Transistor Base Current Help

Discussion in 'General Electronics Chat' started by Ndjs, Sep 26, 2012.

  1. Ndjs

    Thread Starter New Member

    Sep 26, 2012
    18
    0
    Hi guys,

    I am new to this forum and starting to play around with electronics.

    I have a question about BC547 Transistors.

    Using the following data sheet

    http://www.futurlec.com/Transistors/BC547.shtml

    Using a 9 V DC supply and the Emitter tied to ground and the collector having a RED LED and a resistor in series with Vcc.

    Would I be able to turn the led off if I had a big enough Potentiometer? I have tried 100K pot and it dims it only not turning it completely off. So I am guessing the answer is yes.

    My main question is what current do I need the base to be for the transistor to be open? And where does it indicate this on the data sheet.
     
  2. ramancini8

    Member

    Jul 18, 2012
    442
    118
    Please supply a circuit schematic. Zero base current means zero collector current and an LED in the collector circuit is off. Sufficient base current turns the transistor and LED on.
     
  3. Markd77

    Senior Member

    Sep 7, 2009
    2,803
    594
    If you connect the two ends of the pot to ground and Vcc and the centre to the base it is possible to turn it off completely.
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    Make sure you have some resistance in series with the base, otherwise you can zap the transistor when the pot is at the top of its setting.
     
    Markd77 likes this.
  5. Markd77

    Senior Member

    Sep 7, 2009
    2,803
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    Well spotted, I hope there weren't any toasted transistors.
     
  6. ramancini8

    Member

    Jul 18, 2012
    442
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    It is also possible to burn out the transistor.
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    896
    Yes if the potentiometer is turned up too high then the entire power supply will burn out the base-emitter of the transistor.

    As said earlier, the base needs a series resistor to limit the current.
     
  8. Ndjs

    Thread Starter New Member

    Sep 26, 2012
    18
    0
    Thankyou for your posts

    There has to be a cut off point where there is not enough current to turn the transistor on. If I put a 2megohm resistor in series with the base the LED turns off.

    Does that mean the transistor has turned off ?..

    If this is a 9 V supply The current in the base = 9/10^6 = 4.5x10^-6 (4.5uA)

    Or do we just say that IB (No current in base) the transistor does not switch.

    I guess my main point Do we know what current when applied to the base will switche the transistor
     
  9. Ndjs

    Thread Starter New Member

    Sep 26, 2012
    18
    0
    What is the sufficient current point?
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    A transistor is a linear amplifier if it has plenty of collector to emitter voltage so it is not a saturated switch. Then its collector current is hFE x base current. hFE is a range of numbers because each transistor is different and temperature also changes hFE.

    But you want the transistor to be a saturated switch with an extremely low collector to emitter voltage so hFE cannot be used in the calculation.
    The datasheet for nearly every transistor shows that it works very well as a saturated switch when its collector current is 10 times its base current.
     
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