3-wire sensors are sourcing, type "PNP", or sinking, type "NPN", with separate connections for the power supply and for the output. These sensors are protected against overload, short circuit, and reverse polarity. The residual current is negligible. For sensor type PNP, the load is connected between the output and ground potential (L-). For sensor type NPN, the load is connected between the output and L+.
Depending on the type of output-stage transistor used, sinking output stages are often referred to as NPN output stages. Sourcing end stages are referred to as PNP output stages.
A special type of switching 3-wire sensor is the sensor with a digital current output. In its standard version, the 3-wire sensor represents the switch state by the voltage level at the switching output. The special type with digital current output sees the sensor represent the switch state by an applied current. The output thus acts as a current source. When not switched on, the switching output drives an applied current of approximately 5 mA through the connected load. When switched on, this current increases to approximately 10 mA.
A lower sensitivity to external electromagnetic interference is typical and considered a benefit of this output type. Another benefit is the diagnostic capability. Lead breakages and lead short circuits can be detected and evaluated.
The connection of a sensor with a digital current output is the same as for a sensor with PNP and normally-open output. The load is between the output connection and the ground potential.
Sensors with push-pull output combine the characteristics of a PNP output and an NPN output. These sensors correspond to sensor type E (3-wire), in which the switching output is equipped with a PNP and an NPN transistor. This allows the load to be connected between the output cable and L+ or L-.
Sensors with push-pull output are used by Pepperl+Fuchs in the following technologies:
These sensors are usually marked with the following in the third block of the Pepperl+Fuchs order designation:
–EI = special type with digital current output
–EP = sensors with push-pull output
Sensors of type –E are available in the following switching element functions:
Take a look at the range of Pepperl+Fuchs capacitive sensors.