Voltage comparator

Discussion in 'General Electronics Chat' started by aris, May 17, 2010.

  1. aris

    Thread Starter Member

    May 5, 2010
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    Hi all,

    Could someone please provide me with a schematic for a voltage comparator?
    I want to use this with an LM311 and I have 12V DC power supply if those make any difference.If someone could tell me as well how can I choose tha values of resistors and capacitors for that please.

    Thank you
     
    Last edited: May 20, 2010
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    We assume this is a continuation of your earlier thread - http://forum.allaboutcircuits.com/showthread.php?t=38406

    Can you provide some parameters? What is the magnitude of the voltage to be detected, and should the comparator output a high or low for the detection? Is this for one voltage level only?
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    1,728
    Attached is a simplified schematic using an LM111 (same pin-out as an LM311) to sense current through an inductive load.

    When the current through Rsense exceeds 500mA, the output of the comparator goes high.

    R2 gives a very small amount of hysteresis. It can't be decreased or the output of the comparator would be significantly decreased.
     
  4. aris

    Thread Starter Member

    May 5, 2010
    45
    0
    Thank you all for your replies!

    the real spec for what I want to do is as follows:

    12V DC power supply,
    when PC is in use I have a max current consumption of 3.65A
    when pc is off I have a current consumption of 0.14A
    what I want to do is possibly through a comparator when I get a current lower than 1A to get a 12V DC output and when I have a current higher than 1A to get 0V output.
    I believe this could be done by a comparator isn't it?
    could someone provide a circuit with the corresponding resistor and capacitor values for that?
    SgtWookie would the circuit you attached work for what I want to do? are the values shown the appropriate ones?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    It depends - what do you want the 12v output to drive?

    Change R4 from 23.9k to 11.9k; then the threshold will be 1A.
     
  6. aris

    Thread Starter Member

    May 5, 2010
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    I want to drive a delay which is based on a CD4093BCN IC (nand gates along with some diodes,resistors and a capacitor). would that change of the resistance is enough or there is something more needed?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Post the circuit that you are considering.
     
  8. sage.radachowsky

    Member

    May 11, 2010
    241
    38
    Well, you may want to use a smaller current-sense resistor, so that the voltage drop is less for your load. You also want to reverse the connections to the comparator if you want the signal to go high when the current is low -- connect the sense line to the inverting ("-") input rather than the "+" input.


    Sizing the current-sense resistor
    The current-sense resistor uses up some of the voltage from the 12V supply. For example, the 0.1 Ohm will use up 0.1V when there is 1A flowing, or 0.35V when there is 3.5A. Using up 0.35V would leave 11.65V for your load. It's probably okay but think about it. It will also burn up 1.225W in the latter case. That will need to be a hefty resistor. If you change it to a 0.05 Ohm resistor, then at 3.5A it will use up 0.175V and 0.6125W of power.

    The drawback is that the voltages in question will be much less. Your threshold is 1A, so with a 0.05 Ohm resistor, it is 0.05V... 50 mV... that is not very much of a signal to be sensing, as it's close to the ground rail. Op amps and comparators may say "rail to rail" but are not very good near their rails anyway. Accuracy and gain suffers.

    Those are just a few thoughts. Hope it's helpful.

    If your load losing 0.35V is not a problem, then I'd say go with Sargent's circuit as it should work -- just reverse the inputs to the comparator to match your spec on the output signal.
     
  9. aris

    Thread Starter Member

    May 5, 2010
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    as you will see this is the complete circuit I have up to now.
    What I want to do is replace the rs232 feed with the output of the comparator so instead of the signal of the rs232 to be the trigger the comparison of 1A or more to be the trigger. with triggering value to be 1A and lower than this.
     
  10. aris

    Thread Starter Member

    May 5, 2010
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    0
    Just to say thank you to al of you sharing your knowledge with me!
    thanks a lot guys!!!
     
  11. aris

    Thread Starter Member

    May 5, 2010
    45
    0
    SgtWookiecan you upload the file from the circuit maker software? I am trying to simulate it and 1) the LM111 and the inductor give me the following message "selected items in this circuit may not be simulated in the selected simulation mode." and 2) I can not find the oscilloscope you have on the attached picture above..is it right that I have to connect the sensing line to the "-"? how the circuit becomes then?
    and finally at which pins I get the output?
     
    Last edited: May 20, 2010
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