Sinking and Sourcing: Which Connection is Best for Your PLC?

In industrial automation and control systems, Programmable Logic Controllers (PLCs) play a significant role in the control and monitoring of operations. 

An important component of PLC hardware design includes how inputs and outputs are coupled, specifically the sinking and sourcing methods. 

Both phrases refer to the way the device handles current and interacts with other components including sensors, switches, and actuators.

To make the optimal option between sinking and sourcing connections, it’s necessary to understand the fundamental ideas, the applications, and how each type of connection might affect performance, wiring, safety, and troubleshooting.

Sinking Input Module (NPN) in PLC

Sinking Configuration

• Switch Input SW1 is the connection between the positive terminal (+) of the power supply and the positive terminal of the PLC input channel.
• The common (Com) terminal is connected to the negative terminal (-) of the power supply.
• When SW1 is closed (active state), current travels from the power supply through the switch, into the PLC input channel, and ultimately to the ground via the common terminal.

This design is typical of sinking inputs where the PLC input module sinks current to ground when the switch is closed. The load (in this case, the PLC input channel) is linked between the power supply and the input impedance of the PLC, and current flows through the PLC when the input is enabled.

Key Points for Sinking (NPN)

• When the switch is in the active position, current flows from the input device into the ground through the input.
• When the switch is engaged, current flows through the input to ground since it is positioned between the positive supply voltage and the PLC input.
• Because of regulations or preferences for system design, sinking inputs are generally chosen in some regions, such as Europe and Asia.
• A lot of PLCs can work with both sourcing (PNP) and sinking (NPN) inputs. However, it’s important to make sure the wiring is done correctly for the input type.

Sourcing Input Module (PNP)

Sourcing configuration

• Switch Input SW1 is connected between the negative terminal (-) of the power supply and the negative terminal of the PLC input.
• The common (Com) terminal is connected to the negative side of the power supply.
• The positive terminal (V+) of the power supply is connected to the input channel via the impedance.
• When SW1 is closed, current flows from the PLC input channel through the switch, completing the circuit and returning to the negative terminal of the power supply.

In sourcing configurations, the PLC input sources current to the load when the switch is closed. The load (in this case, the input channel) is between the ground and the power supply, and when the input is active, current is supplied to the load.

Key Points for Sourcing (PNP)

• When the switch is active, the input sources current from the positive power supply to the plc load.
• The input signal is placed between the ground and the PLC input, allowing current to flow from the positive supply through the input device.
• Sourcing inputs follow a positive control logic, where a high voltage or presence of current typically signals an active input, making it intuitive for electricians used to positive logic systems.
• Sourcing (PNP) inputs are commonly used in North America, where this type of wiring and control logic is more prevalent.
• Like sinking inputs, many PLCs can handle both sourcing (PNP) and sinking (NPN) inputs, but correct wiring must be ensured to match the system design.

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Key Differences Between Sinking and Sourcing Input Modules

The below table summarizing the key differences between Sinking (NPN) and Sourcing (PNP) PLC  input modules:

Sourcing Output Module (PNP)

In a sourcing output configuration, the output module provides a positive voltage to the load when activated. The module “sources” current from the power supply to the load.

Wiring Configuration

• The PLC Output Channel is connected to the positive terminal of the Switching Circuit inside the sourcing output module.
• The load is connected between the negative terminal (V-) of the power supply and the negative terminal (Com) of the I/O point.
• The positive terminal (V+) of the power supply is connected to the other side of the switching circuit.

Operation

• When the PLC output channel is activated, the sourcing output module switches on, allowing current to flow from the positive terminal (V+) through the load and back to the common (Com) terminal.
• Current flows from the power supply to the load and then back to the module, completing the circuit.

Key Points for Sourcing Output Module (PNP):

• The output module sources current from the positive power supply to the load when activated.
• The load is connected between the ground (negative side) and the output of the module, allowing current to flow from the output to the load.
• This configuration is preferred in systems where a positive control signal is required or expected, meaning the output sends positive voltage to activate the load.
• It is commonly used in North America, where positive control systems are more standard.
• Often used in automation systems that favor positive logic and need clear, direct activation of devices like relays, solenoids, or lights.

