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daq-and-datalogging:documents:current-sensors [2023/06/13 14:56] – created Scandiwebdaq-and-datalogging:documents:current-sensors [2023/07/28 00:52] (current) – Arthur Brown
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-====== 4-20mA current sensors ======+====== 4-20mA Current Sensors ======
  
 Introduction: Introduction:
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 The 4-20mA current loop interface came about because of the need to transmit information over long distances. System designers found that instead of a changing a voltage signal [that is susceptible to noise and voltage drops along the wires] they could transmit a changing current with little downside. For the most part, the information is simply a measurement or a value to control something. For instance, a factory process may need to transmit a pressure value up to the control room that is hundreds of feet away. Likewise the control room my need to adjust the pressure by sending back a 4-20mA signal to the pressure controller that is also hundreds of feet away.  The 4-20mA current loop interface came about because of the need to transmit information over long distances. System designers found that instead of a changing a voltage signal [that is susceptible to noise and voltage drops along the wires] they could transmit a changing current with little downside. For the most part, the information is simply a measurement or a value to control something. For instance, a factory process may need to transmit a pressure value up to the control room that is hundreds of feet away. Likewise the control room my need to adjust the pressure by sending back a 4-20mA signal to the pressure controller that is also hundreds of feet away. 
  
-{{https://files.digilent.com/resources/measurement-computing/Reference Images/4_20mACurrentLoop.png?441x217}}+{{https://files.digilent.com/reference-images/4_20mACurrentLoop.png?441x217}}
  
 Ohms Law: Current times resistance equals voltage.  Ohms Law: Current times resistance equals voltage. 
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 It's best to use a differentially ended input across the resistor but single ended can also be use if care is taken to avoid ground loops. In the picture below, if the power supply negative is the system ground and the DAQ and meter are isolated then single ended will work. However, if the the DAQ is not isolated it will get its ground from the computer and there may be a voltage difference between it and the power supply negative. In this case it's best to use a differential connection. If there is a large difference between the negative and the computer ground it's best to use a DAQ that has isolation.  It's best to use a differentially ended input across the resistor but single ended can also be use if care is taken to avoid ground loops. In the picture below, if the power supply negative is the system ground and the DAQ and meter are isolated then single ended will work. However, if the the DAQ is not isolated it will get its ground from the computer and there may be a voltage difference between it and the power supply negative. In this case it's best to use a differential connection. If there is a large difference between the negative and the computer ground it's best to use a DAQ that has isolation. 
  
-{{https://files.digilent.com/resources/measurement-computing/Reference Images/4_20mACurrentLoop2.png?440x240}}+{{https://files.digilent.com/reference-images/4_20mACurrentLoop2.png?440x240}}
  
 Also care should be taken as to where in the loop the measurement is made. If the DAQ input is sensitive to common mode voltage then put it as close to ground as possible. Elsewhere in the loop the inputs will have 5 or more volts of common mode. Isolated inputs work best in this case. Also care should be taken as to where in the loop the measurement is made. If the DAQ input is sensitive to common mode voltage then put it as close to ground as possible. Elsewhere in the loop the inputs will have 5 or more volts of common mode. Isolated inputs work best in this case.