Strange reverse voltage on RGB LED's

Discussion in 'General Electronics Chat' started by Dewst3r, Jul 16, 2015.

  1. Dewst3r

    Thread Starter New Member

    Jul 16, 2015
    I have read things about RGB LED's using common anode/cathode, but I have a unique situation. In my computer case (phantom 820), there is a circuit board that controls the RGB LED's in the case. It only has a base, solid color mode (works perfectly fine) and I am planning on removing the board and instead using an arduino and transistors to create new color patterns. I have been playing around with the RGB LED's built into the case (testing voltage, trying to get the arduino to work with them).
    The board here has 4-pin connectors for each of the RGB LED's. The white connector is a strip of 5 RGB LED's, while the others are single LED's. One of the black cables has a solid white line on it, making me think that it would be the common ground for the LED while the other 3 would be for the Red, Green, and Blue. This, unfortunately has not turned out to be the case.
    The LED's are nothing special (as seen in the picture). Before I jump into specifics, I would like to say that my arduino code does work perfectly with my RGB LED I have lying around, and is not a programming/wiring fault.

    This is the pins that I believe may be correct on the LED wires
    Pin 1: Ground (it has the solid white line)
    Pin 2: Blue
    Pin 3: Red
    Pin 4: Green
    I am testing this with the header in the left center of the board (above the long header, its the small, 4 pin one)
    Pin 1 is at the top from the view in the picture for reference.

    When I began to measure the voltage to simply see what voltage the LED's ran at I noticed that when the LED color was blue and measured voltage with having my negative lead on pin 1, pins 2, 3, and 4 all resulted in positive voltage (blue beings around 3.3V, red being around 2V, and green around 1.6V). I found it weird that red and green resulted in voltage when the LED's were only displaying blue.

    When I measure the voltage at green I had my negative lead on pin 1 still, and resulted with positive voltage on blue (0.06V nearly off), negative voltage on red (-0.29V), and negative voltage on green (-3.5V)

    When I measure the voltage at red I still had my negative lead on pin 1, and resulted with positive voltage on blue (0.07V nearly off again), negative voltage on red (-3.5V) and negative voltage on green (-0.12V)

    If I flip the pos and neg leads for when the LED's are either green or red, it inverts all of my readings (negative blue, positive red and green)

    After seeing this headache, I tried it with my arduino and it completely flopped compared to when I was smoothly testing it with the RGB LED that I had ( I could not figure out how to get the lights to change colors. Trying to find a different ground only caused more pos and neg voltage readings.

    If anyone happens to know this issue or can help please do!
    Last edited: Jul 16, 2015
  2. wayneh


    Sep 9, 2010
    I think I'd use a 9V battery and a 2.2KΩ resistor to probe the LEDs, to map out which color is which wire and which polarities are required. A 5V USB supply would be even better, to reduce any risk of excessive reverse voltage on the LED.
  3. Dewst3r

    Thread Starter New Member

    Jul 16, 2015
    Yeah I think thats what I'll do. What would you say is the best way to go about that as in finding the ground out of the 4 wires?
  4. Brevor

    Active Member

    Apr 9, 2011
    It's possible there is no ground wire, the led's may be wired common anode and the control circuit grounds the led's.
  5. Dewst3r

    Thread Starter New Member

    Jul 16, 2015
    If thats the case, would I still be able to replace the board and use an arduino with it? I have never encountered something like that before.