11/05/2009 - Volume 2, Issue 14
When using inductive proximity sensors for a control application, it is common to choose a 3-wire DC prox that has a dedicated NPN or PNP control output and bring the output into a PLC input. Choosing between polarities means determining how the common is wired and selecting accordingly. However, if there is an issue with the sensor, and your machines are a mix between NPN and PNP sensors, you'll need to keep a few of each type on hand as spares to prevent a line-down situation.
A simple alternative to 3-wire models: the 2-wire proximity sensor
Instead of having the control output as a separate circuit then the power circuit, the 2-wire solution puts everything in parallel, which essentially consolidates the circuitry into one loop. Figure 1 below shows a typical 3-wire PNP circuit. As you can see, the output wire is specific in its polarity so that it will only function on with a 0V common. The power to the sensor is separate from the output circuit. In figure 2, we see the circuit diagram for the 2-wire variation. The output operation is along the two power wires in the form of a voltage drop, thus making the sensor free to work with either polarity on the common.
Replace NPN and PNP 3-wire models
Because of this, you can use the 2-wire sensor to replace both the NPN and PNP 3-wire models, depending on the way you wire it up. As you can see in figure 3, replacing a three wire sensor is as simple as following the flow of current. Being that in this example you are replacing a PNP, or current sourcing, sensor, you are connecting to an input that has a 0V common. Connecting the 2-wire sensor can be done by merely connecting the blue wire to the input and the brown wire to the 24V source.
The process is the same for replacing a NPN 3-wire sensor. In this case you would just connect the brown wire to the input and the blue wire to the ground of your power supply.
Not only do you have one less wire to install, you have one solution that will work equally as well as both types of 3-wire sensors. This greatly simplifies the usage and replacement of inductive proximity sensors across all applications.