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Wednesday, 25 June 2014

What is Pull Up Resistor ? How Does Pull Up Resistor Works ?

by realfinetime  |  in Pull Up Resistor at  08:57

Pull up resistors are resistors used in electronic logic circuits to maintain the inputs to logic gates at expected logic levels, if external devices are disconnected or high impedance is introduced. They may also be used at the interface between two different types of logic devices, possibly operating at different power supply voltages. Pull up resistor is connected as shown in the following image.

Pull Up resistor operation

When the switch is open, current will flow from the source to the logic gate shown by a red arrow and the voltage at the gate input is pulled up to the level of  Vin. That is, if  external devices are disconnected or high impedance is introduced, pull up resistor will pull the logic gate input voltage to HIGH level.

Similarly, if the switch is closed, current will flow from source to ground through the pull up resistor and the logic gate input voltage will be pulled to ground or LOW level
Applications of pull up resistors.

1. Used in electronic logic circuits to maintain the inputs to logic gates at expected logic levels, if external devices are disconnected or high-impedance is introduced.

2. In open collector circuits, by changing the value of pull up resistance, it can be used either for General Purpose Input/Output (GPIO) or can be used for Inter-Integrated Circuit (I2C). Value of pull up resistance will be 10K, if it is used for General Purpose Input/Output (GPIO) and the value of pull up resistance will be 2K, if it is used for Inter-Integrated Circuit (I2C).

Disadvantages of pull up resistors.

1. Extra power consumed, when current is drawn through the pull up resistor.
2. Speed of pull up resistor is less compared to active current source.

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IMPORTANT NOTICE

All the circuits, published in this blog is only after testing and getting proper results in my private lab. When you try these circuits, you should check the supply voltage, polarity of components, presence of childrens nearby and shorts in the circuits. This website will not be responsible for any harm happened to you or your components caused by your carelessness.

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