I want to use MAX7219 (datasheet) to control bunch of RGB LEDs. With common anode LEDs I should be able to use up to 16 3-color bulbs in following configuration:
simulate this circuit – Schematic created using CircuitLab
There would be probably needed a resistor to protect the pins DIG 6 and DIG 7.
Now the RGB LED has drop voltage 3.2V on green/blue and 2.0V for the red color at 20mA. This would result in noticeable brightness difference of the LEDs. Is it possible to add 60Ω resistor to each red rail to fix the brightness of red color? The resistor is calculated from the voltage difference (3.2V-2.0V=1.2V) and driving current of 20mA. Given that only one LED is on at each time, this should be enough.
My goal is to control at least 16 RGB LED, in such way that I can control color of each bulb.
Best Answer
Now the RGB LED has drop voltage 3.2V on green/blue and 2.0V for the red color at 20mA. This would result in noticeable brightness difference of the LEDsWhy would it? A constant 20mA through the segment is a constant 20mA, regardless of the led Vf or Vs. The internal driver might dissipate more energy but thats mostly negligible.
If you already tested it and thing there is a higher current through the red leds, a silicon diode or two will bring the total Vf of the segment closer to the 3.2V, better than a resistor would, in a constant current driver setup.