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pullup and pull down resistors

Discussion in 'Electronics Homework Help' started by PRIYADHARSHINI, Nov 7, 2014.

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  1. PRIYADHARSHINI

    PRIYADHARSHINI

    137
    8
    Feb 6, 2014
    hi,

    actually this is the basic question...how can we choose the pull down and pull up resistor values for transistor to act as a switch?
    can we calculate from the datasheet.if it so from which parameter we can calculate?

    thanks in advance...
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    11,413
    2,619
    Nov 17, 2011
    R=V/I

    You know the voltage across the resistor and you know the current that is required to drive the load that is connected to the pull-up or pull-down resistor.

    Example:
    Your supply voltage is 5V.
    You use a pull-up to drive a line high.
    One input is connected to this line, the impedancce of the input is 10kΩ.
    A minimum logic level of 3.5V shall be required for the input to recognize a logic high signal.

    From these parameters we get:
    - max. voltage drop acrosss the resistor: Vr=5V-3.5V=1.5V
    - input current into input: Iin= 3.5V/10kΩ (since 1.5V drop across the resistor, the remaining 3.5V are on the input)
    - Rpull-up= Vr/Iin=1.5V/(3.5V/10kΩ)=0,428*10kΩ=4.28kΩ

    Choose the next lower standard value, e.g. 4.22kΩ

    Edit: For safe operation including worst case conditions one would typically use a much lower resistance, e.g. .2.2kΩ. Also to be taken into consideration is the load capacitance (capacitance of the wiring and the input) and required rise time of the signal: The capacitance and the pull-up resistor build a low pass with a time constant of t=R*C. The faster the signal is (short rise time), the smaller R needs to be (given that you cannot easily change the capacitance).
     
    Last edited: Nov 8, 2014
  3. Laplace

    Laplace

    1,252
    184
    Apr 4, 2010
    Absolutely not! The transistor datasheet will give maximum/minimum/typical values for its performance parameters. To choose pull-up/down resistor values one needs to look at the driven load for its device requirements. The transistor datasheet will only help you decide whether it can be used as a switch for the driven load.
     
  4. PRIYADHARSHINI

    PRIYADHARSHINI

    137
    8
    Feb 6, 2014
    thanks...i understood the concept..
     
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