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The fundamentals of 4...20 mA current loops

4…20 mA current is commonly used to connect process signals to a controller in industrial applications. Sometimes, an analog voltage signal or digital communications is used, but there are many advantages to using a current loop to send process values to the controller.

 

Why use current instead of voltage?

 

To understand this, we first need a fundamental understanding of currentvoltage, and resistance:

 

  • Current: Current is the flow of electrons through a circuit. A current of 1 amp equals a flow of 6.24 x 10¹⁸ electrons per second. To measure current, open a circuit and insert an amp meter into the circuit. In this way, all electrons must flow through the meter, ensuring an accurate current measurement. 

    Unit of measurement: Amp, symbol: A.

  • Resistance: Resistance is the opposition to the flow of current. If resistance increases, current flow decreases. To measure resistance, disconnect the component that has resistance from a circuit, then use an ohmmeter to measure the resistance of the component. 

    Unit of measurement: Ohm, symbol: Ω.

  • Voltage: Voltage is the difference in electrical charge between two points of a circuit. A difference in electrical charge exists if the electrons at one point are at a different energy level compared to electrons at the other point, or if there are more electrons of the same energy level at one point compared to the other point. A difference in electrical charge of 1 V is enough energy to drive 1 A current through a resistance of 1 Ω. To measure voltage, connect the voltmeter in parallel with two points of a circuit. 

    Unit of measurement: Voltage, symbol: V.

     

The loop transmitter

 

A loop transmitter measures a process variable, and regulates the loop current between 0.004 and 0.02 A, (4 mA…20 mA), by varying the opposition to current flow of the loop. Loop transmitters can measure nearly any process variable, for example temperature, pressure, level, or flow.

 

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