Tri-colour LED help..

Discussion in 'General Electronics Chat' started by peck68, Feb 14, 2010.

  1. peck68

    Thread Starter Member

    Nov 27, 2009
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    Well, i bought these Tri-Colour LEDs today (red green + yellow) - but they only have 3 poles on them?

    Apparently the middle pole is the common anode (judging by the length compared to the others) and the other 2 are cathodes. I can get the Red and green to show - but i don't know how to get yellow?
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    Light both the red and the green to get the yellow.
     
  3. peck68

    Thread Starter Member

    Nov 27, 2009
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    Hmm i try and do that - but the green turns off and the red stays on. If i took red out then green turns on :(
     
  4. mik3

    Senior Member

    Feb 4, 2008
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    Post the datasheet or part number of the led and your circuit.
     
  5. El3ctroded

    Active Member

    Feb 4, 2008
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    Yellow Red and Green doesn't make sense. You can't get green from yellow and red, and you can't get red from yellow and green, and you can't get yellow from red and green...

    It sounds like they were labeled wrong, you got the wrong thing, or something like that. Most tri-color leds are yellow, red, and orange, or Red, Green, and Blue.

    I think what you got is a yellow-green and Red LED. In other words, the green is slightly yellowish.

    And you may be turning both leds on at once, but when one is one you maybe can't see the other...
     
  6. ELECTRONERD

    Senior Member

    May 26, 2009
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    All the LEDs should share a common cathode or anode. The first ternimal would be used to light up the first color, the second ternimal for the second color. Then by lighting up both the first and second ternimal, you can acheive the third color. All you need to find out is whether they share the same cathode or anode.

    Austin
     
  7. peck68

    Thread Starter Member

    Nov 27, 2009
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    Sorry forgot to say i found out how :)

    Green was also Yellow at a higher voltage (3.5v instead of 2.5v)

    --

    Thanks all
     
  8. ELECTRONERD

    Senior Member

    May 26, 2009
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    That's a very interesting method of acheiving the different colors. I wouldn't exactly say it's propitious in any regard. You'd have to implement some voltage regulator to produce the multiple voltage outputs; which isn't favorable.

    Austin
     
  9. eblc1388

    Senior Member

    Nov 28, 2008
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    The common consensus is that LED, as a diode, lights on the amount of current flowing pass it.

    If one applies a constant 3.5V to the green LED via a voltage regulator, the LED will burnt out soon.

    P.S. To light both red and green LED together, use separate current limiting resistor for each LED. A green LED parallel with a red LED would render the green LED not lit because the voltage across the red LED is lower than the Green.
     
  10. Ron H

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    Apr 14, 2005
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  11. retched

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    Dec 5, 2009
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    Do you have a link to the datasheet on these LEDs? Im interested to see if the voltage drop from powering the red and green at the same time (losing the red) gets the green to the yellow stage. (or yellow to green) . It would be an interesting circuit to see, if that is the case.

    Unless the LED is special use. IE the circuit it was produced for has a "dead battery" voltage of 2.5 showing yellow. An all good "on" voltage of 3.5 showing green.

    Or is could be a two color "yellow-green" and "red" as already stated earlier. You may be overdriving it at 3.5v pushing it into a higher wavelength.
     
    Last edited: Feb 14, 2010
  12. peck68

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    Nov 27, 2009
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  13. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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    Sometimes the term yellow is used, but it's actually orange.

    As an experiment: use a 9V battery and connect a 1K resistor between each cathode and the (-) battery terminal (note that's two 1K resistors). Connect the (+) battery terminal to the Anode. With both LEDs on, the color will be orange.
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    I have some common anode tri-color LEDs I'd bought elsewhere a few years back.

    Just took one and measured the Vf using a 510 Ohm 1/2 Watt resistor with a 12v supply, figuring that:
    1) I wanted about 20mA current to flow through each LED.
    2) The minimum Vf was going to be about 2v.
    3) (12v-2v) / 20mA = 10/0.02 = 500 Ohms; 510 Ohms was the closest standard value >=500 Ohms.
    4) Since power in Watts = Voltage x Current, 10v x 20mA = 200mW, doubling for reliability = 400mW; I happened to have some 1/2W 510 Ohm resistors handy.

    Measured Vf:
    Green:2.23v
    Red: 2.0v

    When I sank current from both cathodes with the single 510 Ohm resistor limiting current at the anode, the green LED was dimly lit, and the intensity of the red LED was reduced by roughly half.

    Anyway, once you determine your LED's Vf using a method similar to what I did, you can then calculate the resistors you'll need for whatever voltage supply you are going to use:
    Rlimit >= (Vsupply - Vf_LED) / 20mA

    For mine, were I going to use a 5v supply:
    RlimitRED >= (5v-2v)/20mA = 3/0.02 = 150 Ohms
    RlimitGRN >= (5v-2.23)/20mA = 2.77/0.02 = 138.5 Ohms; not a standard value. However, I could use a 100 Ohm and 39 Ohm resistor in series, or a number of other combinations in series or parallel, or I could use an E48 140 Ohm resistor.

    Standard decade tables of resistance:
    http://www.logwell.com/tech/components/resistor_values.html
    Resistors in series/parallel calculator:
    http://www.qsl.net/in3otd/parallr.html

    As has already been said, you will need separate current limiting resistors on the cathodes for the red and green in the LED.
     
  15. retched

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  16. Audioguru

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    The datasheet is written in Chinese. Adobe Reader wants me to download many Chinese fonts so I can read it but I can't read Chinese.
     
  17. retched

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    There is not one Chinese character in the whole file...100% English...and not bad English either...
     
  18. Audioguru

    New Member

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    Here is the warning I get from Adobe:
     
  19. retched

    AAC Fanatic!

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    well thats odd. There must be some PDF code or REMARKS in Chinese.

    Here:
     
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  20. SgtWookie

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    Jul 17, 2007
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    I got the same warning, but went ahead and installed the font. Like he said, it comes up in all English.
     
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