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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\$

Circuit Diagram

Forward V-I Curves

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  • \$\begingroup\$ Could you add a schematic to make it more clear what you are trying to do? \$\endgroup\$ Jan 9 at 4:39
  • \$\begingroup\$ Ah sorry updated the post, FET resistors are just placeholders for now \$\endgroup\$
    – trijoco
    Jan 9 at 5:05
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    \$\begingroup\$ Synchronize the PWM so that only ever one of the three LEDs is on. \$\endgroup\$
    – Janka
    Jan 9 at 5:45
  • \$\begingroup\$ core-electronics.com.au/picobuck-led-driver-34054.html looks like one of these is the solution \$\endgroup\$
    – trijoco
    Jan 9 at 6:33
  • \$\begingroup\$ Those FET's are going to be pretty marginal for a power application when the gate voltage is only 5V. \$\endgroup\$
    – mkeith
    Jan 9 at 9:04
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A constant current source is a voltage source with a feedback loop that regulates current. The purpose is to avoid the current limiting resistors that waste energy and/or provide more precise control of brightness without needing PWM.

You've designed a circuit that adds the PWM and current limiting resistors back in, in which case using the constant current source is unnecessary. Either use a constant voltage source, or get a constant current source with a PWM input to control brightness.

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  • \$\begingroup\$ Cool, thanks for the response, had a feeling something was off \$\endgroup\$
    – trijoco
    Jan 9 at 6:18
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PWM can regulate current with current sensing across a sense resistor.

The CC source uses PWM with an inductor and Cap with compensation for stability in the closed loop. It is intended for each color as a DCDC converter with 95% efficiency, not shared. If you want to vary brightness then current must be modulated.

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