Electronic – arduino – draw 30W @ 5v to run LED light string and Arduino from a car battery

12varduino

I've got a set of LPD6803 LED lights controlled by an Arduino that I'd like to connect to a 12v system powered off an auto battery. The lights can run off 5-12V but this particular Arduino board is spec'd for only 4-8V Vin.

The total draw of the lights will be something under 30W. My guess is significantly lower, but I'm basing this on the spec sheet. (In testing I can run them off a 5V/2A AC adapter and it works fine)

Any ideas for the simplest way to power both from the single 12v source? It seems like the lights can run off a 12V circuit while the Arduino runs off a 5V circuit. Or I can run them both off 5V, if the 5V source can provide this power. I'm pretty sure a 12v->USB plug probably wouldn't. Does a 7805 apply here?

(I also have an inverter + AC adapter solution I could use but it's quite a hack!)

Thanks!

Best Answer

You can use a 7805, but only to power the Arduino; the LEDs should then be powered directly from the 12V supply, since the 7805 can only supply 1A. A higher current, if possible, would also require a considerable heat sink.
And that's where a switcher comes in. Since the lights can run on 12V, but also 5V, there will be huge power losses when running off a 12V supply. 30W at 5V is 6A, and 6A over (12V - 5V) is 42W wasted. The best solution would be to have a switcher that gets you the 5V for both Arduino and LEDs, and you won't get this kind of power loss. Switchers for these voltages can reach efficiencies of more than 85%, so consuming 30W will result in a loss of 5W, down from 42W.

National's Webench designer suggests a design around an LM2743 which is even 94% efficient (only 2W loss in regulator).
Linear Technology also lists several devices that can handle the job.

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