Sinking Output Module (NPN)

In a sinking output configuration, the output module “sinks” current to the ground when activated. The module provides a path to the ground, allowing current to flow from the load back to the module.

Wiring Configuration

• The PLC Output Channel is connected to the positive terminal of the Switching Circuit inside the sinking output module.
• The load is connected between the positive terminal (V+) of the power supply and the positive terminal (+) of the I/O point.
• The common (Com) terminal is connected to the negative terminal (-) of the power supply.

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Operation

• When the PLC output channel is activated, the sinking output module switches on, providing a path for current to flow from the load to the ground through the switching circuit.
• Current flows from the power supply through the load and into the module, which then sinks it to the ground (Com).

Key Points for Sinking Output Module (NPN):

•  The output module sinks current to ground when activated, providing a path for current to flow from the load to ground.
• The load is connected between the positive power supply and the output module, allowing current to flow through the load and be grounded by the module.
• This configuration is widely used in applications where grounding is essential for safety, noise reduction, or system stability.
• Sinking output modules follow a negative control logic, where grounding the output activates the load.
• Commonly used in regions like Europe and Asia, where sinking logic is often the standard in control systems.

Key Differences Between Sourcing and Sinking Output Modules

The below table is summarizing the key differences between Sourcing (PNP) and Sinking (NPN) output modules:

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Design Considerations in Sinking and Sourcing in PLC Digital Input Circuits

When designing and wiring PLC systems, understanding the difference between sinking and sourcing configurations for digital input and output circuits is essential. This choice can significantly impact safety, troubleshooting, and overall efficiency in control systems. 

These below topics provide you the design considerations and practical wiring examples using proximity sensors.

Before choosing between sinking and sourcing configurations, it is crucial to consider the requirements of the instruments and sensors involved, such as whether they have PNP (positive) or NPN (negative) transistors. Here are some key points to consider:

• PNP and NPN refer to the types of transistors used in the circuit, which affects the direction of current flow (polarity). PNP transistors source current, while NPN transistors sink current.
• Determine what type of power sourcing is necessary for the circuit and ensure compatibility between the power supply, sensor type, and PLC input module.
• Good design habits involve wiring all input/output (I/O) points with a shared common to either source or sink. This reduces the need for additional terminal blocks and saves cabinet space.
• For DC circuits, use DC power supplies only. Mixing AC and DC circuits can lead to faults because the polarities and operating principles differ.

By planning before and selecting the suitable design, you can ensure compatibility, decrease troubleshooting time, and improve the control system’s safety and reliability.

Sinking (NPN) Input Configuration with PNP Sensor

The sinking input configuration involves a PNP (sourcing) sensor connected to a PLC input module that is designed to sink current to the ground.

Circuit Diagram Explanation:

PNP Proximity Sensor

• The sensor is powered by a DC supply, with the red  wire connected to the positive terminal (+) and the blue wire connected to the negative terminal (-). 
• The yellow wire is the output that switches to a high voltage level when the sensor detects a target.

PLC Input Module (Sinking)

• The input module is configured as sinking, meaning it provides a path to ground. 
• The common (COM) terminal of the PLC is connected to the negative terminal of the power supply.

Wiring Configuration

• The PNP sensor’s output (yellow wire) is connected to the PLC input.
• When the sensor detects a target, the output wire is connected to the positive voltage, allowing current to flow from the power supply, through the sensor, into the PLC input, and finally to the ground via the common terminal.

Key Points for Sinking (NPN) Input Configuration with PNP Sensor

• Current Flow: In a sinking input configuration, the current flows from the positive supply, through the sensor, into the PLC input, and then to the ground.
• Safety and Commonality: This configuration is widely used because it allows a shared negative common, simplifying wiring and reducing the number of terminal blocks needed.

