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2pins IR LED

Discussion in 'LEDs and Optoelectronics' started by vick5821, Oct 2, 2012.

  1. vick5821

    vick5821

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    Jan 22, 2012
  2. GreenGiant

    GreenGiant

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    Feb 9, 2012
    so the diode is reverse biased, so nothing will pass through it, and therefor the output will read nothing, when the diode "sees" the IR the diode essentially lets the voltage through

    Think of it like a transistor, you have 2 arms that wont pass anything but when the third is activated the voltage will pass between the first 2, typically these circuits use a transistor of some sort to create the switch
     
  3. vick5821

    vick5821

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    Jan 22, 2012
    but diode need certain voltage to be forward biased right ?
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    More precisely: Not the voltage but the current.
    When light of the correct wavelength (in this case infrared) falls on the photodiode a small photocurrent is generated within the photodiode. This photocurrent flows from the cathode to the anode and on through the load resistor. Across the load resistor the photocurrent produces a proportional voltage drop. See this explanation.

    Since the photocurrent is generally very small, you need a high resistance toi achieve a good voltage signal. Another drawback of this simple circuit is that the voltage acros the photodiode changes with the photocurrent. Mostly you will add a high input impedance amplifier to decouple the photodiode from the rest of the circuit and to amplify the photodiode's signal.

    Here are a few application examples.
     
    Last edited: Oct 3, 2012
  5. Harald Kapp

    Harald Kapp Moderator Moderator

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    Right, but you don't forward bias a photodiode. You use it in reverse bias.
     
  6. vick5821

    vick5821

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    Jan 22, 2012
    I do not get it ...why we used it in reverse biased ?
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    I told you that. Also read the explanation I linked.
     
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