Pull down resistors ensure that the controller input ports are never left floating. We use as an example the case of an angular encoder which outputs a series of 5 V pulses in response to a change in angular position. Between successive pulses, the encoder output is at 0 V, i.e. there is no signal. When the encoder output is at 0 V, the controller input port is floating, causing it to pick up background noise which is erroneously interpreted as a signal .

To prevent the controller from interpreting background noise as a signal, we channel the noise to ground through a resistor, called a pull down resistor. The reason for using a resistor rather than connecting directly to ground, is to limit the current drawn to ground. However, we do not want to choose an excessively large resistance and this increases the rise and fall times of the voltage on the controller input port.

Illustration of a pull down resistor
The illustration shows an micro-controller (MCU) used to read an encoder. The MCU is grounded through a pull down resistor.

When choosing a pull down resistor, keep the following in mind:

  • A pull down resistor is not a special type of resistor. You can use any standard fixed value resistor.
  • As a rule of thumb, choose a resistance 1/10th of the controller input impedance.
  • The pull up resistor increases the voltage rise/fall time. For faster rise/fall times, opt for lower resistance values.
  • To minimize power dissipation, opt for higher resistance values.
  • Many controllers include built in, programmable pull down resistors so check the relevant data sheets.