Understanding Sinking and Sourcing Sensor Wiring Basics from AutomationDirect
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Sensors play a huge role in automation! If you’re just getting started, one of the first things you need to understand is the difference between sinking (NPN) and sourcing (PNP)it’s crucial for selecting and wiring your sensors correctly to a PLC. Watch this video to learn more about sinking and sourcing.
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Sensors play a huge role in automation! If you’re just getting started, one of the first things you need to understand is the difference between sinking (NPN) and sourcing (PNP), it’s crucial for selecting and wiring your sensors correctly to a PLC. If you're an experienced automation engineer, this might seem like basic stuff. But for beginners, mixing up sinking and sourcing can lead to wiring mistakes, and be difficult to troubleshoot! These concepts only apply to DC I/O circuits because they rely on positive and negative polarities. Before you start wiring, make sure your device is working properly and check what wiring style your circuit requires. If you're using sensors or I/O modules from AutomationDirect, the wiring diagram is printed right on the bag or insert for easy reference. From top-of-the-line Wenglor sensors to our Achieve line, we have many types of sensors available now for you to use in your own applications. If you can’t find the bag or the insert for your sensor, type in your part number in the search bar for all the information on your device! Now, let’s break down sinking and sourcing circuits so you can get it right the first time! If you feel overwhelmed, let’s start simple and use a lightbulb for example. To wire a lightbulb, it needs both positive and negative polarity to activate. A PNP Device circuit is like putting your light switch(device) on the positive wire and connecting the 0v wire directly to the bulb (PLC input). An NPN Device circuit is like putting your light switch(device) on the 0V wire and connecting the positive wire directly to the bulb. Pay attention to the field device you are using! If a sinking (NPN) field device is connected to a PLC, it must be connected to an I/O module with a sourcing input, and vice versa. Some modules support sink/source I/O circuits, meaning current can flow in either direction. However, if you are using a sink/source module, ensure that all I/O points share a common wiring style—either all sinking or all sourcing. In the examples below, we use sensors with the following standard wiring: brown is positive, blue is negative, and black is output. However, wire colors may vary by product. Please refer to your specific product's wiring instructions and take note of any differences. A PNP sensor controls a circuit by opening and closing the connection to the positive side of the power supply. It provides a current-sourcing output and must be wired to a sinking PLC input, like a BRX BX32ND3, whose common is connected to the negative of the power supply. To correctly wire a sourcing (PNP) sensor to a sinking PLC input: 1. The brown and blue wires are the sensor’s power connections. Connect the brown wire to the positive of the power supply and the blue wire to the negative. 2. Connect the negative of the power supply to the PLC input common. 3. The black wire is the sensor output (sourcing); connect it to the PLC input. The sensor controls the positive side of the circuit. When activated, it completes the circuit, and current flows in the direction of the arrow. When wired correctly, your setup should resemble this animation An NPN sensor controls a circuit by opening and closing the connection to the negative side of the power supply. It provides a current-sinking output and must be wired to a sourcing PLC input, like a Productivity P108ND3, whose common is connected to the positive side of the power supply. To correctly wire a sinking (NPN) sensor to a sourcing PLC input: 1. The brown and blue wires are the sensor’s power connections. Connect the brown wire to the positive of the power supply and the blue wire to the negative. 2. Connect the negative of the power supply to the PLC input common. 3. The black wire is the sensor output (sinking); connect it to the PLC input. The sensor controls the negative side of the circuit. When activated, it completes the circuit, and current flows in the direction of the arrow. When properly wired, your circuit should resemble this animation. Sink/source I/O modules combine sinking and sourcing capabilities, allowing you to wire your PLC in either configuration. The common terminal connects to one polarity. The I/O points connect to the opposite polarity through the field device, offering flexibility in power supply connections. The same wiring rules apply. If you are hooking the Sink/Source modules common terminal to DC positive, then all the field devices must be NPN(sinking) devices that source DC negative. If you are hooking the common terminal to DC negative, then all the field devices must be PNP(sourcing) devices that source DC positive. Important: Always wire all connections from a single common as either sinking or sourcing—never mix them! Some Sink/Source I/O modules group their inputs into multiple commons. They may allow sinking devices on one common and sourcing devices on the other. Here are some key takeaways. Sink/source wiring concepts apply only to DC I/O circuits. Sinking (NPN) wiring provides a path to supply common negative. Sourcing (PNP) wiring provides a path to supply source positive. Field devices in modules complement each other. You never use a Sinking (NPN) field device with a Sinking (NPN) I/O module and vice versa. For more information, please contact our free award-winning technical support team! Click here to learn more about the sensors we offer! Click here for more videos!
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