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[on:]

I currently have a project up and running using a single TLC5940 driving 14 3.0v blue LEDs, but I plan on swapping them out for RGB leds and adding another two 5940s to drive them all; my problem now is figuring out exactly how much power it's all going to require.

Would the power dissipation for the 5940, calculated from https://code.google.com/p/tlc5940arduino/wiki/PowerDissipation, plus the power draw of the actual LEDs, give me a rough estimation?  I intended to run it all from a 9v 1A power supply, but the regulated 5v from the Arduino has a 800mA limit, correct? 

Basically, I want to be sure I'm not going to blow anything up before I plug it all in.

Any help would be much appreciated.

[Reply #1 on:]

I currently have a project up and running using a single TLC5940 driving 14 3.0v blue LEDs, but I plan on swapping them out for RGB leds and adding another two 5940s to drive them all; my problem now is figuring out exactly how much power it's all going to require.

All you need to know is the LED current and the number of LEDs. Where the power is actually dissipated doesn't matter (in the TLC5940? In the LEDs? It's all the same to the power supply...)

14 x RGB x 20mA is 840mA.

A 1 amp supply will run it ... but running power supplies near 100% rated load isn't good for them. Get at least double that if you're planning to leave it switched on for long periods of time (eg. 1.5 - 2 Amps).

Would the power dissipation for the 5940, calculated from https://code.google.com/p/tlc5940arduino/wiki/PowerDissipation

That calculation is to make sure the chip doesn't overheat.

(Remember: The datasheet values are where the chip dies, not values to plan for...)

but the regulated 5v from the Arduino has a 800mA limit, correct? 

That's approximately where the thermal cutout will trigger, yes, but is it a good idea to run it that hot...?

[Reply #2 on:]

All you need to know is the LED current and the number of LEDs. Where the power is actually dissipated doesn't matter (in the TLC5940? In the LEDs? It's all the same to the power supply...)

Oh ok, so the TLC5940 doesn't actually use any additional power itself, it's just sinking what the LEDs need?

Because the RGB LEDs current will exceed the 800mA max on the Arduino 5v, that means I'll have to power them straight from the external power supply, yes?  But 9v would raise the power dissipation calculation for the 5940 substantially, so would I have to lower the voltage somehow first?

[Reply #3 on:]

Oh ok, so the TLC5940 doesn't actually use any additional power itself, it's just sinking what the LEDs need?

More like what the LEDs didn't need...

The important thing is how many amps are flowing down the path through the LED to the TLC.

Because the RGB LEDs current will exceed the 800mA max on the Arduino 5v, that means I'll have to power them straight from the external power supply, yes?

Yes.

But 9v would raise the power dissipation calculation for the 5940 substantially, so would I have to lower the voltage somehow first?

Good observation!

IIRC a fully-loaded TLC5940 can safely dissipate about 3V per pin at 20mA. With a 9V supply there'll be about 5.4V left over from the blue/green LEDs and about 6.8V from the reds- too much!

To save the TLC5940 you should add a resistor on each LED to throw some of that away. A 220 Ohm resistor will drop 4.4V, that should be enough.

[Reply #4 on:]

To save the TLC5940 you should add a resistor on each LED to throw some of that away. A 220 Ohm resistor will drop 4.4V, that should be enough.

Would dropping the 9v down to ~4 for the LEDs using something like the cheap LM2596-based DC converter boards all over ebay be a viable option instead?  I'd rather not have to add so many extra resistors--was one of the reasons I went for the 5940 over the cheaper 595s in the first place.

[Reply #5 on:]

Would dropping the 9v down to ~4 for the LEDs using something like the cheap LM2596-based DC converter boards all over ebay be a viable option instead?  I'd rather not have to add so many extra resistors--was one of the reasons I went for the 5940 over the cheaper 595s in the first place.

Sure, or just use a 5V regulator - far cheaper (although not so efficient electrically).