Basic Facts about 4-20 mA current loop in Process Instrumentation

The 4-20 mA current loop, like the one shown in the diagram below is a typical way of transmitting sensor information in most of industrial process monitoring applications.

A sensor in this case is a device used to measure the physical parameters such as liquid flow rates, temperature, pressure, speed, and so on. The 4-20 mA current loop plays an important role especially when the transmitter’s sensor information has to be sent to a remote location over long distances such as 1000 feet or more.

The 4-20 mA current loop can be explained as follows:

A sensor’s output voltage is first converted to a proportional current, with 4 mA normally representing the sensor’s zero-level output and 20 mA representing the sensor’s full scale output. Then, a receiver at the remote end converts the 4-20 mA current back into a voltage which in turn can be further processed by a computing unit or display module.

Nonetheless, transmitting a sensor’s output as a voltage over long distances has several shortcomings. Unless very high input-impedance are employed, transmitting voltages over long distances produces correspondingly lower voltages at the receiving end due to wiring and interconnection resistances. These high-impedance devices however, can be sensitive to noise pickup since the lengthy signal carrying wires often run in close proximity to other electrically-noisy system wiring. Shielded wires can be used to reduce noise pickup, but their high cost may be a prohibitive factor when long distances are involved.

Sending a current over long distances produces voltage losses that are proportional to the wiring’s length. But, these voltage losses also referred to as loop drops do not reduce the 4-20 mA current as long as the transmitter and loop supply can compensate for these drops.

Components of a 4-20 mA Current Loop 

A typical 4-20 mA current loop circuit is made four individual elements:

• Sensor/Transducer.
• Transmitter/signal conditioner.
• Loop power supply.
• Loop.
• Receiver/process monitor.

In loop-powered applications, all the four elements are connected in a closed, series circuit, loop configuration as shown in the figure above.

Sensors measures and provides an output voltage whose value represents the physical parameter being measured for example a thermocouple is a type of sensor which provides a very-low output voltage that is proportional to its ambient temperature.

The Transmitter amplifies and conditions the sensor’s output and then converts this voltage to a proportional 4-20 mA dc current that circulates within the closed series-loop. The receiver/display/monitor, typically a subsection of a panel meter or data acquisition system, converts the 4-20 mA current back into a voltage which can be further processed and/or displayed. 

The loop power supply normally provides all the operating power to the transmitter and receiver, and any other loop components that require a regulated dc voltage. +24 V is the commonly used power supply voltage in 4-20 mA current loop process monitoring applications. This is partly because +24 V is also used to power many other instruments and electromechanical components commonly found in industrial environs. Lower supply voltages, such as +12 V are also popular since they are used in computer-based systems. 

Loop, this is the wire connecting the sensor to the device receiving the 4-20 mA signal and then back to the transmitter.

Last but not least, the receiver/process monitor receives and interprets the current signal.           

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