The source type and sink type of PLCs refer to two different wiring methods for transistor circuits, with the main differences reflected in the direction of current flow and the connection of the common terminal.
Next, we will compare and analyze them in terms of definitions, current direction, common terminal connections, and applicable scenarios:
1. Definition Differences:
Source Type: Refers to the current flowing into the PLC from the input point (e.g., point X), which means the signal is entering. Taking Mitsubishi PLC as an example, the signal flowing into the input point is called the source type connection.
Sink Type: Refers to the current flowing out of the PLC from the input point, meaning the signal is exiting. Similarly, in the case of Mitsubishi PLC, the signal flowing out from the input point is called the sink type connection.
2. Current Direction Differences:
Source Type: The current flows from the positive terminal of the external power supply through the input circuit into the PLC, and then returns to the negative terminal of the power supply. For example, when the input common terminal 1M is connected to the negative terminal M of the PLC’s 24V power supply, the current flows from +24V through the button contact, through the input terminal into the PLC, and then flows out back to the negative terminal of the power supply.
Sink Type: The current flows into the common terminal and exits from the input terminal. For example, when the common terminal is connected to +24V, the current flows into the common terminal, through the input circuit, exits from the input terminal to the external load (such as a sensor), and then returns to the negative terminal of the power supply.
3. Common Terminal Connection Differences:
Source Type: The common terminal is usually connected to the negative terminal of the power supply (0V). For example, in the Mitsubishi PLC source type connection, the common terminal 1M is connected to the negative terminal M of the power supply; in the H5U type PLC source type connection, the common terminal SS0 is connected to 0V.
Sink Type: The common terminal is usually connected to the positive terminal of the power supply (+24V). For example, in the Mitsubishi PLC sink type connection, the common terminal is connected to +24V; in the Siemens PLC sink type input, the common terminal is connected to +24V.
4. Applicable Scenario Differences:
Source Type: Suitable for connecting PNP type sensors or output devices. Because PNP type sensors output a high level (+24V) when activated, they work with the source type input common terminal connected to 0V, allowing current to flow from the sensor into the PLC.
Sink Type: Suitable for connecting NPN type sensors or output devices. NPN type sensors output a low level (0V) when activated, working with the sink type input common terminal connected to +24V, allowing current to flow from the common terminal into the PLC and out to the sensor.
5. Different Brands of PLCs may define source and sink types differently:
Mitsubishi PLC: Uses the input point X as a reference to determine source and sink types, where the signal flowing into point X is the source type, and flowing out is the sink type.
Siemens PLC: Uses the common terminal M as a reference point, where the signal flowing into common terminal M is the source type input, and flowing out is the sink type input.
Summary Comparison Table: Type Definition Current Direction Common Terminal Connection Applicable Scenario
Source Type Current flows in PLC input point External power supply → input point → PLC internal → power supply negative terminal Connect to 0V PNP type sensor
Sink Type Current flows out PLC input point Common terminal → PLC internal → input point → external load → power supply negative terminal Connect to +24V NPN type sensor
The above are the main differences between source and sink types in PLCs. In practical applications, the appropriate wiring method should be selected based on the PLC brand and the type of external devices.