4-20 mA Current Loop - Industrial Analog Signaling

The 4-20 mA current loop is the standard analog interface for industrial process measurement. Pressure transmitters, temperature transmitters, flow meters, level sensors, valve positioners, and many analyzers use it because it is simple, robust, and easy to diagnose over long cable runs.

It is not a digital protocol. It has no device address, packet, clock, or function code. It is an analog current signal where the transmitter regulates loop current in proportion to the measured process value.

Learning Objectives

By the end of this lesson, you should be able to:

Explain why process instruments use current instead of voltage for long field wiring.
Convert a loop current into percent span and engineering units.
Size the receiver shunt voltage for a PLC or ADC input.
Distinguish two-wire loop-powered transmitters from four-wire transmitters.
Recognize underrange, overrange, open-circuit, and short-circuit symptoms.
Explain how HART adds digital communication without changing the DC process value.

The Core Idea

A 4-20 mA loop connects a DC supply, field transmitter, cable, and receiver in series. The transmitter behaves like a controlled current sink or current regulator. If it wants to report 50% of span, it adjusts the loop to 12 mA.

define SUPPLY24 annotation=PS label="24 V DC Supply" {
  pin 1 + right
  pin 2 - bottom
}

define TX2WIRE annotation=TX label="2-wire Transmitter" {
  pin 1 LOOP+ left
  pin 2 LOOP- right
}

define AI annotation=AI label="PLC Analog Input" {
  pin 1 IN+ left
  pin 2 IN- bottom
  pin 3 ADC right
}

PS1: SUPPLY24 value="Loop supply"
TX1: TX2WIRE value="Sensor controls 4-20 mA"
R1: Device:R value="250 ohm shunt" rotate=0
AI1: AI value="1-5 V to ADC"

layout direction=LR gap=90
group SOURCE label="Source" direction=TB {
  PS1
}
group FIELD label="Field device" direction=LR {
  TX1
}
group INPUT label="Receiver" direction=TB {
  R1 AI1
}

PS1.1 --> TX1.1 color=#b91c1c
TX1.2 --> R1.1 color=#b91c1c
R1.1 --> AI1.1 color=#b91c1c
R1.2 --> AI1.2 color=#1d4ed8
AI1.2 --> PS1.2 color=#1d4ed8
AI1.3 --> NC

The same current flows through every series element. Cable resistance consumes some supply voltage, but it does not change the current as long as the supply has enough compliance voltage.

flowchart LR PS["24 V supply"] --> TX["Transmitter\nsets loop current"] TX --> SH["250 ohm shunt\nat PLC input"] SH --> RET["Return to supply"] RET --> PS SH --> ADC["ADC sees 1-5 V"]

Why 4 mA Is Called Live Zero

The lower end is 4 mA, not 0 mA. That design choice solves two practical problems:

Current	Meaning in a typical loop
0 mA	Open wire, dead supply, or disconnected transmitter
3.6 mA or lower	Fault or severe underrange, depending on device configuration
4.0 mA	Valid 0% process value
12.0 mA	Valid 50% process value
20.0 mA	Valid 100% process value
20-21 mA	Overrange region
Above 21 mA	Fault, short, failed transmitter, or misconfiguration

The 4 mA baseline also powers two-wire transmitters. Even when the measured process value is zero, the transmitter electronics still have loop current available.

Converting Current to Percent and Engineering Units

For a standard 4-20 mA span:

Percent span = (I_mA - 4 mA) / 16 mA x 100%
Value = ((I_mA - 4 mA) / 16 mA) x (High - Low) + Low

Where:

I_mA is the measured loop current in milliamps.
Low is the process value represented by 4 mA.
High is the process value represented by 20 mA.

Worked Example

A pressure transmitter is ranged from 0 to 10 bar. The PLC reads 14.4 mA.

Percent span = (14.4 - 4) / 16 x 100%
             = 10.4 / 16 x 100%
             = 65%

Pressure = 0.65 x (10 bar - 0 bar) + 0 bar
         = 6.5 bar

If the same current came from a 2-12 bar transmitter, the value would be:

Pressure = 0.65 x (12 bar - 2 bar) + 2 bar
         = 8.5 bar

Always check the configured range before converting milliamps to engineering units.

