FNIRSI SG-004A Active vs Passive 4-20mA Signal Testing Malaysia
FNIRSI SG-004A Active vs Passive 4-20mA Signal Testing Malaysia
Active vs passive 4-20mA signal testing is an important topic for industrial instrumentation technicians, PLC engineers and factory maintenance teams.
When testing a 4-20mA current loop, technicians must understand whether the circuit requires a current source or an externally powered current simulator.
Selecting the wrong operating mode can cause incorrect readings, prevent the loop from operating or potentially damage connected equipment.
The FNIRSI SG-004A Multifunction Signal Generator offers current signal functions for testing compatible industrial instrumentation.
This guide explains the differences between active and passive current testing, how 24V loop power is involved and what technicians should check before connecting a signal generator.
What Is a 4-20mA Current Loop?
A 4-20mA current loop is widely used in industrial automation and process instrumentation.
It carries a current signal representing a measured or commanded process value.
For example, a pressure transmitter configured for 0-10 bar may use:
| Current Signal | Pressure |
|---|---|
| 4mA | 0 bar |
| 8mA | 2.5 bar |
| 12mA | 5 bar |
| 16mA | 7.5 bar |
| 20mA | 10 bar |
The controller converts the measured current into the corresponding engineering value.
However, not all 4-20mA circuits use the same power arrangement.
Understanding the current loop configuration is essential before testing.
What Is Active 4-20mA Current Output?
Active current output means that the signal source provides the electrical drive required to establish the selected loop current through a compatible load.
A signal generator operating in active current output mode can provide a controlled current to an appropriate receiving input.
Example Application
A technician needs to test a PLC analog input that is designed to receive current from an externally powered signal source.
The technician can use a compatible active current output to generate:
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4mA
-
8mA
-
12mA
-
16mA
-
20mA
The PLC should display the corresponding engineering values when configured correctly.
When Is Active Current Output Useful?
Active current output is useful for:
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PLC Analog Input Testing
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Control Panel Commissioning
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DCS Input Verification
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Industrial Signal Troubleshooting
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Instrumentation Training
Before connecting an active output, confirm that the receiving circuit is suitable and that another active source is not connected across the same terminals.
What Is Passive 4-20mA Current Simulation?
Passive current simulation is used in a suitable externally powered current loop.
Instead of providing the loop's power, the simulator controls the current drawn from an external supply within its supported operating limits.
This arrangement is often used when testing circuits designed for two-wire loop-powered transmitters.
Example Application
A PLC analog input circuit supplies loop power to a field transmitter.
During an approved maintenance test, the transmitter is safely isolated.
A compatible passive current simulator can be connected in the correct loop arrangement to simulate the transmitter's electrical behavior.
The controller can then be checked at selected current values.
When Is Passive Current Simulation Useful?
Passive simulation may be useful for:
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Two-Wire Transmitter Loop Testing
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Externally Powered 4-20mA Circuits
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PLC Inputs with Loop Power
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Process Instrumentation Troubleshooting
The exact wiring depends on the loop design and the signal generator's supported mode.
Active vs Passive 4-20mA Comparison
| Feature | Active Current Output | Passive Current Simulation |
|---|---|---|
| Current Control | Signal generator | Signal generator |
| Loop Power | Supplied by active source | External supply required |
| Typical Application | Compatible passive receiving input | Externally powered transmitter loop |
| Main Requirement | Suitable load and voltage limits | Suitable external supply and loop arrangement |
| Common Use | PLC input testing | Two-wire transmitter simulation |
Important: The terms active and passive can also describe PLC input modules and transmitters. Different manufacturers may use different terminology.
Always confirm the actual electrical function from the wiring diagram rather than relying only on the label.
What Is 24V Loop Power?
Many industrial transmitters operate from a nominal 24V DC supply.
A typical two-wire transmitter uses the same pair of conductors for electrical power and measurement current.
A simplified externally powered loop may contain:
24V DC Supply -> Transmitter -> PLC Current Input -> Supply Return
The actual terminal order and connections depend on the equipment design.
Why Is Loop Power Important?
Without a suitable power supply, a passive two-wire transmitter cannot operate correctly.
Common problems include:
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Missing 24V DC supply
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Incorrect wiring polarity
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Insufficient supply voltage
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Excessive loop resistance
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Incorrect PLC input configuration
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Damaged field wiring
Before using a signal generator, verify the loop power arrangement and the electrical limits of all connected equipment.
Can the FNIRSI SG-004A Supply 24V Loop Power?
The FNIRSI SG-004A is marketed with multifunction signal generation capabilities, including current signal testing functions.
However, technicians should distinguish between:
1. Active 4-20mA Output
A current output function intended to drive a compatible receiving circuit.
2. Passive 4-20mA Simulation
A function intended to operate within a suitable externally powered current loop.
3. Dedicated 24V Loop Supply
A power supply function used to energize a compatible loop-powered instrument.
These are different electrical functions.
Before using the SG-004A as a loop supply or current simulator, confirm the exact operating mode, terminal arrangement, available voltage and load limits in the manual for the specific unit.
Do not assume that every output mode provides an independent 24V power supply.
