Sensing technologies cover a wide range of presence sensing options

Sensing technologies cover a wide range of presence sensing options, the most common being inductive sensors, photoelectric sensors, capacitive sensors and ultrasonic sensors. Making the correct choice of sensor is dependent on the application in which the sensor is being used. Usually there is another consideration as to which sensor to choose, for example, if your application requires an inductive sensor, the next question is what output type do you need, NPN or PNP?

So what is the difference, well firstly one thing to consider is location. NPN is widely used within the Asian region and is the dominant output type, whilst within Europe, PNP is more popularly used than NPN. Knowing this helps a little, but the real question is whether you need a sinking or sourcing output?

  • Transistors are used within the output of a sensor and are either NPN or PNP.
  • NPN are sinking sensors, these allow current to flow into the sensor aand to V-
  • PNP are sourcing sensors and allow current to flow out from the sensor, from V+

 

 

 

 

NPN output

The load is connected to the switching output and V+, therefore V+ is the reference point. With a change in signal state, the transistor turns on, current will flow from V+ through the load via the transistor to V-, thus completing the circuit.

PNP output

The load is connected to the switching output and V- , therefore V- is the reference point. With a change in signal state, the transistor turns on, current will flow from V+ through the transistor and through the load to V-, thus completing the circuit.

In order to clarify the way the sensor’s  electronics behave, the sensor should be thought of as a switch

NPN negative switching

With an NPN sensor the switching occurs on the –V rail. The +V rail forms the common between the device and the sensor. A permanent +V supply will be connected to the device that is to be activated, for instance a PLC or relay.

When the sensor turns on, it switches the –V rail and completes the circuit. Current travels through the sensor transistor into the device, thus turning it on or changing its state.

 

 

 

PNP positive switching

With a PNP sensor the switching occurs on the +V rail. The -V rail forms the common between the device and the sensor. A permanent -V supply will be connected to the device that is to be activated, for instance a PLC or relay.

When the sensor turns on, it switches the +V rail and completes the circuit. Current travels through the sensor transistor into the device, thus turning it on or changing its state.

 

 

 

Something to remember

  • If the DC voltage has a V+ common, an NPN output sensor is needed.
  • If the DC voltage has a V- common, a PNP output sensor is needed.

 

NPN and PNP

The sensors are usually described in terms of the voltage exhibited by the signal wire, when the input is switched. So a PNP sensor will have a signal that ‘goes positive’ when the sensor needs to signal an ‘ON’ state. A NPN sensor will have a signal that ‘goes negative’ when it is in an ‘ON’ state.

PNP is sometimes regarded as the safer option, due to the fact that any earth faults could make NPN sensors give a false switching signal.

 

 

 

 

 

Connection to a PLC                     

When a sensor is connected to an input stage of a PLC, the input stage detects the state the sensor is in (on or off) but it needs to be of the same type as the sensor.

When a PNP sensor is connected to a PNP input, current flows from the sensor to the input. This means that, as a PNP sensor sources current, a PNP input must sink current. Conversely a NPN sensor sinks current and a NPN input sources current.