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.
10 Replies to “Back to the Basics – How do I wire my 3-wire sensors?”
I’m a tool and die maker and we use proxies all the time. But there are times when the black wire doesn;t work and you have to use the white wire. Nobody seems to be able to tell me why.
is there a way to connect a three wire sensor using only two wires?
A three-wire sensor would have to have all three wires to operate correctly and provide an output as the third wire is the output wire.
If two wires are needed you would look at an DC 2-wire sensor.
In an npn speed sensor, will it be damaged if the 12vdc + is connected to the signal cable or wrong polarity connections? Thanks for the answer.
Hi Roger, it is always best practice to review your wiring before selecting the proper polarity sensor. For example, an NPN sensor will sink voltage and a PNP will source voltage. If indeed the sensors are wired up incorrectly it should not damage the sensors as most Inductive Sensors have “reverse polarity protection”. You can verify this when reviewing the datasheet for the sensor in question.
How can I connect the three wires of the Capacitive Proximity Sensor Switch to a usb cable to get power through computer
Common Capacitive Sensors require 10-30vdc power so unfortunately you will not be able to operate a factory automation sensor through a USB port. Additionally, a USB port is typically a 5v power output making the output power too low to operate such a sensor. If you need to demonstrate power to the sensor you may consider a hand held sensor tester. (see below)
Simple explanation, any instrumentation technician can follow this with ease. Thanks for sharing.
Thanks, very helpful.