• Basically, a transducer (a Flow Transducer for example), a power supply and a measurement device (an amperemeter or the analog input of a programmable logic controller for instance) are linked together in a 4-20 mA current loop.
• The analog value of the measure modulates the current in the loop.
• A resistor (which value is between 250 Ω and 1100 Ω) is inserted in the current loop.
• Then, the resistor generates a voltage from 1 volt (for 4 mA in the current loop) to 5 volts (for 20 mA in the current loop).
• The two wires of the analog part of the HART modem must be linked in parallel to the resistor.
• Then, the HART modem uses the voltage to extract the signal.

NOTE - The value of the resistor depends on the global resistivity in the current loop :

• The number of devices (one or more).
• The length of the current loop (up to 3000 meters).
• The technical features of the wires.
• The internal resistivity of each connected devices.

In some special cases, the resistor can have a lower value than the minimal value specified in the HART standard.

  Back to HART modem / Basic cabling chapter.