RGB LED multiple color combination problems

Thread Starter

benb88

Joined Jan 7, 2016
3
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?
 

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Thread Starter

benb88

Joined Jan 7, 2016
3
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?
 

Thread Starter

benb88

Joined Jan 7, 2016
3
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?
 

hp1729

Joined Nov 23, 2015
2,304
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?
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 ?).
 

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ScottWang

Joined Aug 23, 2012
6,861
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?
Normally you should use three 6~10 Ω resistors in series with each led in the cathode.
 

Bernard

Joined Aug 7, 2008
5,368
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.
 

dannyf

Joined Sep 13, 2015
2,197
so I don't completely understand the concept of Vf.
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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