LED Resistor values - Please check my calcs...

Forum 2005-2010 (read only) Hardware Troubleshooting
1

I'm using an RGBB LED (http://docs-europe.electrocomponents.com/webdocs/01ff/0900766b801fff40.pdf) and want to use the correct resistor values (instead of just chucking in a couple of 1k as I usually do!)

Now, using R = (Vsupply - Vled)/I I'm getting some really low values, as I say, I'm used to throwing a couple of 1k onto red LEDs.

At 20mA, with a 5V feed:

Red LED, requires 2V = 150 Ohm
Green LED, requires 2.2V = 140 Ohm
Blue LEDs, require 4.5V = 25 Ohm

Before I start blowing pins up can someone please check my figures?

Many thanks in advance, Easty.

2

the calculations look, although the blue led forward voltage seems a little high

3

Thanks mem, I'll give it a go and see if anything catches fire! ;0)

(Maybe I'll start a little higher)

4

Yes that's what the data sheet says. It is worth remembering that although we typically talk of 20mA per LED this is not normally what you want as it is too bright if on all the time.

A value of 20mA is useful when you are multiplexing the LEDs and they are not on all the time. Also there is not so much current as to blow them up if the multiplexing stopped for any software reason. As it would be the case if you used the higher peak current value.