High current will flow through the pull-up resistor if the resistance value is too low. The value of the resistor near the supply controls how much current you want to flow from VCC through the button, and then to ground. When the button is pressed, the input pin is pulled low. Ideal Resistance Values for Pull-up and Pull-down Resistors Therefore, the small amount of current flows from the 5-V source to the ground using the closed switch and pull-down resistor preventing the logic level pin from getting shorted with the 5-V source. The pull-down resistor which is connected to the ground or 0V sets the digital logic level pin to default or 0 until the switch is pressed and the logic level pin becomes high. Unlike the pull-up resistor, the pull-down resistor pulls the pin to a low value instead of high value. Just like the pull-up resistor in the first figure, the pull-down resistors in this circuit also ensures that the voltage between VCC and a microcontroller pin is actively controlled when the switch is open. Without the resistor, the voltage levels would virtually float between the two voltages. When the switch is closed, the input voltage at the gate goes to Vin. When the switch is open, the voltage of the gate input is pulled down to the level of the ground. Otherwise, it will make the input voltage at the pin on constant logical low value no matter the position of the switch. It should have a larger resistance than the impedance of the logic circuit. It pulls the input voltage down to the ground to prevent an undefined state at the input. The pull-down resistor holds the logic signal near to zero volts (0V) when no other active device is connected. It ensures that the wire is at a defined low logic level even when there are no active connections with other devices. On the other hand, a pull-down resistor is used to ensure that inputs to logic systems settle at expected logic levels whenever external devices are disconnected or of high impedance. Following the principle of Ohm’s law, if there is a pull-up resistance, a small amount of current will flow from the source to the resistors and to the switch before reaching the ground. This is because the pin cannot be directly shorted to the ground or VCC as this will eventually damage the circuit.
If we do not use a pull-up resistor, it will result in a short circuit. At the same time, it will not affect the state of the 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 open. In the figure above, a pull-up resistor with a fixed value was used to connect the voltage supply and a particular pin in the digital logic circuit.
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