Definition
4～20mA is an industrial standard current loop analog signal, where 4mA corresponds to the lower limit of the range and 20mA corresponds to the upper limit. When the line is disconnected, the current is 0, facilitating fault detection. Pressure sensors, gas sensors, light sensors, and other devices often support 4～20mA or Modbus output. The HART protocol overlays digital communication on the 4～20mA loop, enabling smart instrument configuration and diagnostics.
Working Principle and Advantages
4～20mA uses a two-wire current loop: the sensor or transmitter converts the physical quantity into a 4～20mA current. At the receiving end (such as a PLC, DCS, or data acquisition device), a 250Ω resistor is connected in series to convert the current into a 1–5V voltage for measurement. The current signal has strong anti-interference capability; voltage drops during long-distance transmission (hundreds of meters to kilometers) do not affect the current value. Two-wire wiring can simultaneously power and transmit, simplifying wiring. The 4mA zero point (not 0mA) helps distinguish between a disconnected line (0mA) and the normal lower limit. The 20mA full-scale limit reduces the risk of short circuits. In industrial sites, 4～20mA is widely used to connect sensors measuring pressure, temperature, flow, liquid level, pH, gas concentration, and more.
Relationship with HART
HART (Highway Addressable Remote Transducer) superimposes FSK digital modulation (2200Hz represents 1, 1200Hz represents 0) on a 4–20mA analog signal to achieve digital communication without affecting the analog signal. The master device can simultaneously read the 4–20mA analog values and HART digital data (such as device status, range, units, diagnostic information, etc.) and can remotely configure device parameters. HART supports point-to-multipoint mode, allowing multiple smart instruments to be connected to the same bus, suitable for scenarios that require both analog readings and digital configuration.
Acquisition and Conversion
When an IoT gateway or PLC collects a 4–20mA signal, it needs to convert the current into digital values through an ADC. Typical solution: connect a precision resistor in series (e.g., 250Ω) to sample the voltage, then collect it through the ADC and convert according to the formula: Physical Quantity = Lower Limit of Range + (ADC Value - Value Corresponding to 4mA) / (20mA - 4mA) × Range. Pay attention to the range limits, units, and linear/non-linear calibration. For multi-channel acquisition, a multiplexer or dedicated analog input module can be used, supporting protocols like Modbus to report data to the platform.
Use Cases
The 4–20mA signal is widely used in process monitoring in industries such as petrochemical, power, water treatment, metallurgy, and environmental protection. Pressure transmitters output pipeline pressure and liquid level; temperature transmitters output process temperature; gas sensors output concentrations of CO, O2, and combustible gases; flow meters output instantaneous flow; and pH meters output acidity and alkalinity. Industrial gateways commonly provide 4–20mA input interfaces to connect sensor data to protocols like Modbus and OPC UA, uploading it to SCADA, DCS, and IoT platforms for remote monitoring and data analysis.