R calculation in attached circuit - help needed

Discussion in 'The Projects Forum' started by mishra87, Sep 10, 2016.

  1. mishra87

    Thread Starter Member

    Jan 17, 2016
    168
    2
    Hi All,

    please find the attached schematic and let me know how to calculate the R vale for below parameter.

    Les say Q119 =Q120 =BC547
    Vf = 1.9- 2.4V
    IF =15-20 mA

    How do calculate R value !!!

    Thanks in advance !!!
     
  2. mishra87

    Thread Starter Member

    Jan 17, 2016
    168
    2
    Comment awaited still in this particular post...!!!
     
  3. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,951
    387
    First, unless you select matched transistors the currents in the two strings will be different. It would be much easier to dispense with the transistors and use two resistors.

    Maximum voltage across each string of LEDs is 5 * 2.4V = 12V! So you don't have enough suply voltage to feed 5 LEDs in series and leave enough headroom for the voltage drop across the resistor. Note that this problem is even worse if you use the transistors as you lose 0.7V from the supply because of the Vbe of the transistor.

    Minimum voltage across each string of LEDs is 5 * 1.9V = 9.5V
    To restrict the current to a maximum of 20mA, we calcuulate at the maximum voltage across the resistor 12V - 9.5V = 2.5V
    The value of the resistor is then 2.5V / 20mA = 125Ω
    Now we can calculate the voltage available for the LEDs when the current is 15mA,
    12V - 125Ω * 15mA = 10.125, so the voltage for each LED will be ≈2V

    This value of resistor will safely limit the current to 20mA but note that if the LEDs need 2.4V they may not be very bright.
     
  4. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,951
    387
    This won't encourage answers, it just makes it look it has already been answered (1 reply).
     
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