# Safe to use series resistors for powering common anode RGB LED from single constant current source?

I have a 3-9W common anode RGB LED (https://cdn-shop.adafruit.com/product-files/2524/FD-9RGB-Y2.pdf) and want to vary each leg's voltage with PWM through FETs, namely IRLZ44Ns, and an Arduino, in a similar config to this: https://jasonwebb.io/wp-content/uploads/2010/12/schematic.png.

I am hoping to use a single current controlled source (https://www.ledsales.com.au/index.php?main_page=product_info&cPath=200_248&products_id=1508) and run each coloured element of the LED at 700mA by impedance matching each leg but am worried that the 2.1A from the source will just go through the path of least resistance when the source first powers on, and burn out the red LED. I was planning to limit the source to 5V (each element runs at below 4V) and calculated values of 2-5 ohms for each series resistor based on a 5V supply and the LED's different forward voltages.

I've been scouring Arduino and SE posts to find a solution but have come up with conflicting opinions as to the behaviour of the CC source, and also saw that current limiters on the output legs, like LM317s, might interfere with the high speed switching I need from PWM.

So I wanted to confirm if I am safe using series resistors in this configuration or if I should just use a 6-pin RGB LED and multiple CC sources instead. Thank you.

Edit: Added circuit diagram and my own measurements for the LED's forward voltage and current, as the datasheet is incorrect.

Sample resistor calc for red LED at 700mA: $$\ \frac{5-2.36} {0.7} = 3.77\Omega\$$

• Could you add a schematic to make it more clear what you are trying to do? Jan 9 at 4:39
• Ah sorry updated the post, FET resistors are just placeholders for now Jan 9 at 5:05
• Synchronize the PWM so that only ever one of the three LEDs is on. Jan 9 at 5:45
• core-electronics.com.au/picobuck-led-driver-34054.html looks like one of these is the solution Jan 9 at 6:33
• Those FET's are going to be pretty marginal for a power application when the gate voltage is only 5V. Jan 9 at 9:04