Sinking vs. Sourcing Explained: Wiring PLC Inputs the Right Way

If you’ve ever wired a proximity sensor to a PLC input and gotten unexpected results—or worse, damaged a module—you’re not alone. The terms sinking and sourcing are among the most misunderstood in industrial automation, yet they’re critical to getting I/O wiring right.

Whether you're new to PLCs or troubleshooting a system that “should work,” understanding the difference between sinking and sourcing is essential for safe, functional wiring. In this guide, we’ll break down what each term means, how to identify them, and when to use one over the other, with visuals, examples, and a handy comparison chart.

What Is Sinking and Sourcing?

At the most basic level, sinking and sourcing refer to the direction of current flow in a DC control circuit between a field device and a PLC input or output.

📥 Sinking (NPN)

• The PLC or device provides a path to ground.
• Current flows from the load into the PLC.
• Common in Asian control systems and NPN-style sensors.

📤 Sourcing (PNP)

• The PLC or device provides positive voltage.
• Current flows from the PLC into the load.
• Common in North American and European automation.

Sinking vs. Sourcing: Visual Diagram Explanation

Sourcing Input (PNP Sensor → Sinking PLC Input)

[VDC+] ───► [PNP Sensor] ───► [PLC Input (Sink)] ───► [GND]
  

Sinking Input (NPN Sensor → Sourcing PLC Input)

[VDC+] ───► [PLC Input (Source)] ───► [NPN Sensor] ───► [GND]
  

How to Identify Whether Your PLC Input Module Is Sinking or Sourcing

• Check datasheets: Look for "sourcing" or "sinking" terminology.
• Terminal labeling: Which common (COM) terminal is used—+24V or 0V?
• Use a multimeter (only if qualified): Check idle voltage at inputs.
• Brand tendencies: Allen-Bradley often uses sinking inputs; Siemens often uses sourcing.

Choosing the Right Type for Your Application

Use sourcing inputs (PNP sensors) when:

• Your sensors output +24VDC (PNP)
• You’re in North America or Europe
• You want clearer logic ("voltage = ON")

Use sinking inputs (NPN sensors) when:

• Your sensors sink current to 0V (NPN)
• You’re working with legacy systems
• You’re in Asia-Pacific environments or using older control panels

Common Mistakes and How to Avoid Them

• Don’t mix PNP and NPN sensors on one input module.
• DC wiring is not the same as AC—don’t assume!
• Always connect inputs to the correct common terminal.
• Watch for open-collector sensors needing pull-up/down resistors.
• Don’t mix power supplies without proper isolation.

Quick Reference Comparison Table

Feature	Sourcing Input (PNP)	Sinking Input (NPN)
Current Direction	Sensor → PLC	PLC → Sensor → GND
Sensor Type	PNP	NPN
Common Connected To	0VDC	+24VDC
Common In	North America, Europe	Asia-Pacific

Real-World Examples

Example 1: A PNP proximity sensor on a packaging line outputs +24VDC when detecting a target. It's wired to a PLC sinking input, with the common connected to 0V. The input reads “ON” when metal is present.

Example 2: An NPN photoeye in a bottling machine connects to a sourcing PLC input. When a cap is detected, the sensor completes the circuit to ground, triggering the input.

Key Takeaway: Understand the Flow, Protect Your Equipment

Sinking and sourcing aren’t interchangeable—they determine how current flows in your system. Getting it wrong can cause costly downtime or damaged inputs. Match your sensor type with the right input module, and you’ll wire confidently.

Need the Right PLC Input Module or Sensors?

Industrial Automation Co. stocks hundreds of digital input modules, proximity sensors, and field-proven PLC components from trusted brands like Allen-Bradley, Siemens, and Mitsubishi.

Need help? Contact our sales team today—we’ll help you choose the right parts for your project and your budget.