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Author Topic: Current gain (for multiple LED's)  (Read 576 times)
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Netherlands
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Hi all  smiley,

I need to wire up 15 common anode super flux RGB LED's to my Arduino Uno. Which is a problem, considering I'd need 900mA for this to work at it's maximum.

I don't need to control all pins individually, so I don't think I need a shift register. All LED's are meant to get the same color at the same time. (They have to be controlled with the PWM pins, however, because I want to control them very precisely.) There are also 6 to 12 sensors in the circuit, but on other pins, so not directly connected to the LED's. The LED's work at 20 mA per color, 2.0-2.2V R, 3.0-3.2V G, and 3.0-3.2V B. When white, they'd draw 60 mA per LED.

I thought that using an NPN transistor would be a good idea, but I don't know how - electronics has never been my strongest point. I've done my research, but I just can't seem to figure out how to maintain the correct output voltages, but enlarge the current to the value I want, and do it such that it doesn't blow up the Arduino. Any help would really be appreciated!
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It might be easier with a logic-level MOSFET. 

You don't mention the supply voltage.  Note that each individual LED will need its own current limiting resistor
unless you use a constant-current LED driver to control each individually.

For 0.9A there aren't too many transistors that can handle that current and require 30mA or less base current,
you are limited to either a darlington (loses about 1.2V from the effective supply voltage), or a super-beta
transistor (usually surface mount and/or hard to get hold of).

If you common up the red, green and blue LEDs you could use 3 NPN transistors (each at 0.3A), much easier,
a 2N2222 or equivalent would be OK, and you get control of the colour.

(2N2222 with 180 ohm base resistor should handle a 0.3A load)

A logic-level MOSFET with on-resistance of 0.2 ohms or less would be suitable.

Don't forget the individual series resistors.  You'll have to calculate the values from the relevant
voltages (depends on switching device and supply voltage)
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 MarkT's method is the way to go for DIY or R/D, however for mass produce product, the special purpose IC is shine. common anode RGB LED is not some thing you want to start with. Un-committed type is much better to deal. Here is the  try;-




more information is here:

http://arduino.cc/forum/index.php/topic,164552.msg1229401.html#msg1229401
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Maybe these diagrams will help:

common anode:


common ground:



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Wow, your replies really complete each other.

@MarkT: It took some time, but I understand what you mean. Really useful, thanks a lot!

@sonnyyu: I haven't thought about mass produce yet, thanks for that! It'll be added value to the project.

@xl97: Thanks for the schematics! I'm a visual thinker so it helped a lot.
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