What is 4 – 20mA Output？

The 4 - 20mA output refers to an electrical current loop system where the signal is represented by a direct - current (DC) electrical current that varies within the range of 4 milliamperes (mA) to 20mA. Here, “mA” stands for milliamperes, a unit of electric current. In this system, 4mA typically represents the zero or minimum value of the measured variable, while 20mA corresponds to the maximum value. For example, in a temperature - sensing application, 4mA might indicate a temperature of 0 degrees Celsius, and 20mA could signify a temperature of 100 degrees Celsius, depending on the calibration of the sensor and the system.​

The principle behind the 4 - 20mA output lies in its ability to transmit information over long distances with relatively low susceptibility to electrical noise and interference. Unlike voltage - based analog signals, which can be easily affected by resistance in the transmission wires and external electromagnetic fields, current - loop signals maintain their integrity better. This is because the current flowing through the loop remains constant regardless of the wire resistance, as long as the power supply can provide sufficient voltage to overcome the resistance in the loop. A sensor at the source end of the loop, such as a pressure sensor or a flow meter, converts the physical quantity it measures (pressure, flow, etc.) into an electrical current within the 4 - 20mA range. This current then travels through a two - wire or four - wire circuit to the receiving end, where it is converted back into a usable signal, such as a reading on a display or an input for a control system.​

One of the significant advantages of the 4 - 20mA output is its ability to support “loop - powered” devices. In a loop - powered setup, the same two wires that carry the current signal also supply power to the sensor or transmitter. This simplifies the wiring and reduces the installation cost, especially in large industrial plants with numerous sensors and instruments. Additionally, the 4 - 20mA standard allows for easy identification of faults in the system. Since 0mA is not part of the normal operating range (the minimum is 4mA), a reading of 0mA or a current outside the 4 - 20mA range typically indicates a problem, such as a broken wire, a malfunctioning sensor, or an issue with the power supply.​

The 4 - 20mA output finds extensive applications in various industries. In the oil and gas industry, it is used to transmit data from sensors monitoring parameters like pipeline pressure, fluid flow rates, and tank levels to central control rooms. In manufacturing plants, it helps in controlling and monitoring processes such as temperature, humidity, and chemical concentrations in reactors. In the building automation sector, 4 - 20mA signals are employed to manage heating, ventilation, and air - conditioning (HVAC) systems, adjusting parameters based on environmental sensors' readings.​

In conclusion, the 4 - 20mA output is a fundamental and reliable method for analog signal transmission in industrial and process - control environments. Its robustness against noise, support for loop - powered devices, and ease of fault detection have made it a preferred choice for countless applications, ensuring the smooth operation and efficient control of industrial systems worldwide. As technology continues to evolve, the 4 - 20mA standard may integrate with newer digital communication protocols, but its importance in the realm of analog - based industrial automation remains firmly established.