The 4–20 mA current loop is one of the most widely used analogue signal standards in industrial instrumentation. It is commonly used to transmit process measurements such as pressure, level, flow and temperature between field instruments and control equipment.
For maintenance and purchasing teams, understanding the basics of a 4–20 mA loop makes it much easier to identify transmitters, analogue input modules, isolators and replacement instrumentation correctly.
What Does 4–20 mA Mean?
In a conventional 4–20 mA measurement loop, the electrical current represents a process value across a configured measurement range.
The lower end of the measurement range is normally represented by 4 mA and the upper end by 20 mA.
For example, if a pressure transmitter is configured for 0 to 10 bar:
- 4 mA represents 0 bar
- 8 mA represents approximately 2.5 bar
- 12 mA represents approximately 5 bar
- 16 mA represents approximately 7.5 bar
- 20 mA represents 10 bar
The relationship is normally linear unless the instrument or control system has intentionally been configured otherwise.
Why Does the Signal Start at 4 mA Instead of 0 mA?
The 4 mA lower value is often called a live zero.
Using a non-zero current for the bottom of the measurement range provides useful advantages. A valid process measurement at the bottom of the range can still produce current in the loop, while certain wiring or instrument faults may result in a current outside the normal measurement range.
This can help a control system distinguish between a legitimate zero process value and some fault conditions.
However, the exact current used to indicate faults depends on the instrument, its configuration and the receiving equipment. A technician should therefore use the manufacturer's documentation rather than assuming that every value below 4 mA or above 20 mA represents the same fault.
What Is a Current Loop?
A current loop is an electrical circuit in which the measurement is represented by the current flowing through the loop.
A basic loop can contain:
- A DC power supply
- A field transmitter
- Wiring
- A receiving device such as a PLC analogue input, indicator or controller
The exact arrangement depends on whether the transmitter is loop powered or separately powered and on the electrical design of the receiving equipment.
What Is a Two Wire Loop-Powered Transmitter?
A two wire transmitter commonly uses the same pair of conductors for both its operating power and its measurement signal.
The transmitter regulates the loop current according to the measured process variable. Because power and signal share the same circuit, the available loop voltage must be sufficient for the transmitter and all other series voltage drops in the loop.
Two wire transmitters are common in industrial process measurement because they can simplify field wiring.
Two Wire, Three Wire and Four Wire Instruments
Not every 4–20 mA device is a two wire loop-powered transmitter.
A three wire instrument generally uses a separate supply conductor with a shared common arrangement, while a four wire instrument typically has separate power and signal connections.
The exact wiring depends on the device.
When replacing an instrument, do not assume that another 4–20 mA device is electrically interchangeable merely because its output range is the same. Confirm its supply requirements, wiring arrangement and output characteristics.
Why Use Current Instead of Voltage?
Current loops are well suited to many industrial installations because the measurement is represented by loop current rather than by measuring a voltage level directly at the receiving end.
Within the permitted loop voltage and load limits, normal wiring resistance causes voltage drop but does not necessarily change the commanded loop current in the way that voltage drop can directly affect a simple voltage signal.
This contributes to the widespread use of 4–20 mA signals over industrial cable distances.
Good wiring, screening, earthing and installation practice are still important, particularly in electrically noisy environments.
What Is Loop Voltage?
A current loop needs sufficient voltage to operate correctly.
The power supply must provide enough voltage for the transmitter and the voltage drops across the receiving equipment, wiring and any other devices connected in series.
This available voltage margin is sometimes discussed as loop compliance or voltage budget.
If the available voltage is insufficient, the transmitter may be unable to drive the required current through the total loop resistance, particularly toward the upper end of the 4–20 mA range.
Why Loop Resistance Matters
Every series resistance in the loop creates a voltage drop according to Ohm's law.
Sources of loop resistance can include:
- PLC analogue input resistance
- Indicators
- Signal isolators
- Barriers
- Long cable runs
- Other series devices
The total loop must remain within the transmitter manufacturer's permitted load for the available supply voltage.
How Does a PLC Read 4–20 mA?
A PLC analogue input module receives the electrical signal and converts it into a numerical value that the controller can process.
The PLC program or module configuration then scales that raw input to the relevant engineering range.
For a transmitter configured as 0 to 10 bar, the control system can scale the normal 4–20 mA range to 0–10 bar.
The exact raw counts, resolution and configuration procedure depend on the PLC and analogue input module.
Understanding Scaling
For a standard linear 4–20 mA signal, the process percentage can be expressed conceptually as:
Percentage of range = (measured current − 4 mA) ÷ 16 mA × 100
At 12 mA:
(12 − 4) ÷ 16 × 100 = 50%
If the configured process range is 0 to 10 bar, 50% corresponds to 5 bar.
If the configured range is instead −50 to 150 degrees Celsius, the same 12 mA represents the midpoint of that different engineering range.
This is why knowing that a transmitter is '4–20 mA' is not enough. The configured measurement range also matters.
What Happens Below 4 mA or Above 20 mA?
Some transmitters use currents outside the normal measurement range to indicate underrange, overrange or diagnostic conditions.
Industry conventions such as NAMUR NE 43 are used by compatible instrumentation to distinguish measurement limits and failure indications, but actual implementation depends on the device and configuration.
Do not use a single assumed fault threshold for every instrument. Check the manufacturer's manual and the control system configuration.
What Is an Open Circuit?
If a simple two wire current loop is physically broken, current can no longer flow through the circuit.
The receiving equipment may therefore see a very low or zero current condition, depending on the circuit and failure mode.
This is one reason the live zero concept is useful: zero current is clearly outside the normal 4–20 mA measurement range.
