Current sinking refers to a configuration in electrical circuits where a device, such as an LED or transistor, is connected to a power source and "sinks" current into the ground. This means that current flows from the power source, through the device, and into the ground.
Current sourcing, on the other hand, is when a device "sources" current out of a power source and into a load. In this configuration, current flows from the power source, through the device, and into a load, such as a motor or a resistor.
In both cases, the device acts as a switch that controls the flow of current in the circuit. The choice of whether to use current sinking or sourcing depends on the specific requirements of the circuit and the characteristics of the device being used. sinking and sourcing terminology applies only to DC input and output circuits. Input and output points that are sinking or sourcing can conduct current in one direction only.
The figure below depicts a sinking input. To properly connect the external supply, it must be connected so the input provides a path to supply common(-). So, start at the PLC input terminal, follow through the input sensing circuit, exit at the common terminal, and connect the supply (-) to the common terminal. By adding the switch between the supply (+) and the input, the circuit is completed. Current flows in the direction of the arrow when the switch is closed.
The four possible combinations of input/output sinking/sourcing circuits are shown below. The common terminal is the terminal that serves as the common return path for all I/O points in the bank.
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