Pull Up Resistor Basics - Value & Function

In summary, a pull-up resistor is a type of resistor used in digital circuits to ensure that a signal is in a defined state when no input is present. It is typically connected between the input pin and the power supply voltage, and its value is chosen based on the specific circuit requirements. The function of a pull-up resistor is to provide a path for current to flow from the input to the power supply, thus pulling the input voltage up to the power supply voltage level. This ensures that the input is in a known state and prevents any floating or undefined values. Pull-up resistors are commonly used in microcontrollers and other digital devices to improve circuit stability and prevent false readings.
  • #1
zak8000
74
0
hi there

i have been reading up on the function of pull up resistors from the text:http://ti.tuwien.ac.at/ecs/teaching/courses/mclu/theory-material/Microcontroller.pdf and on page 37 it talks about it. in their example of the pull up resistor with the switch. i was wondering if they circuit would still work if the resistor was removed and if not what would be a suitable value for the resistor?
 
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  • #2
zak8000 said:
hi there

i have been reading up on the function of pull up resistors from the text:http://ti.tuwien.ac.at/ecs/teaching/courses/mclu/theory-material/Microcontroller.pdf and on page 37 it talks about it. in their example of the pull up resistor with the switch. i was wondering if they circuit would still work if the resistor was removed and if not what would be a suitable value for the resistor?

No, it wouldn't work.

With the resistor, the supply voltage appears across the switch when it is open, but no voltage appears across it when it is closed. So the voltage changes and the processor can use this to tell if the switch is open or closed.

Without a resistor, there is no voltage across the switch whether it is open or closed, so the processor can't tell if the switch is open or closed.

A typical value of pullup resistor is 4700 ohms.
 

1. What is a pull-up resistor and what is its function?

A pull-up resistor is an electronic component that connects a signal line to a positive voltage source. Its function is to ensure that the signal line is at a known state when no other active components are driving the line.

2. Why is the value of a pull-up resistor important?

The value of a pull-up resistor determines the strength of the pull-up effect. A higher value resistor will provide a weaker pull-up effect, while a lower value resistor will provide a stronger pull-up effect.

3. How do I calculate the optimal value for a pull-up resistor?

The optimal value for a pull-up resistor depends on the specific application and the electrical characteristics of the components involved. It is typically calculated using Ohm's Law, taking into account the desired voltage level and the current flowing through the resistor.

4. Can I use any resistor as a pull-up resistor?

While any resistor can technically be used as a pull-up resistor, it is important to choose a resistor with the appropriate value for the specific application. Using a resistor with too high or too low of a value can lead to undesirable effects, such as unreliable signal levels or excessive power consumption.

5. Are pull-up resistors necessary in all circuits?

No, pull-up resistors are not necessary in all circuits. They are typically used in digital circuits to ensure that signal lines are at a known state, but may not be necessary in analog circuits. It is important to consult the circuit diagram or datasheet to determine if a pull-up resistor is needed for a specific circuit.

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