How to Identify and Use NPN Sensors

In this article, we will discuss the applications and identification of NPN sensors, clarifying why they work and how to properly use them in a control circuit setting.

Most engineers and technicians envision sensors as devices that send process information to a control system such as a PLC, VFD, or similar device. This thought process may be correct, but we should not get too hung up on thinking about the flow of information and the flow of electrical current in the same direction.

NPN sensors, while common, invert the typical mental picture of current flow.

When faced with the task of installing a new sensor or replacing a faulty sensor, the technician or engineer faces a decision. Most typical industrial DC sensors are available in either PNP or NPN polarity.

The PNP variety is generally easier to understand because it follows the conventional directional flow of electrical current. Although NPN sensors still follow electrical principles, they can be more confusing.

Sensors are used in control systems to provide information to a controller, such as a PLC or motor drive. This input information is used to determine if an input condition is true or false, or in other words, to detect if current is flowing or not flowing.

It is important to remember that current flows from positive to negative. This direction of current has no bearing on the fact that information is represented as "coming" to the controller.

Operation of NPN Sensor

To briefly summarize the operation of an NPN polarity sensor, when de-energized, current cannot flow through the load lines. However, when powered up by using any sensing method (inductive, optical, etc.), current will flow into the sensor through the load lines, which will then return the current to ground.

Wiring diagram for a typical NPN sensor. The rectangular symbol of a load connected to +V indicates a sourcing input module that requires a sinking (NPN) sensor.

To complete the internal circuit, current must be sourced from the control module (such as a PLC input module) and enter the sensor.

The concept of current flowing out of the input module is the main reason why NPN sensors appear to flow in the opposite direction of traditional current flow. It is important to remember that the input terminals on the controller only check whether current is flowing, there is no inherent restriction on which direction the current can flow.

A quick indicator as to whether you need an NPN sensor is the common voltage coming into the module.

If the common line of a set of input terminals is connected to +24VDC, then that voltage will appear at each individual input terminal, which can be verified with a voltmeter. Therefore, the module is a current source and is called a "sourcing" module.

In order for a sensor to be compatible with a sourcing input module, the sensor must allow current to flow in through the load wires and return the current to ground. This is called a "sinking" sensor and must be used whenever a sourcing module is present.

In some device structures, the module will be fixed in either sink or source mode and cannot be changed. Other times, changing the destination of the common line is the only change required to switch between source and sink.

NPN sensor wiring diagram

The common voltage of an input module can be identified by observing the common wire or by measuring the voltage at each input. However, identifying the polarity of a sensor by simple observation can sometimes be more difficult.

In another article discussing PNP sensors, several representative device diagrams are shown with +V, -V, and a load line. This load line is connected to a rectangular symbol of the load device, usually a PLC input terminal. The other side of this load rectangle will be connected to power or ground.

In the NPN sensor diagram, the load device will appear connected directly to the +V source.

This connection represents the common voltage line of the input module, which is also connected to the +V source. When the sensor is powered, the input module is connected to +V and the sensor is connected to -V, completing the circuit.

Current is allowed to flow through the complete circuit and the input module detects the energization of the sensor.

Wiring diagram for an NPN sensor with normally open and normally closed outputs. A load device connected to +V shows both outputs as NPN polarity.

To test the polarity of an NPN sensor, remove the load wires from the input module. Measure the open load wires with a voltmeter and observe the voltage when the sensor is powered. An NPN sensor will swing between "open" and ground, which will probably almost always show 0 volts regardless of the power-on state. In contrast, a PNP sensor will switch between 0 and 24 volts.

Construction of NPN sensor

The internal structure of an NPN sensor uses a transistor of the same name, based on the NPN layer structure of semiconductors. Inside this transistor, when the base pin is supplied with a small current, such as that from an actual sensor transducer, a large amount of current flows from the collector to the emitter.

The load wire leading to the PLC is connected to the base. When power is applied, current will flow from the PLC into the collector and return to ground through the sensor.

This way, current flows from the load line into the sensor and returns to ground, which is the characteristic operation of an NPN sensor.

NPN sink sensors are necessary when the interacting input module is sourcing, i.e. continuously supplying voltage to the input terminals. When the sensor is powered, current flows from the module to the sensor and back to ground.