Find LED current in switching circuit

Discussion in 'Homework Help' started by ShowerOf, Nov 17, 2009.

  1. ShowerOf

    Thread Starter Member

    May 4, 2009
    13
    0
    For the switching circuit shown:
    Vcc = 6V
    Base supply voltage (when ON) is 3.6V
    The LED has a forward voltage drop of 2V
    The transistor has a Vbe = 0.7V

    Question: Calculate the current through the LED when the switch is on.
    My calculations:
    Ve = Vbb - Vbe
    Ve = 3.6 - 0.7
    Ve = 2.9V

    Ie = Ve / Re
    Ie = 2.9 / 180
    Ie = 16.11mA

    So can I assume that current through the LED is approximately equal to Ie ?
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    Yes, you can assume that Ic=β/(β+1)*Ie
    So Ie≈Ic in active region.
     
  3. ShowerOf

    Thread Starter Member

    May 4, 2009
    13
    0
    To find the power dissipated by the transistor.
    using calculation based on another thread

    http://forum.allaboutcircuits.com/showthread.php?t=7426

    Vc = Vcc - VLED
    Vc = 6 - 2
    Vc = 4V

    Vce = Vc - Ve
    Vce = 4 - 2.9
    Vce = 1.1V

    P = Vce * Ie
    P = 1.1 * 16.11mA
    P = 17.72 mW

    Is this correct ?
     
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    Yes, look ok for me.
     
  5. ShowerOf

    Thread Starter Member

    May 4, 2009
    13
    0
    Thank you Jony130, I am grateful to have somewhere to get help and
    get my answers checked.

    Cheers :)
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    If you have 3.6V on the base of this transistor you will have 16.7ma through the collector. This configuration is a constant current source (BJT excel at those).

    If the power supply were 2V greater you could use any color LED, as it will still be a constant current of 16.7ma.

    Figure 3.6V (base) - 0.6 (BE drop) = 3.0V at the emitter
    3.0V/180Ω = 16.7ma.
     
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