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Detect lower of two signals

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  • normnet2003
    How to output the lower of two 0-5 volt DC signals? I have two pressure sensors and I need to detect the lowest of the two for this app. Norm
    Message 1 of 11 , Jul 13, 2014

      How to output the lower of two 0-5 volt DC signals?

      I have two pressure sensors and I need to detect the lowest of the two for this app.

       

      Norm

    • redderek
      Use a comparator and a double-throw analog switch. Comparator does the check between two signals and outputs a high or a low. Then use the high or low signal
      Message 2 of 11 , Jul 13, 2014
        Use a comparator and a double-throw analog switch. Comparator does the check between two signals and outputs a high or a low. Then use the high or low signal to tell the switch to pass one or the other signal.

        Derek Koonce
        DDK Interactive Consulting Services
      • normnet2003
        Sounds like a plan! Thanks Norm
        Message 3 of 11 , Jul 13, 2014

          Sounds like a plan!

           

          Thanks

          Norm

        • Jan Kok
          Do you want to _detect_ the lower signal (have a digital output that says A is lower or B is lower ), or do you want to choose the lower signal and _output_
          Message 4 of 11 , Jul 13, 2014
            Do you want to _detect_ the lower signal (have a digital output that says "A is lower" or "B is lower"), or do you want to choose the lower signal and _output_ that voltage?

            The first is easy. Just use a comparator IC such as NTE922 or LM339, etc.

            If you want to choose and output the lower voltage, there are several options:

            - Use a comparator and an analog multiplexor such as 74LVC1G53 or analog switches such as CD4016B (with the 4016 you'd also need an inverter so you turn one switch off when the other is on. A single transistor can be used as an inverter.)

            - Depending on your accuracy requirements and whether you have additional power supply voltages available, you may be able to use a couple of diodes to choose the lower voltage. You can use a third diode to undo the ~.6V voltage offset you'll see across the conducting diode.

            Chances are, the first solution will be easier to implement and give better results. The only downside to the first solution that I can see is that there may be some glitches in the output when the comparator switches states.



            On Sun, Jul 13, 2014 at 8:15 AM, normnet2@... [Electronics_101] <Electronics_101@yahoogroups.com> wrote:


            How to output the lower of two 0-5 volt DC signals?

            I have two pressure sensors and I need to detect the lowest of the two for this app.

             

            Norm




          • redderek
            I believe the OP wanted to pass the signal. Besides, using the comparator, you get the one is higher than the other information since you need to control the
            Message 5 of 11 , Jul 13, 2014
              I believe the OP wanted to pass the signal. Besides, using the comparator, you get the "one is higher than the other" information since you need to control the switch. With a buffer, he could drive a relay just as easily. Options become very numerous and I'll leave it to the OP to figure out the details since none were given.

              Glitches could be minimized with some filtering.

              The diode approach is very simple, but it will pass the higher of the two signals and not the lower.

              Derek Koonce
              DDK Interactive Consulting Services
            • Jan Kok
              On Sun, Jul 13, 2014 at 11:04 AM, derek@dkoonce.com [Electronics_101]
              Message 6 of 11 , Jul 13, 2014



                On Sun, Jul 13, 2014 at 11:04 AM, derek@... [Electronics_101] <Electronics_101@yahoogroups.com> wrote:


                The diode approach is very simple, but it will pass the higher of the two signals and not the lower.

                Just turn the diodes around. ;-)

                The biggest drawback to the diode solution is that it needs additional power supplies to supply current through the diodes. There is also a subtle problem where the output will be slightly lower by a few mV than it should be when the two inputs are approximately equal, due to current sharing between the two input diodes. Also, different diodes of the same part number will have slightly different V-I curves, resulting in different voltage drops. That can be alleviated by using diode array ICs to get better matching between diodes.
              • normnet2003
                I am looking to pass the analog signal. The existing HVAC control monitors only one of the two compressors for a low pressure return condition which in the
                Message 7 of 11 , Jul 13, 2014

                  I am looking to pass the analog signal.

                  The existing HVAC control monitors only one of the two compressors for a low pressure return condition which in the past has resulted in compressor failure due to the fact that at a low pressure Freon is a liquid and doesn't compress well.  

                  I would like add protection for the stage 2 compressor by adding a second pressure sensor to the existing control.

                  By providing the lower of the two sensor signals to the control both compressors can be protected.

                   

                  In addition I am also planning on adding a high pressure sensor to the stage 2 compressor to pass the higher of the two signals to the existing control which operates the HVAC units shared outside fans.

                  This is a little easier as a diode in each sensor output will pass the higher signal level although I will likely utilize a perfect diode circuit to prevent the voltage drop found with a regular diode.

                   

                  A momentary glitch in the signal shouldn't be a problem as the fans change speed relatively slowly.

