Pull-Up Resistor: What is it (And How Does it Work)?

Contents

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Key learnings:

Pull-Up Resistor Definition: A pull-up resistor is defined as a resistor used in electronic circuits to ensure a known voltage state for a signal.
Function: Pull-up resistors prevent ambiguous voltage states in digital circuits by keeping the voltage controlled when a switch is open.
Avoiding Floating Conditions: These resistors help avoid floating conditions where the circuit can’t decide between high or low states.
Short Circuit Prevention: Pull-up resistors prevent short circuits by avoiding direct connections between input pins and ground or supply voltage.
Common Applications: They are used to interface switches with digital circuits, in I2C protocol buses, and in resistive sensors to control current flow.

What is a Pull-up Resistor?

A pull-up resistor is used in electronic logic circuits to ensure a known state for a signal. It is usually used in combination with transistors and switches to ensure the voltage between ground and V_cc is actively controlled when the switch is open (similar to a pull-down resistor).

A pull-up resistor is not a special type; it is a standard fixed-value resistor connected between a supply voltage and an input pin.

This can sound confusing at first, so let’s go into an example.

Digital circuits only understand high (1) or low (0) states.

In a 5V digital circuit, an input voltage between 2 to 5V is considered high, while a voltage between 0 to 0.8V is considered low.

But, due to any reason, if the available voltage at an input pin is between 0.9 to 1.9 V, the circuit will confuse to select a high or low logic state.

To avoid this floating condition, pull-up and pull-down resistors are used.

How Does a Pull-up Resistor Work?

A pull-up resistor is connected between the supply voltage and an input pin. This setup ensures the input voltage is stable when the switch is off.

The gate input voltage is pulled up to the level of input voltage when the mechanical switch is OFF. And the input voltage is directly going to the ground when the mechanical switch is ON.

A pull-up resistor is connected with a switch to ensure the voltage level. The switch controls the input state of the circuit.

In place of a mechanical switch, a power electronics switch can also be used in the circuit.

The pull-up resistor is also used to avoid short circuits because the pin cannot directly be connected with ground or supply. If the pull-up resistor is not connected, it may result in short-circuit or damage to other components of circuits.

Pull-up vs. Pull-Down Resistor

The difference between pull-down and pull-up resistors is as shown in the table below.

How to Calculate Pull Up Resistor

Ohm’s law calculates the value of a pull-up resistor. The formula of a pull-up resistor is as shown below equation.

$R_{pull-up} = \frac{V_{supply} - V_{Hmin}}{I_{sink}}$

Where,

V_supply = supply voltage

V_Hmin = minimum voltage required for high (1) state

I_sink = minimum current sink by the input pin

Applications of Pull-up Resistors

Pull-up resistors are more used than pull-down resistors. The application of pull-up resistors is as listed below.

Pull-up resistors are used as interfacing devices between a switch and a digital circuit.
It is used in the I2C protocol bus to enable a single pin to act as input or output.
In resistive sensors, it is used to control current flow before analog to digital conversion.

	Pull-up Resistor	Pull-down Resistor
Input Stability	It is used to ensure an input terminal is stable at a high level.	It is used to ensure an input terminal is stable at a low level.
Connection	One terminal is connected with V_CC.	One terminal is connected with the ground.
Circuit Diagram
When a switch is open	Current path is V_CC to an input pin. Voltage at input pin is high.	Current path is input to ground, and voltage at an input pin is Low.
When a switch is close	Current path is V_CC to input pin to ground. Voltage at input pin is low.	Current path is V_CC to an input pin. Voltage at input pin is high.
Used	More commonly used	Rarely used
Formula	$R_{pull-up} = \frac{V_{supply} - V_{H(min)}}{I_{sink}}$	$R_{pull-down} = \frac{V_{L(max)} - 0}{I_{source}}$