Determining the proper resistance for row/column led array

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crazyjaw
 
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Joined: Fri Dec 27, 2013 6:38 am

Determining the proper resistance for row/column led array

Post by crazyjaw »

Im working on a project where i am making my own LED matrix, using row/column scanning for selection. This will be connected through an arduino (hopefully a "teensy" if it can handle it).

Im trying to figure out what the proper value for the resistance is. The formula (v=ir) in princple seems easy, and there are a lot of calculators out there, but i've been coming up with inconsistent results that makes me think i misunderstand things.

Its an oddly shaped array, but i am aiming for about a 10x10 of red LEDs:
voltage = 2.0-2.5 max
current = 20 mA

which i believe is a 150 ohm resistor for one LED.

However since there are ten in a column, does this mean i will potentially be drawing 10 x 20 mA = 200 mA? Does that mean I would need a large 1 watt 15ohm resistor, and wouldn't that overwhelm the pins on the arduino (i think i read somewhere they could only handle 70 or 100 mA)?

Just looking for the proper thought process on how to approach this sort of thing

Cheers

-Zak

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zener
 
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Re: Determining the proper resistance for row/column led arr

Post by zener »

That is not quite how it works. This is the best explanation I could find quickly:

http://www.radiolocman.com/shem/schemat ... ?di=105709

There are probably better explanations somewhere.

Basically, you have rows and columns. Often you have all the cathodes of a column connected together, with a NPN transistor at the "bottom".
Then all the anodes of a row are connected together. Since you only have one LED per column on at a time you only need current limiting for one resistor, so your 150 ohm would still apply. The transistors turn on in sequence, and the active rows change each time to create the pattern you are looking for.

Hopefully that makes some sense.

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