LM311 comparitor problems!

Discussion in 'The Projects Forum' started by leewillz, Jan 31, 2010.

  1. leewillz

    Thread Starter Member

    Jan 11, 2009
    hi im building a circuit to compare 2 voltages, i am using a LM311 chip, ive built it on a breadboard and it doesn't work, ive tried building it on strip board and it still doesn't work, the schematic is posted below please can you find any problems that i cant see because i think it should work but im not sure where pin 5+6 go because there Balance and strobe so i left them out of it, and pin 4 ive connceted to ground but it says v- on it :S.
  2. bertus


    Apr 5, 2008

    The LM311 has an open collector output.
    It will need a resistor to the +powersupply to work.
    A resistor of 1K5 from the output to the +powersupply would be a start.


  3. hgmjr

    Retired Moderator

    Jan 28, 2005
    The output of an LM311 is an open-collector. You need a pullup on the output. Look at the datasheet.

  4. SgtWookie


    Jul 17, 2007
    As has already been implied, an open-collector output can sink current, but cannot source current.

    Remove the cathode of your LED from ground, and connect it to the comparator output.

    Remove R4, the 10k resistor from the comparator output and the LED anode.

    10k is much too large for the current limiting resistor R4.
    Re-calculate it as:
    R4 >= (Vsupply - Vf_LED) / Desired_Current

    Your Vsupply is 9v. For the moment, assume your LED has a Vf of 2v at 20mA current.
    R4 >= (9v - 2v) / 20mA
    R4 >= 7 / 0.02
    R4 >= 350 Ohms; not a standard value. However, 360 Ohms is a standard value.
    Table of standard values: http://www.logwell.com/tech/components/resistor_values.html
    Generally, refer to the E24 values in the green columns.

    You could use a somewhat higher if you wanted to; the LED will be more dim, but it will last longer. If you go much over 700 Ohms, it'll be getting pretty dim.
  5. leewillz

    Thread Starter Member

    Jan 11, 2009
    thanks for the help all, and ill do them changes now