How to Calculate LM311 Pull-Up Resistor

The LM311 is a comparator, an electronic device that compares two input voltages. The output can be either high or low, which is interpreted as the Boolean state of true or false. However, a pull-up resistor may be needed to raise the output voltage of the LM311 so it can properly interface with a connecting device. A pull-up resistor is a resistor that connects the output of the LM311 to a voltage source. When the LM311 sends a high or low signal to the output pin, the pull-up resistor can raise the voltage to the necessary level.

Things You'll Need

  • LM311 Comparator Datasheet
Show More

Instructions

    • 1

      Locate the “Output Current” value, which is found in the LM311 datasheet in the form of a table. This table requires that you know the input voltage. A typical input voltage for the LM311 is 15 volts. Find the current values for “Positive Supply – Output Low” and “Positive Supply – Output High.” Assuming a supply voltage of 15 volts, these values are: Positive Supply – Output Low = 4.2 mA; Positive Supply - Output High = 3.2 mA.

    • 2

      Find the difference between “Positive Supply – Output Low” and “Positive Supply - Output High.” The answer for this example is 1.0 mA.

    • 3

      Divide the source voltage, which is assumed to be 15 volts, by the difference in voltage to get the minimum resistance value for a pull-up resistor : 15 volts / 1.0 mA = 1.5 Mega ohm pull-up resistor.

    • 4

      Connect a resistor with at least 1.5 Mega ohm as the pull-up resistor.