In a DC switching circuit, sourcing provides the path from positive supply to a load. Sinking provides the return path from the load toward 0 V. Both sides are needed for a complete current loop.

The terms depend on which device you are describing. A sourcing sensor output connects to a sinking PLC input. A sinking sensor output connects to a sourcing PLC input.

Follow conventional current

This guide uses conventional current, from positive toward negative, rather than electron-flow direction. It covers discrete DC I/O; do not apply these examples directly to AC circuits or assume they describe analog loop powering.

Example 1: a sourcing sensor

When a PNP output conducts, it provides current to the PLC input. A compatible sinking input returns that current through its input circuit to 0 V.

Example 2: a sinking sensor

When an NPN output conducts, it completes the return toward 0 V. A compatible sourcing input supplies the current through its input circuit.

Reference: AutomationDirect’s sinking and sourcing fundamentals.

What about PLC outputs?

A sourcing transistor output switches the positive side of a compatible load. A sinking transistor output switches its return side. Relay outputs are contacts and must be evaluated from their circuit diagram and contact ratings, rather than assumed to behave like a PNP or NPN transistor output.

Why does the common terminal matter?

The common connection completes the input circuit’s path to the power supply. On a configurable sink/source input module, points sharing a common generally need one compatible arrangement for that group. Check the manufacturer’s group boundaries and wiring instructions before mixing signal types.

Reference: Connecting three-wire devices to PLC inputs.

Three common mistakes

• Describing only one device: “sourcing” is incomplete without saying whether you mean the sensor output, PLC input, or PLC output.
• Confusing 0 V with protective earth: they have different functions; bonding depends on the system design.
• Assuming ON means exactly supply voltage: voltage drop, load current, leakage, and input thresholds all matter.

Before working on a real circuit

Use these diagrams to understand the principle. Follow the exact equipment manuals, isolate power before wiring changes, and have a qualified person verify the design. Never bypass a protective function to make an input respond.

Plan your integration project

LPL Industrial Automation in Secunderabad designs and builds panels and develops PLC/HMI applications. Share your controller, sensor, and load details to discuss the required interfaces.