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 devices 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.

A question that often comes up is, “Why not just use predetermined voltages instead of predetermined currents to provide the control signals?”  The answer is that current is always exactly the same in all parts of a series circuit but voltage may vary throughout a circuit according to resistance in the circuit as aforementioned. Hence, the actual voltage at the receiving device would depend on resistance in another part of the circuit and would be less reliable.

Using a precision resistor and an output device that varies its output voltage to produce predetermined currents eliminates error due to resistance. The output device outputs enough voltage to overcome wiring resistance and produce predetermined mA currents, as a result, voltages measured across the resistor may be significantly different but the voltage across the precision resistor depends entirely on the predetermined current. It will always match the mA values in the table 1 below.

Why Precision Resistor is used

A precision 250 ohm resistor in the 4-20 mA circuit (refer to figure 1.1 below) enables the output device to work with virtually no error. The resistor is connected in series with all devices in the circuit, which ensures that the amount of current through the resistor is exactly the same amount of current through all other devices in the circuit.

The output device ‘reads’ mA in the circuit and varies its output voltage to achieve a predetermined mA. Accordingly, when the voltage across the 250 ohm resistor is 1 volt, the current through the resistor is always 4 mA.  When the voltage across the resistor is 5 volts, the current through the resistor is always exactly 20 mA. And when the voltage across the resistor is somewhere between 1 and 5 volts, the current through the resistor is exactly in direct proportional to that voltage.

Table 1: The output device ‘reads’ mA in the circuit and varies its output voltage to achieve a predetermined mA.

If you know the number of mA, simply multiply it by 250 Ω to find how many volts are across the resistor e.g. 4.0 mA (0.004 A) x 250 Ω = 1.0 V.

If you know the number of volts across the resistor, divide it by 250 Ω to find the current or mA e.g. 1.0 V/250Ω = 4.0 mA.

Thus, the precision 250 Ω resistor ensures that a predetermined current in the circuit will always have a predetermined voltage across the 250 Ω resistor. Accordingly, a 4 mA current will always have 1 volt across the resistor even if the voltage in other parts of the circuit may be different due to wiring resistance or other variables. In the same way, a 20 mA current will always have 5 volts across the resistor even though the voltage may be different in other parts of the circuit.

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.           

Related articles:                         

• 4-20 mA Transmitters (2-wire, 3-wire and 4-wire Types)
• Process Instrumentation Diagnostics & Communication Interfaces
• Telemetry in Instrumentation Systems
• NAMUR NE 107 Standard for Modern Instrumentation Diagnostics
• Features & Configuration of a Smart Pressure Transmitter
• Features of HART Communication Protocol
• Pneumatic Signal Transmission System
• Causes of Ground Loops in Instrumentation & How to Avoid Them