RGB LED multiple color combination problems

Discussion in 'The Projects Forum' started by benb88, Jan 7, 2016.

  1. benb88

    Thread Starter New Member

    Jan 7, 2016
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    Hi, I have a common anode RGB LED (591-3501-833F) that I can't get to do color mixing. The anode is limited by a 1k resistor, and each cathode is hooked up to a port on an ATMega328p processor. See the attached schematic. By putting any one of the 3 digital pins LOW in my Arduino program I can get one of the colors to light up. But I can't get multiple colors to light up for color mixing, and if I set the red channel LOW with either of the others also LOW, then only the red shows up. What am I doing wrong?
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    3.3V is too low for the green and blue LEDs.. Their Vf is 3.4 to 4.05 volts.. Use a 5v supply instead
     
  3. benb88

    Thread Starter New Member

    Jan 7, 2016
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    I'm sort of new to circuits, so I don't completely understand the concept of Vf. However, the green and blue LEDs are working when individually illuminated, and are plenty bright. Does the voltage maybe drop too low when more than one LED is opened up?
     
  4. ScottWang

    Moderator

    Aug 23, 2012
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    You may use a 6~10 Ω to try, 1K is too high for the led and 3.3V power.
     
  5. benb88

    Thread Starter New Member

    Jan 7, 2016
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    Just for reference this is a design that I inherited from someone with more circuits experience than I have. I've read some places that using a current-limiting resistor on the common anode is bad because it might be too much current for one of the diodes but not enough for others. If I do want to stick with a resistor on the common anode (since the traces are already made on a PCB), how would I choose a value?
     
  6. hp1729

    Well-Known Member

    Nov 23, 2015
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    Yes, higher voltage than 3.3 V. Maybe the common anode tied to +V and resistors on the cathodes, selected for a maximum of whatever the data sheet suggests (20 mA ?).
     
  7. ScottWang

    Moderator

    Aug 23, 2012
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    Normally you should use three 6~10 Ω resistors in series with each led in the cathode.
     
  8. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    With R in anode, grounding the cathodes brings anode down to lowest Vf which is the red, leaving the B G high & dry. Might connect anode to PS add appropriate Rs to each cathode as post #6. If 3.3 V individually lights all LEDs then you can use 3.3 V PS. At 1 mA red is not very bright ?? Might try 81 ohms for R & 10 ohms for B & G. or what ever gives pleasing brightness.
     
  9. dannyf

    Well-Known Member

    Sep 13, 2015
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    The red led has the lowest Vf so when it is pulled low, it dominates.

    The correct way is to tie each cathode to your mcu through a resistor (so 3 resistors), and tie to the anode to Vcc directly.
     
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