PNP versus NPN, The Comparison
In the construction of a sensor's transistor or the type of transistor used, NPN and PNP are commonly encountered. Their applications can be exemplified as follows:
Wiring a Sensor: In sensor wiring, "N" stands for "Negative," while "P" stands for "Positive." Consequently, an NPN device is capable of switching the negative side of the circuit, while a PNP device switches the positive side.
Flow or Direction of the Current: A PNP sensor, also known as a sourcing sensor, sources positive power to the output. On the other hand, NPN sensors, referred to as sinking sensors, sink ground to the output. The type of transistor employed determines the direction of current flow, with PNP and NPN being technical terms that describe the transistor types. The "P" and "N" refer to the arrangement of semiconductor materials. Note that this only applies to Solid-State Outputs.
Sourcing versus Sinking Circuits: Solid-state devices are associated with PNP and NPN types. A solid-state device typically features three-wire connections for i. +24 Vdc, ii. 0 Vdc, and iii. Switch or sensor signal. The PNP or NPN style dictates how the sensor operates the switched lead. Consequently, for PLC discrete input (DI) types, they must be complementary to the field device type. A sinking PLC DI is appropriate for a PNP field sensor device, looking for +24 Vdc on the DI channel, while a sourcing PLC DI suits NPN field sensor devices, searching for 0 Vdc on the DI channel.
Benefits of PNP versus NPN: In cases where the PNP or NPN cable is damaged, there is a risk of the signal shorting to ground and damaging the sensor. This is where the difference becomes crucial, as programmers and technicians prefer to use and troubleshoot the resulting logic TRUE, which is the outcome when a PNP sensor is damaged.
User Preferences: Based on anecdotal studies, most European and North American designers opt for PNP sensors together with sinking PLC DI modules, while Asian designers prefer NPN sensors with sourcing PLC DI modules. This preference often arises from the availability of readily accessible products in their respective regions.
