help needed with LM311N comparator

Discussion in 'The Projects Forum' started by hunterage2000, Jun 2, 2012.

  1. hunterage2000

    Thread Starter Active Member

    May 2, 2010
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    Hi, I am trying to use an LM311N comparator as a switch for a CMOS 4066 but the breadboard circuit isnt isnt giving me a square wave at the output. I have attached the simulation circuit with the input from an AC source and the output is the square wave. I have also attached the breadboard setup with pins 1 and 3 going to ground, 4 at -15VDC, 8 at 15VDC and 2 10k resistors in series from 7 to 8. Has anyone ever used one and can advise me?
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Connect pin-5 to pin-6.
    Also install a 10kΩ resistor in series with the input at pin-2, assuming you have not blown the chip already.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    MrChips,
    Balance and strobe can be left disconnected.

    Hunterage2000,
    Are you certain that:
    Lowest bus is connected to ground/0v (blue wire on right)?
    Next lowest bus is -15v (green wire on right)?
    Upper bus is +15v (red wire on left)?

    Are you certain that your resistors are 10k each in value? I can't tell by the colors; they look very dark to me. You may have a lot lower value of resistance than you think. Check them with a multimeter/DMM. I have a really tough time reading the color bands on metal film resistors.

    The LM311 can sink up to about 50mA into its' output without having an excessive saturation voltage. If your resistors were too low in value, the output might not budge much below your +V supply.

    Does your comparator share a common ground with your sine wave input? If not, you will need to isolate the DC level by using a capacitor in series with the noninverting input, and a resistor from the noninverting input to ground to keep its average DC level at ground.
     
  4. mlog

    Member

    Feb 11, 2012
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    36
    Maybe I'm reading the TI data sheet wrong, but what I see is the Balance (5) and Balance/Strobe (6) should be tied together if a balance pot is not used.

    http://www.ti.com/litv/pdf/slcs007h
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Where (on what page) do you see that in the datasheet?
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    On page 11

    [​IMG]
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I remember seeing that years ago. I think it is recommended so that any noise on the strobe pins will inject common mode noise into the comparator, which should have minimal effect on the output, as long as the noise is fairly low amplitude. If left unconnected, the noise could be different between the two pins, causing amplified noise to appear on the output.
     
  8. strantor

    AAC Fanatic!

    Oct 3, 2010
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    I just used LM311 a few weeks ago for a project and I shorted out those pins and it worked. I want to say I tried it without the pins connected, and it didn't work, but I'm not 100% sure if that really happened or if my brain is making it up just now.
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    I just tested the LM311 on a breadboard.
    It works fine with pins 5 and 6 unconnected.
     
    strantor likes this.
  10. wayneh

    Expert

    Sep 9, 2010
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    My money's on Sarge's suspicion that "ground" may not be.

    Also, what exactly is the actual incoming signal? Is it capacitively coupled to your comparator? And how do you know the output isn't a square wave - what are you using to look at it and where are you putting the probes? Question everything.
     
  11. hunterage2000

    Thread Starter Active Member

    May 2, 2010
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    Sorry for the late reply but I got it working in the lab but not at home for some reason.
     
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