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Red Green Led 2 Leg

Discussion in 'LEDs and Optoelectronics' started by EdW, Feb 7, 2017.

  1. EdW

    EdW

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    Feb 7, 2017
    I have a circuit that changes polarity and it is in range of 6 to 12 dc. What would be a simple circuit so I could limit the voltage to use this led 3.3v light up red an green .
    Thanks
     
    Last edited: Feb 7, 2017
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Place a 470Ω resistor in series with the LED.
     
  3. Audioguru

    Audioguru

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    3.3V is probably the green forward voltage and the red will be about 1.8V to 2.2V.
    Use a single resistor to ground that is (12V - 1.8V)/20mA= 510 ohms or more. The resistor will dissipate (heat) with 0.2W so use a half-watt resistor.
     
  4. EdW

    EdW

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    Feb 7, 2017
    The led is green when + - and red when - + and the voltage is variable at times.
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Yes, we know
     
  6. EdW

    EdW

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    Feb 7, 2017
    Even with the voltage can be anywhere from 6 to 20v a one size res will work.
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Yes it will. The brightness will vary, but the LED will be safe.

    Does the variation in brightness concern you?
     
  8. EdW

    EdW

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    Feb 7, 2017
    You say res on ground side. Polarity is changing all the time. And how could I use a 5v regulator with the polarity changing every few minutes?
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The resistor can be on either side of the LED, it doesn't matter.

    The following circuit will limit the LED current to 10mA for an input voltage between +/-6V and probably +/-30V.

    To operate correctly at 6V the diodes should be Schottky diodes.

    IMG_20170208_095503.jpg

    Sorry about the quality.

    Edit: transistors can be BC548 or similar.
     
  10. EdW

    EdW

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    Feb 7, 2017
    Are you saying to just use a 6v Schottky. I can't find 6v Schotty do you mead zener?
     
    Last edited: Feb 8, 2017
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    No, I mean the 4 diodes shown on that circuit need at be Schottky diodes.

    Having them rated for maybe twice the max input voltage would be safe. In this case 20V or more would be fine.
     
  12. EdW

    EdW

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    Feb 7, 2017
    Thanks
     
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