Three-wire sensors are used in various applications from detecting parts to locating position of the actual machine. They can come in all different technologies such as inductive sensors, photoelectric sensors and capacitive sensors, just to list a few. Although the sensor technology may differ, all 3-wire sensors are wired the same way.
A three-wire sensor has 3 wires present: two power wires and one load wire. The power wires will connect to a power supply and the remaining wire to some type of load. The load is a device that is being controlled by the sensor. The most common type of load would be a PLC (programmable logic controller) DC input. Other examples of a load could be a relay or machine alarm. Just make sure the load rating of the sensor is not exceeded. A typical 3-wire DC sensor’s output has a rating of 100mA to 200mA.
As an example, let’s reference an inductive proximity sensor. When a target (the object that a sensor is detecting) comes within sensing range of the sensor, the sensor output turns on and current flows. A 3-wire sensor typically is color coded with one brown wire, one blue wire and one black wire. The brown wire is the +VDC wire that connects to the positive (+) side of the power supply and the blue wire is connected to the common terminal of the power supply — this is the negative (-) terminal that is present on the power supply. The black wire is the output (load) wire of the sensor. 3-wire DC sensors can have a PNP (sourcing) or NPN (sinking) output. Just make sure the correct sensor part number is selected for the correct transistor circuit.
There you have it! As you can see wiring a 3-wire sensor is not too difficult.