Actual troubleshooting should still consider the power supply, transmitter, wiring, barriers, terminals and analogue input rather than assuming the transmitter itself has failed.
Active and Passive Analogue Inputs
The terms active and passive are used when discussing current-loop equipment, but they can be confusing because terminology varies with perspective.
Some analogue input arrangements provide loop power, while others expect an externally powered current loop. Similarly, transmitters can be loop powered or separately powered.
Before connecting equipment, use the manufacturer's wiring diagrams to establish which device supplies loop power and how the circuit is intended to be connected.
What Is a Signal Isolator?
A signal isolator or signal conditioner can be used between field instrumentation and control equipment for functions such as galvanic isolation, signal conversion, duplication or conditioning.
Applications can include:
- Reducing unwanted electrical interaction between circuits
- Converting one signal type to another
- Splitting one process signal to more than one receiving system
- Providing isolation where required by the system design
The isolator itself must be correctly selected for input signal, output signal, power supply and isolation requirements.
What Is a 4–20 mA Loop Isolator or Splitter?
A loop isolator can reproduce an input current signal while electrically isolating the input and output circuits. A signal splitter can provide multiple isolated outputs representing the same process measurement.
This can be useful where, for example, one transmitter measurement needs to be supplied to separate control and monitoring systems.
Whether this is appropriate depends on the control architecture and the specific equipment.
4–20 mA vs 0–10 V
Both are common industrial analogue signal types, but they are not interchangeable.
A 4–20 mA signal is represented by current. A 0–10 V signal is represented by voltage.
Current loops are widely used for field instrumentation, particularly where cable distances and industrial environments are considerations. Voltage signals are also common for equipment such as drives, actuators and control devices.
The correct choice depends on the equipment and application.
Can You Connect a 4–20 mA Transmitter to a 0–10 V Input?
Not directly unless the equipment specifically supports the required conversion or the circuit has been designed to convert the signal appropriately.
Likewise, a voltage output should not simply be connected to an input configured only for current.
Where signal types differ, an appropriate signal converter or conditioner may be required.
4–20 mA and Variable Speed Drives
Many VSDs provide analogue inputs that can accept a 4–20 mA speed reference or process feedback signal, depending on the drive.
For example, a PLC could send a 4–20 mA speed reference to a VSD, or a pressure transmitter could provide process feedback to a drive's built-in PID controller in a suitable pumping application.
The VSD analogue input must be configured for the correct signal type and scaling.
4–20 mA and Process Instrumentation
Typical instruments using 4–20 mA outputs can include:
- Pressure transmitters
- Differential pressure transmitters
- Level transmitters
- Flow transmitters
- Temperature transmitters
- Analytical instruments
- Position transmitters
Some modern instruments also superimpose digital communications such as HART on a conventional analogue current loop where supported.
What Is HART?
HART is a communication technology used with compatible process instrumentation. It can allow digital information to be exchanged while retaining the conventional 4–20 mA analogue process signal.
Depending on the instrument and system, HART can be used for configuration, diagnostics and access to additional device information.
HART capability should not be assumed simply because an instrument has a 4–20 mA output.
Common 4–20 mA Troubleshooting Problems
When a process reading is incorrect, the transmitter is only one possible cause.
Potential issues can include:
- No loop power
- Incorrect polarity
- Open circuit wiring
- Loose terminals
- Insufficient loop voltage
- Excessive loop resistance
- Incorrect PLC input configuration
- Wrong signal range selected
- Incorrect process scaling
- Faulty signal isolator or barrier
- Transmitter configuration error
- Instrument or sensor failure
Testing live process instrumentation should be performed according to site procedures by appropriately competent personnel.
Common Selection Mistakes
A request for 'a 4–20 mA pressure sensor' is usually not enough information to identify the correct replacement.
Important details can include:
- Manufacturer
- Exact model or part number
- Measured variable
- Measurement range
- Process connection
- Wetted materials where relevant
- Supply voltage
- Two wire, three wire or four wire arrangement
- Electrical connection
- Environmental or IP rating
- Accuracy requirement
- Temperature range
- Hazardous area approval where applicable
- Required communications such as HART where applicable
Two transmitters can both provide a 4–20 mA output while being completely unsuitable substitutes for one another.
Replacing an Existing 4–20 mA Transmitter
When replacing installed instrumentation, provide the exact manufacturer and model number wherever possible.
Clear photographs of the product label, process connection and electrical connection can help identify the existing device.
Also provide the configured measurement range if known. A replacement instrument may have the same physical sensing range but still need to be configured for the process range expected by the PLC or control system.
A Practical Planning Checklist
For a 4–20 mA transmitter enquiry, collect as much of the following as possible:
- Manufacturer: ___
- Exact model or part number: ___
- Measured variable: pressure, level, flow, temperature or other
- Measurement range: ___
- Output: 4–20 mA
- Supply: ___ V DC
- Wiring: two wire, three wire, four wire or unknown
- Process connection: ___
- Electrical connection: ___
- Wetted materials: ___ where applicable
- IP or environmental requirement: ___
- Hazardous area approval: ___ where applicable
- Communications: HART or other, where required
- Quantity: ___
- Photographs attached: yes or no
If you are replacing an existing transmitter, send the manufacturer, exact model number, measurement range and clear photographs of the instrument label and connections where possible.
Technical Note
This article provides general instrumentation information. Current-loop design, hazardous-area requirements, functional safety, intrinsic safety, wiring, configuration and process instrument selection must be assessed for the specific application. Manufacturer instructions, applicable standards and site procedures take precedence, and installation or modification should be undertaken by appropriately competent personnel where required.
Related catalogue examples: PLCs & Programmable Controllers. For an equipment enquiry, contact Softcore Group Industrial Solutions at info@softcoregroup.co.za.