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I'm working on a hobby project where I will be using an Arduino board and a TLC 5940 to control 5 different groups of RGB LED's. The number of LED's in any particular group will not necessarily match the number in another group i.e. there may be 20 in one group but 24 in another. I'm trying to figure out the best way to wire up these LED's. I'm probably going to have on the neighborhood of 50+ LED's total so using one TLC pin for each pin on the LED's is completely out of the question. If I can do it I would like to only use one TLC chip though that is not a requirement; I will start to run into space issues if I need to use more than 2 or 3 though. Ideally with 5 groups of LED's and 3 pins per group I could do it with only one TLC chip since they have 16 channels. I'm pretty new to electronics so I'm trying to figure out the best way to wire this up. They are all going to be powered from a hobby RC battery pack, probably from an RC helicopter or something.

My LED's want 30mA, 3.2v, and the TLC is a constant current driver so if it was just one LED I'd rig up the correct resistor to make it 30mA and go. But since I need to run a bunch of LED off of one pin I have a couple choices.

If I have a 20 LED group and run them parallel then I would need to set the TLC for 600mA correct? And if I run them all series I would need a 64 volt power source; is that correct?

So do I want to do those or some combination of series-parallel. Current RC battery backs like the ones I'm considering seem to be 7v or 11v.

As I said this is a hobby project so absolute stability is not an overwhelming concern. That in mind is the TLC enough of a current limiter for the application or will I want to be adding additional ones inside whatever circuit I work out? Like these LM317 chips: http://users.telenet.be/davshomepage/current-source.htm

Also am I correct in assuming that if one of my groups has 24 LED's and another has 20 I can have the second group mimic the first by adding in 4 resistors that drop 3.2v each? That way I could have both groups run off the same TLC chip since they would need the same current limit.

My guess is that I'll be best off running them all parallel and once I know exactly how many LED I will have I'll need to do the heat dissipation math for the TLC to figure out how many chips I need to spread it across. Is there a better way?

As I said I'm still new to electronics. Your patience and support is appreciated. If I'm missing any obvious considerations or am completely off in my understanding of how this works please let me know.

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What you have guessed is correct -- if you want to run the LEDs in parallel, then you need to be able to source 600mA. If you run in series, then your power supply needs to supply at least 64V. I don't know the specifics for selecting the ideal power rail, but I would certainly shoot for >64V and put in the current limiting resistor.

A resistor on its own doesn't "drop 3.2V each" -- the voltage drop across a resistor is IxR, so the actual drop depends upon the current going through the circuit. You can calculator or measure the current going through your other group(s) of LEDs, and then pick the appropriate resistor for the mismatched group so that the current is the same.

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I think you are right running them in parallel; it will be much simpler. On the 20 vs 24 issue, I would just pick the current for the 20; the 24s will be dimmer but it likely won't be noticeable. If it is, a single resistor that pulls 120mA at 3.2V would work.

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  • \$\begingroup\$ Good note on the 20 vs 24 issue. I will actually probably have one group that is only 4 though so there will be a 4, 20, 24, etc. I'll definitely need to put a couple resistors on the group of 4 to bring it closer to the other groups but if the others are all within a few LED of each other I'll just put the current for the smallest group and see how it works out in a test setup. \$\endgroup\$ – William Sep 30 '12 at 15:39

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