Introduction

Analog 4–20 mA current loops are one of the most used communication buses in industrial environments, where environmental variables, process sensing, and control is required. This bus type is named after its maximum and minimum current values across the line; 4 mA and 20 mA. Current loops are used not only for receiving information from sensors and field instrumentation, they are also used for transmitting control signals to actuators or other devices to regulate a controlled action.

Current loops, such as 4-20 mA, have been used since the 1950s in many industrial applications. The bus is appealing due to the low implementation cost, high noise immunity due to the non-voltage dependence, and the ability to be used effectively over long distances.

These characteristics are the main reasons for using a 4-20 mA loop for signaling and control. Because of this, there are several industrial sensors that implement a 4-20 mA output to indicate their measurements. Field instrumentation sensors such as pressure sensors, temperature sensors, level or flow sensors, pH sensors, and gas sensors implement a 4-20 mA connection to their outputs. Also, there are several actuators, such as valves, which can be controlled via a 4-20 mA loop.

There are several different brands of commercial IC’s that implement a 4-20 mA bus receiver. 

In this project, a GreenPAK™ is designed as an oxygen sensor with a 4-20 mA output and is used to detect dangerous concentrations of oxygen for human beings in industrial environments. The project implements the analog measurement and digital logic required to implement an integrated concentration-level alarm. This application also has a configurable limit value to detect out-of-level concentrations. To implement this 4-20 mA bus receiver, an SLG46621V is used. 

Below we described steps needed to understand how the sensor interface has been programmed. However, if you just want to get the result of programming, download GreenPAK software to view the already completed GreenPAK Design File. Plug the GreenPAK Development Kit to your computer and hit the program to design the device.

4-20 mA Output General Characteristics

As described before, in industrial and noisy environments, analog 4–20 mA current loops are one of the most used buses for electronic measurement and control. These buses are used not only for controlling valves or other actuators, they are also used for transmitting sensor information from field instrumentation to the control center.

The main advantages of the 4-20 mA current loop are:

– The data transmission is not affected by the voltage drop over the interconnecting wiring, meaning it can be used over long distances with much less degradation of the signal.

– It allows the system to implement a self-monitoring routine because currents less than 4 mA and higher than 20 mA can indicate a fault in the circuit.

– The 4-20 mA current loop, often, is used also to power the remote device.

– The "live" or "elevated" zero of 4 mA allows powering of the device even with no process signal output from the field transmitter.

The 4-20 mA communication bus is based on the principles of Kirchhoff’s current law, which states that the sum of currents flowing toward a point is equal to the sum of currents flowing away from that point. With this principle, all current present at the beginning of a loop must reach the end.

This law is the basic principle to explain the behavior of the 4-20 mA loop. The current at any position of the system can be measured and found to be the same result. This is also the main reason for using current loops with low impedance within industrial applications; they provide the benefit of obtaining a longer data path with better noise immunity. A scheme of a 4-20 mA loop can be seen in Figure 1.

One of the most commonly used and simple circuits for measuring...

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