Calculation – Pull Up/Pull Down Resistors

How to calculate value of resistors ?

It is obvious that every electronics device consume some current to operate and have some input impedance. And therefore voltage drop occurs across these resistors. These resistors should not be too low or high value otherwise it will result in misbehaving of the circuit you want to operate or you will not get the desired output.

The value of resistors should not be too high to restrict the current required by device and it should not be too low to allow excess current to flow through it else device will be damaged.

Example:

An IC input consumes 100uA current and Vcc is +5V. Calculate pull up and pull down resistors.

Solution:

Select Pull Up voltage = +4V

Current = 100uA=100×10^(-6) A

So you have voltage and current and you can calculate resistor using Ohm’s Law.

Resistor Relation

R=4/(100×10^(-6))

R = 400000 = 40Kohms

Pull Up Resistor

Pull UP resistors will be added as image below:

Image1: Pull Up Resistors

Select Pull Down voltage = +0.5V

Current = 100uA=100×10^(-6) A

So you have voltage and current and you can calculate resistor using Ohm’s Law.

R=0.5/(100×10^(-6))

R = 5000 = 5Kohms

Pull Down Resistor

Pull Down resistors will be added as image below:

Image1: Pull Up Resistors

Now you can calculate value of Pull UP/Pull Down resistors by yourself for your circuit.

NOTE: To calculate for yourself please make sure to read datasheet of the component you use for current consumption.

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