What is Sink and Source in PLC | Sourcing vs Sinking Explained with Examples
Published on Jun24, 2025 | Category: sinksourceShare this Page:
The terms sourcing and sinking describe the direction of current flow in DC circuits, specifically in relation to sensors, actuators, and control devices. These concepts are essential for understanding how digital signals are managed in PLC systems.
Sourcing: A sourcing device supplies current from the positive (+24V) side of the power supply to the PLC input or output terminal, allowing current to flow out of the PLC.
Sinking: A sinking device connects the load to the negative (0V) side of the power supply, allowing current to flow into the PLC terminal.
Although these concepts may seem confusing at first, they become intuitive when applied to digital PLC I/O logic circuits. These circuits operate using binary signals (ON/OFF) and rely on the correct direction of current flow. Common components involved include push buttons, limit switches, transistors, diodes, and relays.
In this article, we will clearly explain the difference between sinking and sourcing I/O logic and how they are applied in digital DC PLC systems. But first, let’s briefly review what digital inputs and outputs are in a PLC.
What is Sinking in PLC?
Sinking refers to a method of wiring in DC control systems where a device (such as a sensor, switch, or output module) connects the load or signal line to the negative terminal (0V or ground) of the power supply. In this setup, the current flows from the positive side of the power source, through the load or PLC input, and finally to ground through the sinking device.
Why Is It Called “Sinking”?
The term “sinking” comes from the idea that the device “sinks” or receives current into its terminal and routes it to 0V. This means the current enters the device (PLC input or output) and exits toward ground. The opposite concept is “sourcing,” where the device provides or “sources” current from the positive voltage.
Key Features of Sinking Circuits:
- Current Direction: Current flows into the PLC input or output terminal.
- Power Reference: The device is connected to 0V (ground) of the DC power supply.
- Sensor Compatibility: Requires PNP (sourcing) sensors or switches, which send positive voltage when active.
- Common Usage: Frequently used in European and industrial automation standards.
- Logic Behavior: The input is considered ON when voltage is present due to current flow into the terminal.
How a Sinking Input Works (Example):
Imagine a proximity sensor connected to a PLC input. In a sinking input configuration:
- The common terminal of the PLC input is wired to the +24V power supply.
- The PNP sensor has its output connected to the PLC input channel (e.g., I0.0).
- When the sensor detects an object, it sends +24V to the PLC input.
- The current flows from +24V (common), through the PLC input circuit, into the sensor, and finally down to 0V ground.
- This current flow activates the PLC input and registers it as an ON signal.
Advantages of Using Sinking Inputs:
- Simplified wiring when using sourcing (PNP) sensors, which are commonly available.
- Easy to diagnose faults based on voltage presence.
- Standardized wiring in some regions and industries (especially Europe).
Important Notes:
- Sinking and sourcing concepts apply only to DC circuits, not AC systems.
- A sinking input must always be used with a sourcing (PNP) field device, or it will not operate correctly.
- Miswiring can cause circuits to fail or become damaged—always match sensor type with PLC input configuration.
What is Sourcing in PLC?
Sourcing refers to a wiring method in DC PLC circuits where a device (such as a sensor, switch, or output module) provides current from the positive terminal (+24V) of the power supply to a connected load or PLC terminal. In this configuration, current flows out from the sourcing device and continues through the PLC input or output channel, finally reaching the ground (0V).
Why Is It Called “Sourcing”?
The term “sourcing” is used because the device “sources” or supplies the current into the circuit. That means it acts as the provider of positive voltage. The device sends +24V into the circuit when active, and this current continues toward a sinking component, such as a PLC input or another grounded load.
Key Features of Sourcing Circuits:
- Current Direction: Current flows from the device (sensor or PLC output) into the load or PLC input terminal.
- Power Reference: The sourcing device is connected to the +24V side of the power supply.
- Sensor Compatibility: Requires NPN (sinking) sensors or switches, which pull the signal down to 0V when active.
- Common Usage: Frequently used in North American industrial automation systems.
- Logic Behavior: The input is considered ON when current is sourced into the terminal from the field device.
How a Sourcing Input Works (Example):
Let’s consider a simple PLC circuit using an NPN proximity sensor. In a sourcing input setup:
- The PLC input common terminal is connected to 0V (ground).
- The NPN sensor's output wire is connected to a specific PLC input channel (e.g., I0.0).
- When the sensor detects an object, it connects the signal wire to 0V (sinks current).
- Current flows from the +24V power supply, through the PLC input circuitry, into the sensor output, and finally down to 0V.
- This current flow is detected by the PLC, which registers the input as active (ON).
Advantages of Using Sourcing Outputs:
- Compatible with NPN (sinking) sensors, which are common in many countries.
- More robust in noisy environments because the switching happens on the ground side.
- Preferred in systems where multiple devices need to share a common 0V reference.
Important Notes:
- Sourcing and sinking concepts only apply to DC systems because polarity matters.
- To avoid miswiring, always pair sourcing outputs with sinking inputs, and vice versa.
- Connecting two sourcing devices together without proper grounding may lead to electrical faults.
Sinking vs. Sourcing Comparison Table
| Feature | Sinking | Sourcing |
|---|---|---|
| Current Direction | Into the PLC input or output terminal | Out of the PLC input or output terminal |
| Wiring Connection | Device connects to 0V (DC-) | Device connects to +24V (DC+) |
| PLC Common Terminal | Connected to +24V | Connected to 0V |
| Sensor Type Used | PNP (Sourcing sensor) | NPN (Sinking sensor) |
| Signal When Active | 0V (low signal) | +24V (high signal) |
| Regions Where Common | Europe, Asia | North America |
| Common Application | Field device pulls input down to 0V | Field device drives input with +24V |
| Typical Use Case | Inputs that expect a 0V signal from device | Inputs that expect a +24V signal from device |
Key Points to Remember
- Sourcing = Positive side of the circuit.
- Sinking = Negative side of the circuit.
- Never connect sourcing to sourcing or sinking to sinking directly — it won’t work.
- Check manufacturer manuals to determine if the PLC supports sourcing, sinking, or both (universal input).
Applications of Sourcing and Sinking
- Sourcing is more common in North American systems (positive logic).
- Sinking is more common in Asian/European systems (negative logic).
- Both have pros and cons based on safety, noise immunity, and wiring standards.
DC Circuits Only
Sinking and sourcing only apply to DC input and output circuits due to the reference to + and - polarities. These configurations are not applicable to AC circuits.
One-Way Current Flow
Sinking and sourcing I/O points allow current to flow in only one direction. If connected improperly, the circuit will not operate. Understanding these concepts ensures correct wiring every time.
Conclusion
Understanding the concept of sink and source in PLC systems is crucial for proper wiring and signal compatibility. Always match the PLC module type with the device type (sourcing or sinking) to ensure reliable operation and safety.
The image shows two side-by-side wiring diagrams:
- Left: Sinking Sensor with PLC Sourcing Input — A 3-wire NPN sensor connects its black signal wire to the PLC input I0.0. The PLC provides +24V via the +COM terminal, and the sensor sinks current to 0V when active.
- Right: Sourcing Sensor with PLC Sinking Input — A 3-wire PNP sensor connects its black signal wire to the PLC input I0.1. The PLC provides 0V via the -COM terminal, and the sensor sources +24V to the input when active.