Comparator Not Working Properly

Discussion in 'The Projects Forum' started by KaguraHime, Sep 25, 2013.

  1. KaguraHime

    Thread Starter New Member

    Sep 25, 2013
    7
    0
    The 6V and 11V are generated by the wipers of potentiometers. I've traded the LT1716 comparator for an LM339 IC. 2N2222 was used for the NPN transistor. And the R2 represents my load. A 6V motor for my case.

    In this project, I'm trying to use the (-) of the comparator to program the ref voltage. And attached to the (+) of comparator is the input voltage, which varies. Now when the (+) of the input voltage exceeds the (-), the NPN transistor lets current flow from the collector to the emitter and drives the load. On LTspice, it works. And as you can see, voltage measured at the node below the NPN is 11V<.

    Only problem is I've done the circuit iRL and it doesn't work. Help!
     
    • Ku.jpg
      Ku.jpg
      File size:
      35.6 KB
      Views:
      47
    • Ku2.jpg
      Ku2.jpg
      File size:
      29.1 KB
      Views:
      42
  2. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    The LM339 has an open collector output. It requires a pullup resistor on its output to provide a HI output signal.
     
  3. KaguraHime

    Thread Starter New Member

    Sep 25, 2013
    7
    0
    Yes, I thought the 2k2 resistor (R2) would be it.

    Actually I've wired my load directly to the output thinking it would act as the resistor I need. I'll have to put the 2k2 in for the actual circuit too I guess.
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,447
    3,363
    No. The 2k2 resistor does not supply any current to the base of the transistor.
    Put a 1k pullup resistor from the output of the comparator to Vcc.
     
  5. KaguraHime

    Thread Starter New Member

    Sep 25, 2013
    7
    0
    Alright, thanks! Will see if it fails again tomorrow.
     
  6. newbies_hobbyist

    Member

    Jun 4, 2010
    67
    8
Loading...