Reading the Loop with a PLC

Many PLC analog input modules measure voltage internally. A precision shunt resistor converts loop current to voltage:

V_shunt = I_loop x R_shunt

With a 250 ohm resistor:

Loop current	Shunt voltage	Meaning
4 mA	1.0 V	0%
12 mA	3.0 V	50%
20 mA	5.0 V	100%

Use a resistor with suitable tolerance and temperature coefficient if it is external. A 0.1% resistor gives much better scaling accuracy than a 5% resistor. Also check input burden: every receiver, barrier, indicator, and cable resistance consumes voltage from the loop supply.

Loop Compliance Check

The supply voltage must be high enough for all series voltage drops:

V_supply >= V_transmitter_min + I_max x R_total + safety_margin

For a 24 V loop, transmitter minimum of 12 V, 250 ohm input resistor, and 100 ohm cable/barrier resistance:

Voltage drop at 20 mA = 0.020 A x (250 ohm + 100 ohm)
                      = 7.0 V

Required supply = 12 V + 7 V
                = 19 V

A 24 V supply has about 5 V of margin in this example.

Two-Wire, Three-Wire, and Four-Wire Devices

Type	Wiring	Typical use	Key point
Two-wire loop-powered	Same two wires carry power and signal	Field transmitters	Most common process instrument style
Three-wire	Separate supply positive, common return, signal output	Some sensors and small actuators	Signal and power share a reference
Four-wire	Separate power pair and signal pair	Analyzers, powered instruments, valve controllers	Device can consume more power than the loop allows

Two-wire devices must operate from the loop current itself. Four-wire devices can power heaters, displays, relays, or measurement circuits independently while still outputting a 4-20 mA signal.

HART on a 4-20 mA Loop

HART adds a small frequency-shift-keying signal on top of the DC loop current. The waveform averages to zero, so the analog 4-20 mA value remains unchanged.

flowchart LR A["4-20 mA DC value\nPrimary process variable"] --> L["Same loop wires"] H["HART FSK overlay\n1200 baud\nzero average"] --> L L --> R["PLC input plus HART modem"]

HART is used for configuration, diagnostics, trim, tag information, and secondary variables. The plant can keep reading the analog current while a handheld communicator or asset-management system talks digitally to the transmitter.

Practical Checks

Confirm the loop supply polarity before connecting the transmitter.
Measure current in series, not voltage across the loop, when diagnosing the actual signal.
Check whether the PLC input already includes a 250 ohm shunt before adding an external one.
Verify loop compliance at 20 mA, not only at 4 mA.
Confirm the transmitter range in the instrument configuration and the PLC scaling block.
Use shielded twisted pair where the site standard requires it, and ground the shield according to the plant grounding practice.
In hazardous areas, use approved barriers or isolators and follow the site safety documentation.

Common Mistakes

Treating 4 mA as a fault instead of valid 0% process value.
Assuming 12 mA always means a specific engineering value without checking the configured range.
Adding a second shunt resistor and overloading the loop.
Forgetting cable, barrier, and indicator voltage drops in the compliance calculation.
Measuring loop current by putting the meter across the supply, which can short the loop.
Ignoring NAMUR-style fault current behavior configured in the transmitter.

Summary

A 4-20 mA loop is an analog current interface for industrial process values. The live-zero 4 mA point powers two-wire transmitters and separates valid zero from broken wiring. A receiver usually converts current to voltage with a precision shunt, commonly 250 ohm for a 1-5 V signal. Correct commissioning depends on scaling, loop compliance, polarity, fault thresholds, and safe measurement practice.

Mind Map

mindmap root((4-20 mA Loop)) Core concept Series current loop Transmitter controls current Live zero at 4 mA Applications Pressure Temperature Flow Level Valve command Formulas Percent equals I minus 4 over 16 Value equals span fraction times range plus low V equals I times R Supply covers transmitter plus burden Design rules Check compliance at 20 mA Use precision shunt Keep polarity correct Use approved barriers Practical checks Measure current in series Confirm range Check open wire Verify PLC scaling Common mistakes Treating 4 mA as fault Double shunt Wrong range No voltage margin