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pullup and pull down resistors

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

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    Feb 6, 2014

    actually this is the basic 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

    Nov 17, 2011

    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.

    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


    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.


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