Pull-Up And Pull-Down Resistors

Mon Nov 21 20:25:43 CST 2022

Pull-Up-And-Pull-Down-Resistors-HornmicLink

Pull-Up And Pull-Down Resistors

Most digital electronic circuits contain pull-up and pull-down resistors. They are used to properly bias the inputs of digital gates to stop them from floating randomly when there are no input conditions. Implementing pull-up and pull-down resistors in your circuit will allow you to reach a "high" or "low" state. If you do not implement it and nothing is connected to your pins, your program will read the "floating" impedance state.

Pull-up Resistors

The pull-up resistor is used to create additional loops on critical components while ensuring that the voltage is clearly defined even when the switch is open. It is used to ensure that the wire is pulled to a logic high level in the absence of an input signal. It is not a special type of resistor. 

They are simple fixed value resistors connected between a voltage source and the appropriate pin to define the input or output voltage in the absence of a drive signal. When the switch is open, the voltage at the gate input is pulled up to the level of the input voltage. 

When the switch is closed, the input voltage to the gate is directly grounded. When you have a low default impedance state and want to pull the signal to "high", you need to use a pull-up resistor.

Pull-down-Resistor-Schematic

Pull-up Resistor Schematic

In the diagram above, a fixed value pull-up resistor is used to connect the supply voltage to a specific pin in the digital logic circuit. The pull-up resistor is paired with a switch to ensure that the voltage between ground and VCC is actively controlled when the switch is turned on. At the same time, it does not affect the state of the circuit. If pull-up resistors are not used, a short circuit will result. This is because the pins cannot be directly shorted to ground or VCC, as this would eventually damage the circuit. Following the principle of Ohm's Law, if a pull-up resistor is present, a small amount of current will flow from the source to the resistor, then to the switch, and then to ground.

Pull-down resistors

And a pull-down resistor is used to ensure that the input to the logic system is stable at the expected logic level when an external device is disconnected or at a high impedance. It ensures that the connection is at the defined low logic level, even when there is no active connection to another device. 

The pull-down resistor holds the logic signal at near zero volts (0V) when no other active devices are connected. It pulls the input voltage down to ground to prevent an undefined state at the input. It should have a resistance greater than the impedance of the logic circuit. Otherwise, the input voltage on the pin will be at a constant logic low regardless of the switch position. 

When the switch is open, the voltage at the gate input is pulled down to ground level. When the switch is closed, the input voltage to the gate becomes Vin. Without this resistor, the voltage level would actually fluctuate between the two voltages.

Pull-up-Resistor-Schematic

Pull-down Resistor Schematic

Just like the pull-up resistor in the first diagram, the pull-down resistor in this circuit ensures that the voltage between VCC and the microcontroller pins is actively controlled when the switch is turned on. Unlike the pull-up resistor, the pull-down resistor pulls the pin to a low value instead of a high value. A grounded or 0V pull-down resistor sets the digital logic level pin to its default value or 0 until the switch is pressed and the logic level pin goes high. As a result, a small amount of current flows from the 5V supply to ground using the closed switch and pull-down resistor, thus preventing the logic level pin from shorting to the 5V supply.

Ideal resistance values for pull-up and pull-down resistors 

For pull-up resistors, a resistor that is at least 10 times smaller than the input pin impedance value needs to be used.

1. For logic devices operating at 5V, typical pull-up resistor values should be in the range of 1-5 kΩ.

2. For switch and resistive sensor applications, typical pull-up resistor values should be in the range of 1-10 kΩ.

For pull-down resistors, it should always be greater than the impedance of the logic circuit. Otherwise, it will pull the voltage down too much and the input voltage on the pin will remain at a constant logic low value regardless of whether the switch is open or closed.

Ideal-resistance-values-for-pull-up-and-pull-down-resistors



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By HornmicLink_Henry @221121 20:30