\endgroup\$ – Kamil Apr 7 '14 at 11:26

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