Скачать или смотреть PLC sinking and sourcing concept

PLC Sinking and Sourcing: Comprehensive Description

Core Concept

Sinking and sourcing describe two fundamental wiring configurations for connecting DC input/output devices to Programmable Logic Controllers (PLCs). These terms define the direction of conventional current flow relative to the PLC's terminals. The concept applies specifically to DC circuits and determines how sensors, switches, lights, relays, and other field devices are connected to the PLC's digital modules.

Current Flow Fundamentals

In electrical engineering, conventional current flows from positive to negative. Sinking and sourcing configurations define whether the PLC provides a path to ground (sinking) or a path to positive voltage (sourcing) for this current flow. This distinction is critical because it affects how all external devices must be wired to the PLC.

Detailed Explanation of Sinking

Sinking Configuration (often called NPN logic in sensor terminology) refers to a setup where the PLC provides a path to ground (0V). In this configuration, current flows into the PLC input or output terminal. The PLC essentially "sinks" the current to ground.

For input modules in sinking configuration, the common terminal is connected to the positive DC supply voltage. Field devices like sensors or switches are wired between the PLC input terminal and ground. When the device activates, it completes the circuit by providing a path from the positive supply, through the device, into the PLC input, and out through the PLC's common terminal to ground.

For output modules in sinking configuration, the common terminal is connected to positive voltage. Load devices like indicator lights or relay coils are connected between the PLC output terminal and ground. When the PLC activates the output, it provides a path from the load, through the PLC output, to the positive voltage on the common terminal.

Detailed Explanation of Sourcing

Sourcing Configuration (often called PNP logic in sensor terminology) refers to a setup where the PLC provides a path to positive voltage. In this configuration, current flows out of the PLC input or output terminal. The PLC "sources" the current from the positive supply.

For input modules in sourcing configuration, the common terminal is connected to ground (0V). Field devices are wired between the positive DC supply and the PLC input terminal. When a device activates, it provides a path from the positive supply, through the device, into the PLC input, and the PLC provides the path to ground through its common terminal.

For output modules in sourcing configuration, the common terminal is connected to ground. Load devices are connected between the positive DC supply and the PLC output terminal. When the PLC activates the output, it provides a path from the positive supply, through the load, into the PLC output, and out through the common terminal to ground.

Sensor Compatibility

The sinking/sourcing concept directly relates to sensor types. NPN sensors are compatible with sourcing PLC inputs, while PNP sensors are compatible with sinking PLC inputs. An NPN sensor switches the ground side of the circuit—when active, it connects its output to ground. A PNP sensor switches the positive side—when active, it connects its output to positive voltage.

This compatibility is crucial: an NPN sensor must be connected to a sourcing PLC input, and a PNP sensor must be connected to a sinking PLC input. Mismatching them will result in the sensor not working properly with the PLC.

Practical Wiring Considerations

When implementing sinking or sourcing configurations, several practical considerations emerge. All devices connected to a single common terminal on a PLC module must be of the same type—all sinking or all sourcing. Mixing types on the same common will create a short circuit through the PLC's internal circuitry.

The power supply polarity must be carefully observed. In sinking configurations, the positive voltage connects to the common terminal. In sourcing configurations, the positive voltage connects to the field devices, and the common connects to ground.

Load current ratings must be checked. Each PLC output has a maximum current capacity, and the common terminal typically has a higher rating since it carries the combined current of multiple outputs.

Industry Preferences and Regional Variations

Different regions have developed preferences for sinking versus sourcing configurations. North American industries often prefer sourcing configurations (using PNP sensors), while European and Asian manufacturers frequently use sinking configurations (using NPN sensors). Japanese automation equipment traditionally uses sinking configurations. These preferences are historical and relate to early semiconductor manufacturing trends in different regions.