Instrumentation · Lesson 2 of 10

Analog signals: 4–20 mA and 0–10 V

Most transmitters in the world speak one language: a current from 4 to 20 milliamps. In this lesson you will understand why current and not voltage, why it starts at 4 and not zero, how to convert mA to a value, how to measure the loop properly, and how to tell the fault from the meter reading.

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1Why current and not voltage?

Imagine a transmitter 300 metres from the panel. If it sends a voltage (0–10V), the long wire has resistance, and any current flowing in it causes a voltage drop, so the PLC reads less than the truth. The long cable also picks up noise from motor and VFD cables.

But in a series circuit the current is the same everywhere (Kirchhoff's law). The transmitter sets the current itself, so the wire resistance does not change it as long as the supply is sufficient. That is what lets 4–20 mA travel long distances and resist noise.

	4–20 mA	0–10 V
Distance	Hundreds of metres, no problem	Short (inside the panel or nearby)
Wire resistance	No effect (if the supply is sufficient)	Causes a reading error
Broken wire detection	Yes: 0 mA = fault	No: 0 V = a zero value
Used for	Field transmitters	Potentiometers, VFDs, nearby devices

2Why 4 and not zero? (live zero)

If the signal is 0–20 mA and you read 0 mA, you can't tell: is the value really zero, or has the wire broken? But with 4–20, the lowest real value = 4 mA. So if you read much less than that, there is definitely a fault. Those 4 mA also power a 2-wire transmitter (it lives on them).

Current	Meaning per NAMUR NE43
Below 3.6 mA	Fault: broken wire or the transmitter signalling a fail-low alarm
3.8–4 mA	Slightly under range (value below LRV) but the signal is healthy
4–20 mA	Normal measurement
20–20.5 mA	Slightly over range but healthy
Above 21 mA	Fault: a short or the transmitter signalling a fail-high alarm

32-wire, 3-wire and 4-wire transmitters

2-wire (loop powered)

The same two wires carry the power and return the signal. The transmitter lives on the 4 mA. This is the most common: pressure, temperature, level.

3-wire

+24V and 0V are shared, and the third wire is the signal output. Common on devices that draw more than 4 mA (small flowmeters, sensors with a display).

4-wire

Two separate power wires (usually 230V AC or 24V) and two signal wires. Like magnetic flowmeters, analysers and indicators. The output is usually active: it drives the current itself.

Active or passive? Whatever supplies power to the loop is active. Exactly one side must be active: either the card supplies 24V (a 2-wire card) and the transmitter is passive, or the transmitter is active (4-wire) and the card is passive. If both are active or both passive, the loop won't work (or something burns).

4The 24V supply and the 250Ω resistor

An analog card usually has a 250Ω resistor (or close) inside. The current flows through it and creates a voltage per Ohm's law: V = I × R. So 4 mA ⇒ 1 V, and 20 mA ⇒ 5 V. That is what the card actually measures (which is why you hear "a 1–5V signal").

V(250Ω) = mA × 0.25

A 2-wire transmitter needs a minimum voltage at its terminals (usually 10.5 to 12 V; read the datasheet). So the supply must cover the transmitter + all the resistances in the loop at the highest current:

V supply ≥ V min(TX) + 0.020 × R loop

Example: 24V supply, transmitter needs 10.5V ⇒ maximum loop resistance = (24 − 10.5) ÷ 0.020 = 675 Ω. So 250Ω for the card + 250Ω for a HART resistor + the wire, and all is fine. But at 12V ⇒ (12 − 10.5) ÷ 0.020 = 75 Ω only! Try it in the simulator below.

5Converting mA to a value and back

The 16 mA (from 4 to 20) represent the whole span. So the equation is:

PV = LRV + (mA − 4) ÷ 16 × (URV − LRV)

mA = 4 + (PV − LRV) ÷ (URV − LRV) × 16

Example 1: a 0–10 bar pressure transmitter giving 12 mA ⇒ 0 + (12 − 4)/16 × 10 = 5 bar.
Example 2: a −50 … 150 °C temperature transmitter giving 8 mA ⇒ −50 + 4/16 × 200 = 0 °C. Note that 8 mA here means zero degrees!
Example 3: level 0–4 m with the level at 3 m ⇒ 4 + 3/4 × 16 = 16 mA.

Scaling calculator

Scaling drill

Inside the PLC: the card converts the mA to a raw number. In Siemens, for example, 4 mA = 0 and 20 mA = 27648, and a broken wire = 32767. Allen-Bradley and others use different numbers. Then a scale block (like NORM_X / SCALE_X) converts the number to the engineering value.

6Loop simulator: create the faults yourself

This is a real loop: a 24V supply, a 2-wire pressure transmitter with a 0–10 bar range, a multimeter on mA wired in series, and a PLC card. Change the pressure and watch the current, then create the faults one at a time and see what the meter and the PLC say.

4–20 mA loop

7How to measure loop current properly

Inform operations and put the loop in Manual (the reading will drop out for a moment when you open the loop).
Meter on mA DC with the red lead in the mA socket (not the 10A socket).
Open the loop: undo one wire from a terminal and put the meter in series in its place.
Read the current and compare it with the value expected from the equation.
Put the wire back, tighten it well, and return the loop to Auto.

Don't: never put the meter on mA in parallel across the transmitter or card terminals. Its resistance is very low, so it shorts them: the reading is wrong, the PLC sees overrange, and the meter fuse may blow. In parallel you measure volts only.

Without opening the loop: many transmitters have a TEST terminal with a diode: put the meter across it and read the current without undoing anything. There is also the mA clamp (process clamp meter) that measures 4–20 mA around the wire. Or measure the voltage across the 250Ω resistor and divide by 0.25.

HART: a protocol that superimposes a small digital signal (a 1200/2200 Hz tone) on top of the 4–20 mA without changing the average. With a HART communicator you can read the tag, range and diagnostics and change the range from the panel. It needs a loop resistance of at least 250Ω to work.

8The cable: shielded, twisted and grounded at one end

Use a shielded twisted pair cable (a twisted pair with an aluminium foil or braid shield): the twist cancels magnetic noise, and the shield catches electrical noise.
Ground the shield at one end only, usually at the panel (the instrument earth bar), and cut back and insulate the other end inside the transmitter. If it is grounded at both ends, current can flow in it due to ground potential differences (a ground loop) and cause noise.
Keep signal cables away from power and VFD cables: separate trays, and if they must cross, cross at 90°.

9Common field faults

Meter reads	PLC says	Most likely
0 mA	Wire break / −25 %	Broken wire, blown loop fuse, reversed polarity, or no power
3.6 or 21+ mA	Fault	The transmitter itself is reporting a fault (check its display or use HART)
Above 22 mA and steady	Overrange	A short in the cable or water in the junction box
Stuck at a number and won't rise	A fixed value	Weak supply or high loop resistance (low voltage at the transmitter)
Unstable	The reading jumps around	Shield not grounded, cable next to a VFD cable, or a loose terminal

?Lesson quiz

Get at least 5 of 6 right to mark the lesson as done.

The numbers (10.5V, 250Ω, 27648) are common values, not fixed ones: the transmitter and card datasheets are the reference. Your progress is saved on this device only.
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