Why Process Control Systems Use the 4-20mA Analog Signal Transmission Standard

The 4-20mA DC (1-5V DC) signal protocol is the analog signal transmission standard adopted by the International Electrotechnical Commission (IEC) for process control systems. China also adopts this international standard signal protocol, using 4-20mA DC for instrument signal transmission and 1-5V DC for signal reception, thereby employing a system that transmits via current and receives via voltage. The function of the multi-functional electrical quantity transmitter.

Three-Phase Electric Quantity Transmitter: As industrial applications commonly require the measurement of various non-electrical physical quantities such as temperature, pressure, speed, and angle, these must be converted into analog electrical signals to be transmitted to control rooms or display devices several hundred meters away. The reason for using current signals is that they are less susceptible to interference, as the noise voltage in industrial environments can reach several volts, but the noise power is weak, so the noise current is typically less than nA.

Thus, the error introduced by the 4-20mA transmission is minimal.

Due to the high internal resistance and constant current output of the current source, at the receiving end, we only need to place a 250Ω resistor to ground to obtain a voltage of 0-5V. The benefit of a low-input impedance receiver is that nA-level input current noise only produces a very weak voltage noise. The upper limit is set at 20mA due to explosion-proof requirements: the spark energy caused by the on-off of 20mA current is not sufficient to ignite gas. The lower limit is not set at 0mA to enable wire break detection.