RGB LED

Other Hardware LEDs and Multiplexing
1

I have a Common Cathode LED. Every thing I see says to use a current limiting resistor on each of the Anodes, that's three resistors. Why should I not use only one higher wattage resistor on on the Cathode and none on the anodes?

2

Hi, and welcome to the forum.

Vf (working voltage) of LED colours is not the same.
Red might be ~2.4volt, and blue ~3.3volt.
If you parallel a red LED with a blue LED, only the red one will be on.
Three resistors ensures even currents through the three colours.
Leo..

3

  1. Higher wattage resistor can be a problem.

  2. Current limiting resistors are intended to set the amount of current. The amount of current you want for one color being on is probably different from the current that you want for all three colors being on.

  3. The calculation for resistance depends upon the forward voltage of the LED. Forward voltage of a red LED is different from the forward voltage of a green LED is different from the forward voltage of a blue LED. Three resistors allow you to set things up independently for the three LEDs.

You might be able to get away with a single resistor rather than three resistors but resistors are cheap.

There are probably additional reasons but first I would want to know things like how you are powering the LEDs, how you are controlling the LEDs, and what is the brand and model number of the LEDs.

Have you done the calculations? What did you find out?

4

The RGB LED I am using is from a company called CHANZON. ON the package is states that the voltage drop for Red is 2.0-2.2V. FOr the Green and Blue is is 3.0-3.2V. I am driving these anodes directly from an Arduino UNO using PW outputs to adjust the brightness. My current limiting resistors are 220 ohms. Since I am controlling each color with software they all have the same resistor value. Yes resistors are cheep but they also take up space. I have had no issues with using only one resistor on the cathode lead and have used the same strategy with common anode LEDs. My question is mainly so that I might better understand what I am doing.
Thank you for your responses.

5

If what you are doing works fine with one resistor and that resistor can handle the heat dissipation then you can continue doing what you are doing.

6

My question is mainly so that I might better understand what I am doing.

The answer was in reply#1.

With only one resistor you turn on the red and none of the other two colours will turn on. You might be able to get the green and blue on at the same time but the brightness of one will depend on if the other one is on or not, because of the common resistor.

Their is NO problem with power dissipation on a standard 20mA LED.