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  • Sal Aiello (UK)
    Hi All, I ve got a small project on the go and need some advice. It concerns the use of voltage comparators, in particular the LM311 and LM393. I ve got a
    Message 1 of 3 , Dec 6, 2006
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      Hi All, I've got a small project on the go and need some advice. It
      concerns the use of voltage comparators, in particular the LM311 and
      LM393. I've got a breadboarded ( plywood and cup screws!! ) test setup
      and I've managed to get both comparators to work including some
      hysteresis, but I am still having a problem in understanding why they
      won't switch around the reference voltage. The comparators are set up
      with single supply, +5V from a 7805 regulator and 0V ground. The
      reference voltage is to +ve input at 2.5V with about 0.1V hysteresis.
      I vary the input voltage to the -ve input using a cermet
      potentiometer. I expected the comparators to switch around 2.5V (
      allowing for hysteresis), but I am finding that it will only switch a
      full 0.5V below this value at around 2V. What am I doing wrong? Any
      help much appreciated.

      Thanks,
      Salvatore.
    • Bernie Fernandez
      Hi, I had used an LM393 of my project before but the supply I had used was 12V and I never try this with 5V but I think there is no problem with as long as the
      Message 2 of 3 , Dec 7, 2006
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        Hi,
         
           I had used an LM393 of my project before but the supply I had used was 12V and I never try this with 5V but I think there is no problem with as long as the supply is with in it parameter based on datasheet. If you can post your circuit is better so I could help you out. You need to place a pull up resistor on its output(lm393) because this device is an open collector. Do not test your circuit using breadboard, this is based on my experience, that comparator circuit will not work properly using breadboard connection.
         
        Regards,
        Bernhix

        "Sal Aiello (UK)" <saiello@...> wrote:
        Hi All, I've got a small project on the go and need some advice. It
        concerns the use of voltage comparators, in particular the LM311 and
        LM393. I've got a breadboarded ( plywood and cup screws!! ) test setup
        and I've managed to get both comparators to work including some
        hysteresis, but I am still having a problem in understanding why they
        won't switch around the reference voltage. The comparators are set up
        with single supply, +5V from a 7805 regulator and 0V ground. The
        reference voltage is to +ve input at 2.5V with about 0.1V hysteresis.
        I vary the input voltage to the -ve input using a cermet
        potentiometer. I expected the comparators to switch around 2.5V (
        allowing for hysteresis), but I am finding that it will only switch a
        full 0.5V below this value at around 2V. What am I doing wrong? Any
        help much appreciated.

        Thanks,
        Salvatore.



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      • Sal Aiello (UK)
        Hi Bernhix, To keep things simple, I ve reduced the circuit to not include hysteresis. I ve uploaded a schematic in Photos under the folder Sal s Stuff . The
        Message 3 of 3 , Dec 7, 2006
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          Hi Bernhix,

          To keep things simple, I've reduced the circuit to not include
          hysteresis. I've uploaded a schematic in Photos under the
          folder 'Sal's Stuff'. The circuit is literally the LM393, +5V supply
          to the comparator and two pots, one 1Mohm for the -ve reference
          input and a 10K for the +ve input, all connected to ground. The
          output of the comparator has a 2K pull-up resistor. I've tabulated
          the results with varying reference voltages and corresponding input
          voltages that start to switch the comparator output to 'high' ( more
          or less +5V):

          Ref (-ve) Input (+ve)
          0.5V 0.55V
          1.0V 1.20V
          1.5V 1.85V
          2.0V 2.5V
          2.5V 3.10V

          I've also replaced the 1M pot with a lower value 1K pot as has been
          suggested by those more expert in electronics than myself and also
          changed the comparator for another one in the small chance that it
          was faulty, but I still get exactly the same results as above. The
          point at which the input starts to switch the output seems to be
          proportional to the reference voltage, increasing as the reference
          voltage increases. The circuit is practically the same as the many
          examples on the internet but it just doesn't seem to behave in the
          way that they say, i.e. to switch around the reference voltage! I
          can normally work things out for myself even if it means spending
          days trying to get a particular component to work, but this one has
          me totally baffled! ;o) Another thing that may shed some light is
          the fact that I have made the same circuit with a LM311N voltage
          comparator and I get more or less the same behaviour...

          Thanks,
          Salvatore

          --- In home_electronics@yahoogroups.com, Bernie Fernandez
          <bernie_a_fernandez@...> wrote:
          >
          > Hi,
          >
          > I had used an LM393 of my project before but the supply I had
          used was 12V and I never try this with 5V but I think there is no
          problem with as long as the supply is with in it parameter based on
          datasheet. If you can post your circuit is better so I could help
          you out. You need to place a pull up resistor on its output(lm393)
          because this device is an open collector. Do not test your circuit
          using breadboard, this is based on my experience, that comparator
          circuit will not work properly using breadboard connection.
          >
          > Regards,
          > Bernhix
          >
          > "Sal Aiello (UK)" <saiello@...> wrote:
          > Hi All, I've got a small project on the go and need some
          advice. It
          > concerns the use of voltage comparators, in particular the LM311
          and
          > LM393. I've got a breadboarded ( plywood and cup screws!! ) test
          setup
          > and I've managed to get both comparators to work including some
          > hysteresis, but I am still having a problem in understanding why
          they
          > won't switch around the reference voltage. The comparators are set
          up
          > with single supply, +5V from a 7805 regulator and 0V ground. The
          > reference voltage is to +ve input at 2.5V with about 0.1V
          hysteresis.
          > I vary the input voltage to the -ve input using a cermet
          > potentiometer. I expected the comparators to switch around 2.5V (
          > allowing for hysteresis), but I am finding that it will only
          switch a
          > full 0.5V below this value at around 2V. What am I doing wrong?
          Any
          > help much appreciated.
          >
          > Thanks,
          > Salvatore.
          >
          >
          >
          >
          >
          >
          > ---------------------------------
          > Want to start your own business? Learn how on Yahoo! Small
          Business.
          >
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