NPN and PNP type of sensors,two sensor types used especially in industrial automation environment .These specification related to three wire sensors like proximity sensors. Selection of this type of sensors are so important .
We know NPN ,and PNP are the specification of two types of transistors .The letter N and P represents negative and positive sides of PN junction .
NPN type and PNP type sensors got their name from these transistors .Because switching circuits are designed either of these types .
In NPN and PNP type transistors the current flow direction with respect to its terminals is different from each other .Also switching behaviour is entirely opposite from each other when they used in switching circuit . obviously the PNP and NPN proximity sensors also shows this type of differences .
NPN and PNP sensors differs from each other by their circuit design and the type of transistors used .NPN transistor or PNP transistor.
Sourcing Type sensors -PNP
If a sensor switches a positive side of power supply to the load it is called sourcing sensor .It sources the current to the load . Sensors uses transistor for switching function.And PNP transistor switches positive side .So sensors designed with PNP transistors comes under this category.Means they functions to provide positive side switching. .And such type of sensor are PNP type sensors .
See figure above A.three wire sensor is wired to the power supply and load as shown .Here the sensor circuit is designed with PNP type transistors . So it switches the positive side of the power supply to the load .On examining the wiring we can see that the load is already wired to the negative side of the power supply or ground (0 V).And it requires a positive side to make itself activated .And this positive side is given by the sensor through switching . The load may be a input card of a PLC , indicating device ,lamp etc .
A schematic representation of PNP type sensor circuit is shown below .
It has three wiring connections .Or we can say it is a three wire device .A popular colour code is
BN – Brown lead for connecting positive side of the DC power.
BK – Black lead for connecting switched output . PNP switches +ve side of power to load
BU -Blue lead for connecting -ve side of the DC power .(GND)
Sinking Type sensors --NPN
If a sensor so designed such that it switches negative side of power to the load ,such type of sensors are called sinking type sensors .NPN sensors come under this category .NPN type sensors are constructed with NPN type transistors .
See figure above . A three wire sensor is wired to the power supply and load as shown .It used NPN transistor for switching. It switches the negative side of the power supply to the load .See the wiring. we can notice that the load is already wired to the positive side of the power supply .And negative side is given by the sensor through switching .PLC input card or indicating device ,lamp etc can be a load .
A schematic representation of PNP type sensor circuit is shown below .
It has three wiring connections .Or we can say it is a three wire device .A popular colour code is
BN – Brown lead for connecting positive side of the DC power.
BK – Black lead for connecting switched output .NPN switches -ve side of power to load
BU -Blue lead for connecting -ve side of the DC power .(GND)
Connecting PNP and NPN Sensor to control panel
Selection of sensors for a particular type of input card or PLC is important .For example we have a PLC with sinking type input module .Then what type of sensor should be used ?
A NPN or PNP ????
The following concept you need to remember always
A sourcing type device is connected to sinking input channel
A sinking type device is connected to sourcing input channel.
PNP connection
These sensors are connected to the sinking type I/o channel .And they switches in the positive side of power supply to the load .The conventional direction of current flow is from sensor to the load .(Load here means the I/o channel or indicating device) .
See figure above . A three wire proximity sensor is connected to the PLC DI module .Here the device is a sourcing device or PNP type . And the module is sinking type input module .On examining the wiring you can see that the sensor switches the positive side to the load. .
NPN connection
These sensors are connected to the sourcing type I/o channel .And they switches in the negative side of power supply to the load .The conventional direction of current flow is from load to the sensor .(Load here means the I/o channel or indicating device) .
See figure above . A three wire proximity sensor is connected to the PLC DI module .Here the device is a sinking device or NPN type . And the module is sourcing type input module .On examining the wiring you can see that the sensor switches the negatiive side to the load. .
Why NPN sensors switches negative and PNP sensors switches positive
This is because these sensors are constructed with NPN and PNP transistors .Their biasing and resulting current flow direction is exactly opposite with each other with respect to its terminals (emitter,base,collector).The direction of current flow through the collector is opposite from each other in these transistors .This is why switching circuit connected to them differs by this nature .
Points to remember........
NPN Sensors
NPN sensors are sinking type sensors
NPN Sensors used with sourcing input module
NPN Sensors Switches negative side of power supply to the load or input module.
PNP sensors
PNP sensors are sourcing type sensors
PNP sensors used with sinking input module
PNP sensors switches positive side of the power to the load or input module .
(Note: Many people have a misconception about these PNP and NPN sensors .They thinks like this . PNP provides NO contacts and NPN provides NC contacts or vice versa .This is absolutely wrong .These sensor types have nothing to do with these NO/NC contact provisions . There is no specific relationship between NPN and PNP sensor types with NO/NC contacts .All these sensor type can be designed so as to provide NO or NC contacts or both.)
