LM311N comparator HELP please.

Discussion in 'General Electronics Chat' started by b nelson, Sep 3, 2010.

  1. b nelson

    Thread Starter New Member

    Sep 3, 2010
    2
    0
    I am confused by the national datasheet on the LM311n. I dont understand the maximum ratings.... the datasheet http://www.national.com/ds/LM/LM111.pdf shows 36v max power supply voltage but states that out put to neg supply is 40v maximum. Can someone please let me know what maximum voltages as referenced from the ground pin that each pin on the device can handle safely. I especially need to know what the maximum voltage on the pull up resistor for the output (pin7) can be. I want to supply the device with a 12v power supply (pin8) but switch a 24v output (pin7). Both supplies will have a common negative. Thanks in advance.
     
  2. mossman

    Member

    Aug 26, 2010
    131
    3
    Sounds like you're below the maximum's so you're fine. The pull-up resistor is for the next stage of your circuit since the output is open collector. It allows you to pull the output to positive voltage when the comparator is "off" since this is not done internally to the chip (this is what "open collector" means). As long as the current flowing into pin 7 isn't higher than the maximum sinking current, then you'll be fine. It shouldn't be anywhere near the maximum. You want to choose a resistor that's high enough to minimize current flow, but can also successfully pull the output to a positive level.
     
  3. b nelson

    Thread Starter New Member

    Sep 3, 2010
    2
    0
    Thanks, Mossman.

    Can I externally short the open collector pin 7 to ground (pin4) without harming the device regaurdless whether or not the output is high or low? This will allow the next stage circuit to be controlled independent of the comparator.

    Also, can the datasheet shows absolute max vcc at 30vdc, but shows normal operation at 15vdc, can I safely run it at 24vdc?
    thanks again.
     
  4. mossman

    Member

    Aug 26, 2010
    131
    3
    Sure, you can short pin 7 to ground, but what would be the point of that?

    If 36V is the max supply voltage, then anything below that would be fine.
     
  5. DonQ

    Active Member

    May 6, 2009
    320
    11
    Realize that there are 2 different "grounds" on a 311. Pin 4 is more properly known as V-, which may or may not be at ground potential. Pin 7 is labeled as GND, but is basically just the emitter of the open-collector transistor. This is so that it can be connected to a different voltage than the V- of the analog circuitry. Often pin 4 and pin 7 are connected to Ground in a single supply circuit (and that is how the multi-comparator versions are wired internally).

    In that case, touching the open collector to ground does absolutely nothing. In other configurations, it could easily destroy the output.

    What you want is 4&7 to ground, V+ to your supply (12V?), and the open collector connected to the relay coil, with the other end of the coil to 24V. You also want a freewheeling diode across the coil to stop "inductive kick" from punching through the output transistor.
     
  6. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Pin7 to Ground? :confused:

    What you'll need to do is connecting pin1 and pin4 together as 0V. The 24V load can be connected to pin7.

    [​IMG]
     
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