What does the following BJT configuration does ?

Discussion in 'General Electronics Chat' started by sanuzr, Jul 7, 2014.

  1. sanuzr

    Thread Starter New Member

    Mar 29, 2012
    7
    0
    Can anyone elaborate the following schematic (Attached) ? It is used to control the Backlite of an LCD. The Collector of the Transistor is connected to the cathode of LCD backlight. The LCD_BLED is the control Pin and VCC_SYS is the Supply voltage of the LCD (Anode of the LCD Backlight is also connected to VCC_SYS). What is the need of making the driving circuit so complicated,a simple MOSFET can not be used instead ?
     
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  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,501
    380
    hi,
    A NMOS FET with a logic level Vgs, would easily control the typical 100mA to 200mA of a LED back light.
    The circuit as posted has a number of problems.
    E
     
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  3. RichardO

    Well-Known Member

    May 4, 2013
    1,230
    382
    I think that the intent of the circuit is to supply a constant current to the backlight LED's. If you use a MOS-FET don't forget to include a current limiting resistor.
     
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  4. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    Sometimes for the stock of EE parts preparing to get more easier, and when the required quantity is enough, and then this is the way to do, but if just for some personal applications, maybe this is not a good way to do.

    [​IMG]
     
  5. sanuzr

    Thread Starter New Member

    Mar 29, 2012
    7
    0
    I was thinking, this circuit is to limit the Maximum Voltage delivered to the Back-Light.
     
  6. sanuzr

    Thread Starter New Member

    Mar 29, 2012
    7
    0
    Thanks ScootWang for the nicely description of the circuit. I liked the way you explained. Got a clear idea about the working of the circuit. Thanks again ! Cheers !!
     
  7. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    The zener voltage of the third circuit, it just labeled that for the zener function, normally, if we can't get a zener voltage we want then we will using the normal diode or schottky(0.4V) diode as a zener shows in the second circuit.

    I only can get the zener voltage the smallest as 2.4V in the local EE store, but i found the mouser has offer 1.8V zener diode.
     
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