open collector problem

Discussion in 'Homework Help' started by braddy, Oct 4, 2008.

  1. braddy

    Thread Starter Well-Known Member

    Dec 29, 2004
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    0
    HI,
    I have his problem on open collector and I don't know how to deal with it:

    In the figure attached, the first transistor is driven by an output of the MCU, with the goal of turning the LEDs(several in parallel) on and off.
    Compute the values of R1,R2,R3 required to make the circuit work properly, if it can.

    Please can I have some suggestions meaning the step I need to do. Especially with the first resistor R1: If the output is low, The transistor inside the output of the MCU will sink current, letting T1 off I am right?

    Thank you
    B
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Yes you are right. But your LEDS won't light unless they have all the same characteristics (the important one is same voltage drop across them). This is almost impossible so you have to use a current limiting resistor for each led individually.
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    896
    The MCU sinks and sources 10mA. But it cannot source current when it is open collector. So the text does not make sense.

    It is easy to use simple arithmatic to calculate the value of R1 if it is needed and R2. Details about the forward voltage of the LEDs is required to calculate the value of R3.

    The spelling is wrong. It should be hFE, not hEF. Usually the hFE is spec'd for a linear amplifier transistor that has plenty of collector voltage. A higher base current is spec'd to saturate a transistor that is used as a switch like these transistors.
     
  4. braddy

    Thread Starter Well-Known Member

    Dec 29, 2004
    83
    0
    I am stuck with the case where the output of the MCU is 'on'. How do we compute the currents for the base current for T1 and at R1. I believe that the curre4nt at entering the MCU will be 10 mA.
    pleas can I have some help?
    thank you
    b
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    896
    For the LEDs to light then T2 has a collector current of 400mA and a base current of 20mA or 40mA. The base voltage is 0.9V so R2 is (5V - 0.9V)/20mA= 205 ohms or 103 ohms.

    T1 has a collector current of (5V - 0.1V)/205 ohms or 103 ohms which is 23.9mA or 47.8mA. The base current of T1 is 0.239mA or 2.39mA.
    R1 is (5V - 0.7V)/0.239mA= 17.99k or 1.799k.
     
  6. braddy

    Thread Starter Well-Known Member

    Dec 29, 2004
    83
    0
    Thank you Audioguru,
    but how do you know that T1 is in saturation?
     
  7. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    If you assume T1 as a short circuit the the maximum current through R2 is 5/R2.
    If T1's base current times its gain at saturation is more than 5/R2 then the transistor is in saturation.
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    896
    The gain of a transistor is not spec'd when it is saturated. Its gain is spec'd when it is a linear amplifier that has plenty of collector voltage (5V).

    Some small signal high gain transistors like the BC547 has a current gain of 300 and more but has its max saturation voltage loss listed when its base current is 1/20th of its collector current.

    Other small signal high gain transistors like the 2N3904 has its max saturation voltage loss listed when its base current is 1/10th of its collector current.
     
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