# 250rgb leds with arduino duemilanove

Hi, I need conect 250 rgb leds in one arduino duemilanove using 74HC595 shift register. So what power supply I need and how to conect everything. Thanks.

You didn't say what kind of leds do you have (web link?), most common 20 mA version would require: 250 x 3 x 20 mA = 15 A, this is a maximum, probably you could be satisfy with less as not all leds powered simultaneously ( you always can control in software such limiting factor). http://www.elcojacobs.com/shiftpwm/

To minimize the number of data pins you'd generally want a square matrix. With 750 total LED's that would be roughly a 28x27 matrix. 27 is divisible by 3 so that makes it a good axis for RGB: 28 rows of 9R, 9G, and 9B. Since the 74HC595 is an 8-bit device it might make more sense to make the matrix 32x24 (4*8 rows and 3*8 columns). The row drivers will each have to handle current for 24 LED's (480 mA?). Since the 74HC595 can only source 35 mA you will need some transistors for current handling.

IMHO, there is a mistake:

With 750 total LED's that would be roughly a 28x27 matrix.

250 rgb leds must be considered indivisible, so the best matrix is 16 x 16 = 256. 16 rows and 16 x 3(rgb) columns. For switching a row current goes up 48 x 20 mA = 960 mA, and would require transistor or sourcing chip , some of them support parallel channel configuration.

Magician: IMHO, there is a mistake:

With 750 total LED's that would be roughly a 28x27 matrix.

250 rgb leds must be considered indivisible, so the best matrix is 16 x 16 = 256. 16 rows and 16 x 3(rgb) columns. For switching a row current goes up 48 x 20 mA = 960 mA, and would require transistor or sourcing chip , some of them support parallel channel configuration.

Half as many rows and twice as many columns as I recommended means the row drivers would have to push twice as much current. That will limit the choice of drivers.

Well, matrix 16 x 16 is square, and 32 x 8 is rectangular.
Itâ€™s also mistake to calculate a current in this manner, have to confess, I follow the same wrong path.
With matrix 16 x 16 current flows only 1/16 period of time , which is very low and to keep brightness more or less adequate, impulse of current must be in order of 16 times higher than nominal 20 mA. LED would not withstand more than 10 times overload of current, so about 60 % maximum level of brightness could be reached.
But with matrix 32 x 8 this duty cycle becomes even lower, 1 / 32 , which means brightness would fall to less than 30 %.
It is misconception, there is no gain in current flow w/o losses in brightness

To get close to 100%, better to configure matrix as 8 x 32 , where 32 is triple rgb. 13 IC, plus high current drivers, pumping impulse about :
96 x 20 mA x 8 = 15.360 A