                   

                  Norm

                • Jan Kok
                  Freon is liquid at HIGH pressure, don t you mean? You can get min or max from diodes, depending on which way you orient them. Remember you need to flow some
                  Message 8 of 11 , Jul 13, 2014
                    Freon is liquid at HIGH pressure, don't you mean?

                    You can get min or max from diodes, depending on which way you orient them. Remember you need to flow some current through the diodes to get them to turn on. That current has to come from some power supply, or ground.

                    If the threshold voltages for turning the compressors or fans off and on are in the range of about 1.5 to 3.5V (well away from 0 and 5V), then you may not need extra power supplies. Note that there will be two thresholds in each case, one for turning the compressor or fan ON, and another threshold for turning the device OFF.


                    On Sun, Jul 13, 2014 at 12:36 PM, normnet2@... [Electronics_101] <Electronics_101@yahoogroups.com> wrote:


                    I am looking to pass the analog signal.

                    The existing HVAC control monitors only one of the two compressors for a low pressure return condition which in the past has resulted in compressor failure due to the fact that at a low pressure Freon is a liquid and doesn't compress well.  

                    I would like add protection for the stage 2 compressor by adding a second pressure sensor to the existing control.

                    By providing the lower of the two sensor signals to the control both compressors can be protected.

                     

                    In addition I am also planning on adding a high pressure sensor to the stage 2 compressor to pass the higher of the two signals to the existing control which operates the HVAC units shared outside fans.

                    This is a little easier as a diode in each sensor output will pass the higher signal level although I will likely utilize a perfect diode circuit to prevent the voltage drop found with a regular diode.

                     

                    A momentary glitch in the signal shouldn't be a problem as the fans change speed relatively slowly.

                     

                    Norm




                  • normnet2003
                    Freon can be a liquid or a gas at either high or low pressure depending on the temperature. Air Conditioning - Basic Refrigeration Cycle
                    Message 9 of 11 , Jul 13, 2014

                      Freon can be a liquid or a gas at either high or low pressure depending on the temperature.

                      Air Conditioning - Basic Refrigeration Cycle 

                      If not enough heat is transferred from the inside air to the refrigerant in the evaporator coil due to low inside air flow or to much heat is transferred from the refrigerant in the condenser coil to the outside air due to the outside fans operating to fast the Freon returns to the compressor too cool and in a liquid state which is sometimes referred to as "slugging" the compressor.

                       

                      I am outputting the higher of the two high pressure signals to an existing control which provides a 0-10vdc to 2 inverters which control the outside fans speed.

                      Likewise the lower of the two low pressure signals is output to the existing control which if determines a "slugging" condition is occurring will open a contact to shut off the cooling.

                       

                      The purpose is to provide the control with pressure readings from both compressors in place of just one.

                       

                      Norm

                    • tstark0529
                      Something doesn t seem right. How old is this HVAC unit? If it is only a 2 stage system, in theory if the 2nd stage is running you are calling for maximum
                      Message 10 of 11 , Jul 13, 2014
                        Something doesn't seem right.  How old is this HVAC unit?  If it is only a 2 stage system, in theory if the 2nd stage is running you are calling for maximum cooling and shouldn't have to worry about "slugging" either compressor.  The same goes for the condenser fans, if running both stages, they should be running flat out.

                        I've also never heard of a vari-freq drive on the condenser fans.  Usually they are either on or off.  It seems strange that the control system is sophisticated enough to have a variable speed drive on the condenser fans but not sophisticated enough to calculate the phase of the refrigerant after the condensers????? In order to do this the system would need to know both the pressure and the temperature of the refrigerant.  Knowing just pressure is not enough to know if liquid refrigerant is getting pulled into the compressor.  Is the system set up with a hot gas reheat for de-humidification?

                        Can you please post the make ,model, and capacity of the unit? you have me intrigued.      
                      • normnet2003
                        Normally when the 2nd stage is calling you would be correct the system is calling for maximum cooling and there should be adequate inside air flow however in
                        Message 11 of 11 , Jul 13, 2014

                          Normally when the 2nd stage is calling you would be correct the system is calling for maximum cooling and there should be adequate inside air flow however in the event the first stage pressures are off due to a first stage refrigerant leak or first stage tripped its breaker for example the outside fans wouldn't be correct as presently only the first stage head pressure is sensed by the outside fan speed control.

                           

                          Although this unit draws in outside air when the outside temperature is less than 55-60 degrees (an economizer) many don't and require a low ambient kit to maintain a minimum head pressure which is accomplished by either cycling the outside fans on and off or varying their speed to maintain head pressure.

                          A vari-freq drive on the condenser fans is nice because the head pressure remains fairly constant as opposed to cycling the condenser fans on and off.

                          A vari-freq drive on the condenser fans is also helpful when only a small zone is calling (low inside air flow).   

                           

                          Slugging the compressor can be caused by either low head pressure (over cooling the outside coil) or low demand (low inside air flow because of only a small zone calling) or both.

                           

                          No gas reheat for dehumidification.

                           

                          Norm

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