AiVibe4–20 mA Current Loop
The 4–20 mA current loop is the long-established analogue signalling standard in process instrumentation, in which a transmitter represents a measurement as a DC current between 4 mA at the bottom of its range and 20 mA at the top.
In a current loop, a power supply, a transmitter and a receiver such as a PLC analogue input are connected in series, so the same current flows through every element. The transmitter regulates that current in proportion to the measured value, and the receiver measures it, often as a voltage across a precision resistor. Because current is the same at every point in a series circuit, cable resistance does not change the reading as long as the supply voltage is sufficient.
The live zero at 4 mA lets receivers distinguish a genuine bottom-of-range value from a broken wire or failed instrument, which reads near 0 mA. It also allows two-wire transmitters to draw their operating power from the loop itself. These properties have made 4–20 mA the default for pressure, flow, level and temperature transmitters and for valve positioner commands in process plants. The HART protocol superimposes digital communication on the same pair of wires.
Loop design must account for the supply voltage, the total loop resistance and the minimum voltage each transmitter needs. Many devices signal faults by driving the current outside the normal range, and NAMUR NE 43 is a widely used recommendation that defines such failure signal levels. Instrument loops in hazardous areas require certified intrinsically safe or otherwise protected designs, with OEM documentation and qualified personnel governing installation.
Key points
- Measurement is sent as current from 4 mA (range low) to 20 mA (range high)
- Series loop makes readings insensitive to cable resistance within limits
- Live zero at 4 mA distinguishes a true zero from a broken wire
- Two-wire transmitters draw their operating power from the loop
- NAMUR NE 43 is a widely used recommendation for failure signal levels
Where AiVibe comes in
AiVibe designs and manufactures the AiAmbA AI Factory, whose edge devices and AI agents let people talk to PLC, CNC and robot controllers in plain language. Agents only propose changes, a trained operator confirms each one, and the AI Factory never replaces a machine's control or safety systems.