What are Pull-Up or Pull-Down Resistors?

A pullup or pulldown resistor prevents the voltage on a wire from "floating" and giving false indications. When the switch is open, a pullup resistor "pulls" the voltage to the power supply, a pulldown resistor "pulls" the voltage down to ground potential.





Pull-up or Pull-down resistors are resistors whose job it is to pull and Integrated Circuit's (IC) input voltage either up to the power supply voltage, or down to circuit ground. The IC's input is usually high resistance; high enough that the moderate resistance on the Pull-up or Pull-down resistor acts like a short to the power supply (Pull-up resistor) or a short to circuit ground (Pull-down resistor).

The resistors are there to solve a problem. The problem is that without something to hold the IC's input voltage high or low, the input voltage moves on its own, or floats up or down.

In this schematic, there's also a switch. The switch is used to change the IC's input voltage.

Pull-up Resistor

With the switch contacts open, the Pull-up resistor is the only pathway for current. The input to the IC has very high resistance so its current is very small. Because the current from the IC's input (which also routes through the Pull-up resistor) is small enough, there isn't much voltage drop across the resistor. To the IC, it looks like its input is shorted to the power supply.

Switch to Pull the Voltage Down to Circuit Ground Potential

With the switch contacts closed, the voltage at the IC's input is essentially shorted to the circuit ground.

Another point of view is the current through the switch and the Pull-up resistor is large so the voltage across the Pull-up resistor becomes the same as the power supply voltage, and the resistor pulls the IC's input voltage down to circuit ground.

Because the current through the IC's input is so small, it isn't shown here.

Pull-Down Resistor

A Pull-down resistor does the opposite of a Pull-up resistor but it works about the same way.

With the switch contacts open, the Pull-down resistor is the only pathway for current. The input to the IC has very high resistance so its current is very small. Because the current from the IC's input (which also routes through the Pull-down resistor) is small enough, there isn't much voltage drop across the resistor. To the IC, it looks like its input is shorted to circuit ground.

Switch to Pull the Voltage Up to the Power Supply Voltage

With the switch contacts closed, the voltage at the IC's input is essentially shorted to the power supply.

Another point of view is the current through the switch and the Pull-down resistor is large so the voltage across the Pull-down resistor becomes the same as the power supply voltage, and the resistor pulls the IC's input voltage up to the power supply.

Because the current through the IC's input is so small, it isn't shown here.

Pull-up or Pull-down

A Pull-up or Pull-down resistor, when nothing else is connected (or the switch contacts are open), pulls and holds the IC's input voltage up to the power supply voltage or down to circuit ground voltage.