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Power issues
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Power issues

by Atrum on Thu Jun 26, 2008 3:05 pm

My current project involves seven LEDs sucking out 200mA out of a pin. I know thats not possible, so I need to use a transistor.

Question is, do I need a transistor per LED or since its all parallel do I only need one per pin.

Secondly can the +5 power rail give off around 600 mA as long as I'm using the Arduino's power adapter?
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by magician13134 on Thu Jun 26, 2008 6:53 pm

If the LEDs are in parallel then the total current would be I1 + I2 + I3 + ... + I7, so you would need 200mA * 7, or 1.4 Amps from your transistor. I would recommend 7 transistors. Also, what kind of LEDs are they? 200mA would kill most LEDs, I think, unless they're being pulsed...
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by Atrum on Thu Jun 26, 2008 8:17 pm

They are 30mA LEDs but I'm running them in parallel so it needs around 200mA total per pin (channel). Im basiclly doing mood lighting with three channels Red, Blue and Green.
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by oPossum on Thu Jun 26, 2008 8:19 pm

One transistor per pin. A single 2N4401, 2N2222 or PN2222A can easily switch 200 mA.

Each LED should have it's own resistor.
Last edited by oPossum on Thu Jun 26, 2008 9:02 pm, edited 1 time in total.
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by oPossum on Thu Jun 26, 2008 8:24 pm

I think the voltage regulator would get quite hot with 600 mA continuous draw. An easy solution would be to power the LEDs directly from the AC adapter's output.
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by Atrum on Thu Jun 26, 2008 8:58 pm

I don't want one LED per pin, it will be a whole lot more coding, on top of that I'd want to put in Four buttons to control each LED and a setting where it just runs through each possible color.

There are seven LEDs per Channel. Both Green and Blue will take 175mA each and Red runs 102mA and a grand total of 452mA.

Oh in case your woundering Blue and Green run 3.3volts and 30mA each while Red needs 2volts and 30mA.

Blue and green channels require 68ohm resistors on each LED and Red needs 120ohm resistor.

I know I need to use resistors, but the main question is can I pull 452mA out of that +5 rail if the wall wart is plugged in.
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by magician13134 on Thu Jun 26, 2008 9:39 pm

Oh, it's 200mA total? I didn't get that. I thought it was 200mA per LED. Right.
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by Atrum on Thu Jun 26, 2008 11:00 pm

So can the power rail provide 542mA of power if theres and attached wallwart?
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by mtbf0 on Fri Jun 27, 2008 10:17 am

make sure what regulator you've got. an lm7805 is rated at 1amp, (good), an lm78l05 at 100ma, (oops). attach a heat sink. i've got a boarduino driving 18 leds and the regulator gets mighty warm.
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