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I want to control a lot of LEDs (like 128) in an area of about 1 m2.

Instead of using separate wires, could I use a 28 AWG flat cable, and split/cut it wherever needed? E.g. split it in two wire groups and cut it wherever I want to place a LED?

Btw, I'm intending to use two MAX7219 ICs to control them using multiplexing.

According to the AWG table, I can use 0.226 A for each wire, which is enough (the LEDs I used are simple 3 mm LEDs and not intending to give them more than 0.2 A of current).

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    \$\begingroup\$ Depending on your application, controllable LEDs like WS2811 might be an easier solution. \$\endgroup\$ – David Dec 17 '18 at 23:29
  • \$\begingroup\$ @David True, however, I want only about 1 or 2 LEDs per square dm2, so I would need too many WS2811 and use only a few LEDs from them. \$\endgroup\$ – Michel Keijzers Dec 18 '18 at 0:43
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I think that you will be fine.

You should expect a voltage drop of ~90mV/meter at 0.2A.

According to :
Fermi National Accelerator Laboratory
Ampacity Test of 28 AWG Ribbon Cables (1990)
lss.fnal.gov/archive/test-tm/1000/fermilab-tm-1657.pdf

28AWG Ribbon cable with parallel runs has been tested to 2.5A per conductor in free air, leveling off at about 40C.

Try it and find out for yourself.

Add a sprinkling of multiplexing to reduce the duty-cycle, and even more headroom is added.

Go for it.

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  • \$\begingroup\$ Thanks for your answer, what do you mean with 'sprinkling of multiplexing' ? Like not giving all wires a current at the same time? That's the plan. \$\endgroup\$ – Michel Keijzers Dec 17 '18 at 13:16
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    \$\begingroup\$ Yes. One can also 'flash' the entire led array fast enough so leds appear to be constantly on. This can save on power, and heating, at the expense of complexity and, perhaps, noise. \$\endgroup\$ – Chris Knudsen Dec 17 '18 at 13:43
  • \$\begingroup\$ Ok, I don't think that will be needed, but good to know it is possible. \$\endgroup\$ – Michel Keijzers Dec 17 '18 at 13:45
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You need 16 wires from each MAX7219. 8 wires going to the 8 common cathode groups, and 8 wires going to the 8 anode groups. Only a max of 8 LEDs from each MAX7219 will be on at any one time (the 8 whose common cathode is low). I have wired up discrete LEDs with 30 AWG wire wrap wire with no issue, so 28 AWG will be fine. You might consider the same - spread out the LEDs in your 1m square, put on a header to receive the 16 signals from the MAX7219, and then connect everything up with wirewrap wire from there. Many LEDs have square legs on the bottom, you can wirewrap right on them, and limit the soldering to tacking them down to a island of holes perfboard, which is also safer for the LEDs. Point to point soldering with 28 AWG will be awkward.

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  • \$\begingroup\$ Thanks ... never used wirewrap before ... maybe I should try to find a 1 meter flat ribbon cable which has already a header connector on it. \$\endgroup\$ – Michel Keijzers Dec 17 '18 at 15:33
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    \$\begingroup\$ Take a look at peconnectors.com They have wirewrap wire and tools, I use the gold colored tool myself. Pick up some 34-core wire, couple of headers to crimp for whatever length you are using, search fr IDS34, and some male-male pins with longer legs to plug the cable onto, and wirewrap to on the other side. \$\endgroup\$ – CrossRoads Dec 17 '18 at 15:50
  • \$\begingroup\$ Looks interesting, I will check how it should be used. \$\endgroup\$ – Michel Keijzers Dec 18 '18 at 0:44

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