How Two-Wire 4–20 mA Loop-Powered Current Sensors Work
The latest version of the EQSP32 has been improved by the addition of 4–20 mA analog input support and wired Ethernet, further cementing its role as a powerful and flexible IIoT controller for industrial environments.
This article explores the simple brilliance of the 4–20 mA signaling method – a standard born in the 1950s (with earlier roots) that enables the reliable transmission of sensor signals – and often, their power too – over long distances using just two wires.
A short history of the 4–20 mA standard
The 4–20 mA current loop became the industrial standard for analog signal transmission in the mid-20th century. Its rise coincided with the development of low-power microelectronics, which made it possible for sensors and transmitters to operate reliably on just a few milliamps of current.
The standard offered several practical advantages: it was inherently noise-resistant, well-suited for long cable runs, and compatible with simple 24 V DC power supplies. Crucially, its use of a live-zero (4 mA = 0%) made it easy to detect sensor failures or broken wires—something voltage signals or zero-current systems couldn’t do reliably.
These features helped 4–20 mA gain rapid adoption in process control, instrumentation, and industrial automation—and it remains dominant in many industries to this day.
Why 4–20 mA analog sensing?
The widespread use of 4–20 mA is no accident—it offers multiple benefits for industrial environments:
- Immunity to voltage drop and noise: Current loops maintain consistent signal integrity even over long wire runs.
- Simple wiring: Only two wires are needed for both signal transmission and power delivery.
- Live zero: A signal of 4 mA represents zero output, allowing systems to detect cable breaks or sensor failures.
- Built-in power delivery: Sensors can be powered directly from the current in the loop – so long as they draw less than 4 mA.
- Standardized integration: Most PLCs, controllers, and industrial devices (like EQSP32) support 4–20 mA inputs directly.
How does a loop-powered (2-wire) sensor work?
It is possible to power a sensor and transmit analog information over the same two wires using 4–20 mA DC, if the transmitter is loop-powered.
In this setup, the sensor behaves like a current regulator, not a true current source. The current is driven by a remote supply (typically 24 V DC) and passes through all loop components-including the sensor and a load resistor in the controller (470 Ω in the case of the EQSP32). The sensor regulates the current to represent the measured value (e.g., pressure, temperature), but draws its own operating power from the loop itself.
Visualizing the loop: A 10 mA example
The diagram above shows a real-world example of a 4–20 mA current loop where an industrial sensor is connected to an EQSP32 controller. Here’s what happens:
- A 24 V DC power supply powers both the EQSP32 and the sensor loop.
- The loop current flows through the sensor, then through a current-sense resistor inside the EQSP32.
- Inside the sensor:
- The sensor interface and amplifier electronics consume 2 mA to operate.
- A current sense resistor that measures the total loop current
- A transistor shunts 8 mA to regulate the total loop current to 10 mA.
- The EQSP32, configured in CIN mode, measures this total loop current precisely.
This example shows a loop current of 10 mA, which represents a value partway between the minimum (4 mA) and maximum (20 mA) limits.
Converting current to percentage
To determine what process value a current corresponds to, we calculate its normalized percentage within the 4–20 mA span:
So for a 10 mA signal:
10 mA = 37.5% of the measurement range
If the sensor represents 0–100 bar pressure, then 10 mA corresponds to 37.5 bar.
The trade-off: power limits
Because the sensor must operate on just 4 mA and ~19 V, this limits what can be done inside the sensor. Devices requiring heaters, solenoids, or embedded computers (e.g., chromatographs or spectrometers) simply can’t function with this little power.
These devices require 4‑wire transmitters, where the signal lines (typically voltage or 4–20 mA) are separate from the power supply.
EQSP32 support for 4–20 mA
With the latest update, EQSP32 supports 4–20 mA inputs via the CIN mode, available on analog-capable pins (1–8). When configured, the EQSP32:
- Reads current in hundredths of a milliamp
- Detects overcurrent at > 21 mA and triggers protection
- Converts readings to mA easily using the CONVERT_CIN() macro
This makes it an ideal controller for 2‑wire loop-powered sensors across industrial sectors—whether you’re measuring pressure, flow, or temperature in remote or noisy environments.