Sourcing (PNP) Input Configuration with NPN Sensor

The sourcing input configuration uses an NPN (sinking) sensor connected to a PLC input module that is designed to source current from the power supply.

Circuit Diagram Explanation:

NPN Proximity Sensor

• The NPN sensor is also powered by a DC supply, with the red wire connected to the positive terminal (+) and the black wire connected to the negative terminal (-). The yellow  wire is the output that connects to ground when the sensor detects a target.

PLC Input Module (Sourcing)

• The input module is configured as sourcing, meaning it supplies a positive voltage to the sensor when the circuit is completed. 
• The common (COM) terminal of the PLC is connected to the positive terminal of the power supply.

Wiring Configuration

• The NPN sensor’s output (yellow wire) is connected to the PLC input.
• When the sensor detects a target, it connects the output to ground, allowing current to flow from the PLC input (sourcing current) through the sensor to the ground.

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Key Points for Sourcing (PNP) Input Configuration with NPN Sensor

• Current Flow: In a sourcing input configuration, the current flows from the PLC input, through the sensor, and back to the power supply’s negative terminal.
• Safety and Commonality: Sourcing input configurations are common in North America, where positive control logic is often preferred.
• Common Usage: This configuration is used in applications that require a positive voltage for the sensor to activate, and it is intuitive for electricians accustomed to working with positive control signals.

Choosing Between Sinking and Sourcing

• In industrial automation, the choice between sinking and sourcing configurations depends on the specific requirements of the application, such as grounding considerations, compatibility with existing equipment, and regional preferences. Both configurations have their advantages:
• Sinking (NPN) Inputs: Often used where grounding is a priority and common in Europe and Asia.
• Sourcing (PNP) Inputs: Preferred in systems requiring a positive control signal, common in North America.
• A well-planned wiring design will match the circuit to the application requirements, ensuring safety, reliability, and easier troubleshooting. Modern PLCs offer flexibility in choosing either configuration, but it is important to consider the shared “common” wiring scheme to ensure compatibility and optimal performance.
• Understanding these principles will help you make informed decisions in designing PLC systems and wiring sensors and actuators correctly, reducing risks and saving time during installation and maintenance.

FAQ in PLC sinking and Sourcing 

What is the Difference Between Sinking and Sourcing Digital Input?

Sinking and sourcing digital inputs differ in current flow between the input device and the PLC.

• Sinking Digital Input: The input device provides a path to ground, while the PLC supplies positive voltage. Current flows from the PLC to the device, and then to ground.
• Sourcing Digital Input: The input device supplies positive voltage to the PLC, which acts as the ground. Current flows from the device to the PLC.

What is Current Sinking Digital Input?

A current sinking digital input refers to a device or input module that allows current to flow into it, usually connected to ground or a negative terminal. When a sinking digital input is active, the current flows from the positive side of the PLC’s power supply, through the load (or input device), and into the sinking input module.

What is PLC Sinking and Sourcing?

PLC sinking and sourcing refer to how the PLC manages the current flow for its digital input/output modules:

• Sinking PLC: In a sinking PLC configuration, the PLC input or output module acts as the path to ground. It allows current to flow from the external device (which sources positive voltage) to the PLC, completing the circuit. For example, in a sinking input module, the external device supplies positive voltage, and the PLC input sinks the current to ground.
• Sourcing PLC: In a sourcing PLC configuration, the PLC input or output module provides positive voltage, and the external device acts as the ground or sinking element. Current flows from the PLC to the device. For example, in a sourcing input module, the PLC supplies the positive voltage, and the device sinks the current to ground.

What is the Difference Between Sourcing and Sinking Current?

The difference between sourcing and sinking current lies in the direction of current flow in the circuit:

• Sourcing Current: The device or module provides (sources) positive voltage, and the current flows from the device to ground (negative terminal). The device is responsible for sending current to the load.
• Sinking Current: The device or module provides a path to ground, allowing current to flow into it from a positive voltage source. The device receives (sinks) the current.

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