Open Collector (NPN / PNP) vs Push Pull Outputs

In factory automation, sensors not only need to be specified appropriately for the sensing task, but also need to consider what “type” of output and what output “configuration” is required by the PLC or machine to be compatible with it.  PLC’s or machines will typically require either a form of AC input from an AC powered sensor, or a “type” of DC input and a DC input “configuration” from a DC powered sensor.  Since the issues regarding the AC powered sensor and its compatibility with the PLC or machine are relatively straight forward, only DC inputs are discussed here since they are the most popular, and often the most misunderstood.

For PLC’s or machines that require a DC input (DC output from the respective sensor) the typical single question that is often asked is: “Does the PLC or machine need an NPN/Sinking or PNP/Sourcing “type” output from the sensor?”   Although the answer to this question is usually sufficient to select the right sensor, it assumes that the output “configuration” of the sensor is compatible with the input “configuration” needs of the PLC or machine.  It would be prudent to also ask if the existing sensor has a push-pull output configuration?  If so, then likely the PLC or machine is designed to accept only sensors with a push-pull output configuration.

NPN/Sinking type outputs attempt to take their output to equipment ground or power supply common (near zero volts) when the sensor turns ON its output.  PNP/Sourcing outputs attempt to take their output to near positive (+) of the supply voltage when the sensor turns ON its output.

Note that if what “type” of DC input the PLC or machine needs (i.e. NPN/Sinking or PNP/Sourcing) is unknown, then generally it is safe to try an NPN/Sinking sensor type output, and if the PLC or machine does not recognize it, disconnect it, and try the PNP/Sourcing sensor type output, or vice-versa.

The two DC output “configurations” from a sensor are:

  1. Open-Collector output configuration:
    The DC powered open-collector sensor may provide either NPN/Sinking type output or open collector PNP/Sourcing type output, or the DC powered sensor may provide dual type outputs (open-collector NPN/Sinking and open-collector PNP/Sourcing).   If dual type outputs are provided, it will be via two separate, dedicated output wires.  Most DC sensors in the industry provide open-collector output configurations, and most PLC’s and machines are designed to be compatible with open-collector sensor configurations.  Note that not all manufacturers clearly indicate in their spec sheets that their sensor’s output is in fact, an open-collector configuration.
  1. Push-Pull output configuration:
    A sensor with push-pull output configuration has both an NPN and a PNP type transistor in its final output stage.  Because of the required common connection between the two within the sensor, a single output wire from the sensor can provide both Sinking and Sourcing output.  Many sensor manufacturers will indicate that their sensor’s output is push-pull, if in fact it is.  Chances are that if a manufacturer does not say the output configuration is push-pull, that it is open-collector.

Where is the concern?:
Sensors with open-collector NPN/Sinking or PNP/Sourcing output configurations do not supply any voltage to their output transistors.  They depend on obtaining the voltage that is needed to make them function from the voltage supply of the PLC or machine that they are connected to.  The advantage of the open-collector configuration is that the sensor will be compatible with a wide variety of PLC or machine supply voltages.

While push-pull configuration sounds like the ideal situation, the two transistors are connected to a supply voltage within the sensor, and that voltage reference is experienced in the single output that it provides.  This output voltage provided by the sensor, must be compatible with the supply voltage for the PLC or machine that it is connected to.

In short, good advice would be that sensors that provide open-collector output configurations should be used with PLC’s or machines that are designed to accept open-collector input configurations, and sensors with push-pull output configurations should be used with machines that are designed to accept push-pull input configurations.

If the PLC or machine required configuration is unknown and if, for example, is push-pull, a sensor with an open-collector configuration may operate and sense the task, but the machine may or may not accept the sensor’s signal, or may respond unpredictably solely because of the difference in configurations of the sensor versus the PLC or machine, and vice-versa.

A specific solution to a specific incompatibility:
To use a sensor with open-collector NPN/sinking output configuration and be compatible with a PLC or machine that is designed to accept a push-pull NPN/Sinking input configuration, a “PULL-UP” resistor of approximately 1k ohm value would have to be added.  The resistor needs to be connected from the output wire of the sensor to the PLC or machine’s positive (+) supply.  This will provide the necessary operating voltage for the sensor’s NPN transistor in the open-collector configuration, and facilitate compatibility between sensor and PLC or machine.

Eagle Sensors & Controls - A Division of Excel Automation LLC