loading

Resistor values for common anode RGB LED?

Hi,  I have 10w RGB common anode LEDs. Forward Voltage is R:6-8v, G:9-12v, B:9-11v. Forward current is 300ma.  I calculate resistor values as R:15ohms, G:1Ohm, B:3.9Ohms.  This is based on max Forward Voltage.  I was told I wouldn't need resistors but clearly that is not the case as in one of the LEDs the red has failed.  Is it as simple as putting the correct value resitor between each cathode and supply?  Also, if I were to increase the resisors to lower the voltage a little (i.e. R:7v=18Ohm,  G:11v=3.9Ohm B:10v=6.8Ohm) would that help keep the heat down?

Many thanks,

sort by: active | newest | oldest
These big LEDs are not well controlled with resistors, you would be better using a circuit like the one I've attached, and an LM317 for each channel. You have no choice about the heat, its an inevitable consequence of what you have to do. Unless you use switching regulators, all you can do is add a heatsink.
trimmable current source.JPG
Summerly (author)  steveastrouk4 years ago
Thank you for your kind response. One last question does the LM317 need heat sinking in this application?

Just for info: I found a cheap RGB colour changing lamp (see image). The colours were controlled by IR remote. I wanted to sync the colours using DMX so I removed the ir circuit which just left the RGB Led and the fitting (which is aluminium and acts as a heat sink). After getting the specs and being assured it was OK by the supplier I applied the decoder output directly to the LED chip, but the red channel became damaged. Also the colours were slightly different to the lamps designed for purpose. Hence my question. Hopefully I can make up three circuits, pop one on each cathode and squeeze them into the fitting. I'll let you know how I get on. Thanks again.
RGB.jpgDecoder.jpg
I need to see the suppliers full notes to work out what's happened: whether the LED'd dead, or the controller channel.
Summerly (author)  steveastrouk4 years ago
Hi, The supplier only gave me the forward voltage for each channel and the forward current. The actual LED has 3 chips per colour and it was one of the chips on the red channel that failed (See image). So I still have some red channel but it is not as bright. As it was ruined I continued to run it directly from the controller and no further damage has been done (About 8 hours so far on a colour cycle). The DMX controller works fine as I tested it with some RGB strips.

One of the things that confused me since you pointed me in this direction was that constant current driver diagrams I have looked at since were based on pin3 on the anode, but these LEDs are common anode so I need to apply it to the cathode. I am now fascinated and can see many hours slipping away in search of understanding.....

Thanks again for your time.
LED.jpg
+1

There are dedicated led driver solutions, but this is the quickest cheapest option. (shy of a gigantic resistor)
BETTER than a gigantic resistor: remember the idea is to approximate a constant CURRENT source. The regulator is, as near as damn it, a perfect current source, in its compliance range, a resistor, unless its very big and the supply rail very high, is not.
Here's the maths
Copy of maths.jpg
Rsense = 1.22/ 300mA = 4 Ohms.