# pullup and pull down resistors

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

1. ### 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 KappModeratorModerator

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

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

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

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