How to Test a PLC Analog Input Using Active Current Output
Step 1: Check the PLC Specifications
Confirm the analog input supports the intended 4-20mA signal and determine whether it supplies loop power.
Step 2: Review the Wiring Diagram
Identify the current input terminals, polarity and input common.
Step 3: Isolate the Existing Signal Source
Follow approved maintenance procedures before disconnecting the original transmitter.
Step 4: Select Active Current Output
Use the appropriate SG-004A mode only if the receiving circuit is compatible.
Step 5: Connect the Instrument
Follow the manufacturer's terminal and wiring instructions.
Step 6: Apply Test Values
Generate 4mA, 12mA and 20mA.
Step 7: Check the PLC Reading
Compare the displayed values with the expected engineering range.
For a linear 0-100% input, these currents should correspond to approximately 0%, 50% and 100%.
How to Test an Externally Powered Loop Using Passive Simulation
Step 1: Identify the Existing Loop
Confirm that the circuit is designed to supply power to a compatible two-wire transmitter.
Step 2: Check the External Supply
Verify the supply voltage and permitted loop operating conditions.
Step 3: Isolate the Original Transmitter
Follow the approved procedure for the equipment.
Step 4: Select Passive Current Simulation
Confirm that the SG-004A unit supports the required externally powered simulation mode.
Step 5: Connect According to the Manual
Use the specified terminals and polarity.
Step 6: Apply Known Current Values
Set appropriate current values within the supported operating range.
Step 7: Verify the Controller Reading
Check whether the PLC or DCS responds correctly.
Do not connect a passive simulator as though it were an independent current source.
Common Active and Passive Loop Testing Mistakes
Mistake 1: Connecting Two Active Sources Together
Connecting incompatible powered outputs can damage equipment.
Always identify which device provides loop power.
Mistake 2: Using Passive Simulation Without External Power
A passive simulator requires a suitable externally powered loop.
Without the required supply, the circuit may not operate.
Mistake 3: Incorrect Wiring Polarity
Incorrect polarity may prevent the loop from functioning.
Check the wiring diagram before testing.
Mistake 4: Ignoring Loop Resistance
Every current loop has electrical load limitations.
Excessive resistance can prevent the signal source from maintaining the required current.
Mistake 5: Assuming All PLC Analog Inputs Are Identical
Some PLC inputs are passive, while others include loop power or configurable input arrangements.
The correct test method depends on the module specifications.
Troubleshooting Active and Passive Current Loops
| Problem | Possible Cause | Recommended Check |
|---|---|---|
| No current reading | Missing loop supply | Verify power arrangement |
| Current cannot reach 20mA | Insufficient compliance voltage or excessive load | Check supply and loop resistance |
| PLC reading stays at zero | Wrong mode or wiring | Check input and simulator configuration |
| Signal unstable | Poor wiring or interference | Inspect connections and grounding |
| Simulator does not operate | Incorrect active or passive selection | Review the manual and circuit |
| Incorrect displayed value | PLC scaling error | Test known current points |
Who Needs Active and Passive 4-20mA Testing?
This knowledge is important for:
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Industrial Instrumentation Technicians
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PLC Automation Engineers
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Factory Maintenance Engineers
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Process Control Technicians
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Control Panel Manufacturers
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System Integrators
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Industrial Maintenance Contractors
Understanding current loop power arrangements helps technicians troubleshoot industrial equipment more safely and efficiently.
Important Safety Precautions
Before testing a 4-20mA circuit:
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Identify the loop power source.
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Confirm the correct SG-004A operating mode.
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Check the permitted voltage, current and load limits.
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Verify terminal polarity and wiring.
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Never connect incompatible powered outputs together.
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Obtain authorization before disconnecting operating instruments.
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Prevent unintended machinery operation.
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Follow the equipment manufacturer's instructions.
Safety Note: Do not interfere with safety instrumented systems, emergency shutdown circuits or critical process protection without an approved procedure and authorization.
For traceable calibration, use suitable calibrated reference instruments.
FNIRSI SG-004A Signal Generator Supplier Malaysia
MTM Precision Sdn Bhd supplies industrial testing and measuring instruments for factories, automation contractors, engineering companies and maintenance teams throughout Malaysia.
For customers working with active and passive 4-20mA signals, 24V current loops, PLC analog inputs and industrial transmitters, the FNIRSI SG-004A provides a practical multifunction signal testing solution when used within its specified capabilities.
Not sure whether your PLC input requires active or passive current simulation?
Customers may WhatsApp us a photo of their PLC analog input module, transmitter wiring diagram or existing signal generator for product selection assistance.
MTM Precision Sdn Bhd
Showroom & Service Centre
No. 29-1 & 2, Jalan Bandar 18,
Pusat Bandar Puchong,
47160 Puchong, Selangor, Malaysia.
Tel: 03-8080 7172
WhatsApp: +6016-660 7346
Email: mtmpre@yahoo.com
Website: www.mtmpre.com.my
We serve customers throughout Kuala Lumpur, Selangor, Johor, Penang, Melaka, Negeri Sembilan, Perak, Pahang, Kedah, Kelantan, Terengganu, Perlis, Sabah and Sarawak.
08 Oct 2026
