Learn Automation with MINATA #36: Analogue Signals, 0–10V and 4–20mA — Reading Continuous Values Without Guessing

Analogue Signals, 0–10V and 4–20mA: Reading Continuous Values Without Guessing

A digital input only answers ON or OFF. When the system needs to know how much pressure there is, where the tank level sits, how the temperature is moving or what speed has been set, it uses an analogue signal. The two common standards are 0–10V and 4–20mA. They become trustworthy only when the module, the wiring type and the scaling in the PLC and HMI all agree.

How 0–10V and 4–20mA differ

0–10V: a voltage proportional to the measured value. Common for speed references, distance sensors and devices inside the panel. It is more sensitive to voltage drop and to noise over a long run.
4–20mA: a current proportional to the measured value. The 4mA point is a live zero, so a value below it can reveal a broken loop depending on the configuration. This standard usually suits long cable runs and noisy industrial environments better.

You cannot convert 0–10V into 4–20mA by changing the wire. The sensor, the input module, the supply arrangement (two-, three- or four-wire) and the measurement configuration all have to match.

Scaling the value inside the PLC

The module returns a raw value; the program has to turn it into engineering units. Take a 4–20mA transmitter measuring 0–10 bar: 4mA is 0 bar and 20mA is 10 bar. The linear formula has to use the module's actual raw range, and the HMI then shows bar, the alarm limits and the filtered value.

Three things belong in the documentation together: the sensor range, the module range and the display range. If the HMI says 0–10 bar while the program scales 0–16 bar, a machine can run wrongly and still look entirely plausible.

Wiring and noise immunity

Check the manufacturer's diagram for whether the input is active or passive and where the COM sits.
Route analogue wiring away from motor and drive cables; use shielded cable where it is required.
Bond the shield according to the system rule — do not simply tie both ends and create an earth loop.
Supply the sensor with the correct arrangement, and check the loop current for 4–20mA.
Add just enough digital filtering: too much slows the control, too little makes the HMI number jitter.

The analogue standards side by side

Standard	Character	Noise and long runs	Broken-loop detection
0–10V	Proportional voltage, common inside panels	Weaker, sensitive to voltage drop	Hard — 0V looks like a low value
0–20mA	Proportional current	Good	No — 0mA is a valid value
4–20mA	Proportional current, live zero at 4mA	Good, preferred in the field	Yes — below 4mA suggests a break or fault
±10V	Bidirectional voltage, used for signed speed references	Moderate	Hard

The scaling formula, with numbers

Convert the module's raw value into engineering units (EU) by linear interpolation:

EU = EU_min + (raw − raw_min) × (EU_max − EU_min) / (raw_max − raw_min)

Take a 4–20mA transmitter measuring 0–10 bar, with an analogue module returning 0–32000 across the 4–20mA range. At 12mA — mid-range — raw is about 16000, so EU = 0 + 16000 × (10 − 0) / 32000 = 5.0 bar. Always re-check the real raw_min and raw_max of the specific module, because each manufacturer numbers the raw range differently.

Two-, three- and four-wire transmitters

Two-wire (loop-powered): supply and signal share one 4–20mA pair. It saves wiring but needs enough loop voltage.
Three-wire: separate supply with a shared 0V for the signal; common on 0–10V or 0–20mA sensors.
Four-wire: supply and signal entirely separate; used by instruments that need their own power.

The supply arrangement decides whether the input module must be active or passive — read the manufacturer's diagram before wiring.

A worked engineering situation

A compressed-air receiver has a 4–20mA transmitter. The PLC uses the scaled value to display pressure, to raise low and high alarms, and to hold a cycle until the pressure is sufficient. During commissioning the technician compares the PLC value against a reference gauge at several points across the range, not only at a single mid-scale point.

Alarm thresholds, tolerance and the earthing architecture must follow the process requirement, the actual instruments and the safety documentation.

Analogue checklist

[ ] The standard — 0–10V or 4–20mA — the sensor type and the module are all defined.
[ ] Engineering range, raw range, unit and alarm limits are recorded.
[ ] Wiring and shield are routed away from noise sources.
[ ] Scaling is verified against reference points, not one single value.
[ ] Out-of-range and broken-loop indication matches what the module can actually detect.

Analogue is where power quality, wiring and software meet. After this, #37 goes into the variable frequency drive — a device that uses both digital and analogue control constantly.

Read more automation knowledge at MINATA: https://minatavn.com/en/blog/industrial-automation

Previous — #35: PLC timers and counters: https://minatavn.com/en/blog/automation-35-plc-timers-counters

Next — #37: The variable frequency drive: principles and limits: https://minatavn.com/en/blog/automation-37-vfd-principles

View all MINATA technical articles