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The LED is lightly glowing when switch off state. Please explain

Discussion in 'LEDs and Optoelectronics' started by KRK, Jan 9, 2018.

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  1. KRK

    KRK

    27
    0
    Jun 20, 2017
    Hi,

    Please find the file. When V1 is 3.3V, the output of the transistor Q2 will ON and get the 3.3V at the collector and D2 LED will glow.

    When V1 is 0V, the output of the Q2 transistor will low and D2 LED glow very little on that. But I need 0V apply it should off perfect.

    Please help and why this happen to glow very little. image1.png
     
  2. dorke

    dorke

    2,342
    665
    Jun 20, 2015
    Looks like you don't quit understand this circuit.

    "When V1 is 3.3V, the output of the transistor Q2 will ON and get the 3.3V at the collector and D2 LED will glow."
    WRONG!
    But D2 will glow.
    That is because it will conduct through the collector of Q1.
    Q1 is on,Q2 is off.

    "When V1 is 0V, the output of the Q2 transistor will low and D2 LED glow very little on that.."
    WRONG!
    But D2 will glow very little .
    That is because it will conduct through the base of Q2.
    Q1is off,Q2 is on.
     
    Last edited: Jan 9, 2018
  3. WHONOES

    WHONOES

    952
    249
    May 20, 2017
    Correct. With Q1 not conducting current will pass through the base emitter junction of Q2. This is because in this state base emitter of Q2 is analogous to a forward biased diode.
     
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