Electronic – What value resistors should I use for this LED

ledrgb

I'm trying to figure out the actual value resistance I need for the RGB LED I bought from here: http://www.adafruit.com/products/314#Description

According to most of the online calculators, for the Red LED I need 120R, Green needs 24R, and Blue needs 24R.

Can anyone confirm that these are suitable values to get both bright and even illumination? Also, I calculated these values taking the maximum voltage drop into account, so 2.5 for Red and 4.5 for Green and Blue. Is this the best way of calculating the values?

Best Answer

This is running on the assumption that you are using a supply of 5v.

For the resistor calculations you want to use Ohms law V = IR, where I is the current you want to flow through the LED and V is the voltage difference between what you are supplying and what the LED is specified for. The voltage you want to use is the typical voltage listed on the datasheet, so for the case of red you have (5v - 2.1v) = 20mA * R -> R = 2.9v/20mA = 145 Ohm. The green and blue channels are calculated similarly using a voltage of 3.8v and 2.7v respectively.

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