Introduction to sinking and sourcing - Ladder Logic Tutorial

- [Tutor] Industrial sensors fall within the field level of automation systems. There are several commonly used circuits when connecting sensors to PLCs. For example, sinking and sourcing circuits are common for digital devices. And for analog devices, four to 20 million current loop circuits are typical. There are other ways to connect sensors to PLCs depending on the sensor type or communication networks, for example, PROFIBUS devices. Now, when working with digital sensors or choosing the type of input or output modules for your PLC, it is important to understand sinking and sourcing concepts to properly design and wire up the system. In industrial settings, the terms sourcing or PNP and sinking or NPN refer to the direction in which the current flows through the input or output modules in DC circuits. When working with PLC IO modules, as sourcing PNP module, is when current flows out of the PLC port. A sinking NPN module is when current flows into the PLC port. This is usually mentioned in the documentation when working with various PLCs. Here is an example from the Allen Bradley CompactLogix Controllers. It shows that this controller has 16 sinking digital input points and 16 sourcing digital output points. Another example is Micro820 PLC, here are the same sinking and sourcing configurations. And the devices that connect to the PLC DC input and output modules are also labeled sourcing and sinking. With a sourcing module, the device connected to it is a sinking device. In sinking devices, current flows into the device from the module. The module provides or sources the current a sinking device always attaches to a sourcing IO module. For a sinking module, the device connected to it is a sourcing device. In sourcing devices the current flows out of the device into the module. The module receives or sinks the current, a sourcing device always attaches to a sinking IO module. So simply, you have two options, sourcing that is connected to a sinking and vice versa. You cannot have two sinking or two sinking parts. The terms sinking and sourcing or NPN, NP and P are both in industrial settings when working with PLCs and sensors. But for simplicity, we refer to PLC IO modules as sinking and sourcing and sensors as NPN and PNP sensors. However, both terminologies are correct and you will see them both in documentation. There are ways to remember what PNP and NPN stand for. One method is to count the letter N, PNP has one N so sourcing, NPN has two Ns, so sinking. Another way to remember the terms is P stands for power. So the PNP outputs power representing sourcing, NPN is for no power, which is sinking. These two methods are just to help remembering the terms. Feel free to come up with your own if you prefer that.