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RGB LEDs in series - turn Green on

  1. Oct 8, 2008 #1
    Hello,
    I have a high power RGB LED with common anode.
    I want to have 3 of them in series, and put them on 15V power supply.
    And I want to switch green color on, and red color as well (something like a normal traffic light)

    Here is the problem:
    Given that three LEDs are in series, and are common anode, how can I switch color separately?
    I was thinking about placing 6 transistors between them (on cathode of each LED - e.g. 1LED= 2 transistors for each color). I assume P-channel MOSFET (i.e. IRF9250) can do the job?
    However, this is quite bad solution, does anyone have anything better?


    I hope I explained it well,
    I will post a drawing later on.
    Thank you in advance,
    M.
     
  2. jcsd
  3. Oct 8, 2008 #2
    here is the schema, sorry for making it by hand.
    hopefully it is understandable.
    All I want is to switch between all green and all red.
     

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    Last edited: Oct 8, 2008
  4. Oct 8, 2008 #3
    Why do you need six switches? You could connect the green LEDs in series to one switch at the ground, and likewise with the reds.
     
  5. Oct 9, 2008 #4
    Thank you for the reply.
    unfortunalely, thats cant be done since RGB LEDs are "common anode".
    If I did what you suggest, green and red LEDs would light at the same time.
    Dont forget that left and right diodes are in one package (denoted as slashed rectangle on the drawing), having common anode.
    Or am I missing something?
    Cheers
     
  6. Oct 9, 2008 #5
    Never mind, I missed that. The next simplest option would be to go parallel. Tie all the anodes together and connect them to plus of the power supply. Then connect the greens together to a resistor and to the on/off switch and the ground. Same with the reds.
     
  7. Oct 13, 2008 #6
    you are right, but the leds are high-power (current 0.4A) so the resistor would get hot pretty much.
    What I tried to do is the following scheme. But T3 is getting very hot when the current 0.4A flows through it. T1 is ok - they both are connected in the same manner (at least i think).
    Does anybody know what the problem is? I probably miss something. Are the transistors wired properly? The gate of the resistor is still on +15V. When I ground it, the transistor should switch on, am I right?

    thanks in advance,
    regards M.
     

    Attached Files:

  8. Oct 13, 2008 #7
    you might get some insight if you measure Vgs and Vds on each
     
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