Can a Proximity Switch Be Directly Tested with a Multimeter?
① Methods for Testing Proximity Switches with a Multimeter We know proximity switches fall into two types: NPN and PNP. For PNP proximity switches, the output signal is active-high. That means when an object to be detected approaches, the signal output terminal generates a high level; when the object moves away, the output returns to a low level. By contrast, NPN proximity switches have active-low outputs. Therefore, proximity switches are switching devices that output either a high level or a low level during operation.
My routine test procedure for proximity switches is as follows. First, supply 24 V DC power to the proximity switch. Move the target object toward and away from the sensor. Use the voltage range of the multimeter to measure the signal output terminal. For an analog multimeter, observe whether the pointer deflects. For a digital multimeter, check for voltage switching between high and low levels. If voltage switching occurs, the proximity switch works normally. If the measured voltage remains constant, the switch is faulty and unusable. The display effect is shown in the figure below.
There are two common multimeter test methods for proximity switches. One measures the output voltage signal when the sensor is powered on. The other measures the resistance between the signal output terminal and power terminals with the power disconnected. Most commonly used proximity switches are 3-wire NPN or PNP types. When measuring output voltage with a multimeter, you must identify the function of the three wires. The three wires are brown, blue and black. The brown wire connects to the positive terminal of the 24 V DC supply; the blue wire connects to the negative terminal; the black wire serves as the signal output line.
② Testing Proximity Switches without a Multimeter When installing and debugging opto-mechanical equipment, I often test proximity switches without a multimeter. There are several types of proximity switches, including inductive, capacitive and photoelectric types. Different proximity switches only respond to targets of specific materials. For example, in opto-mechanical material sorting equipment, inductive proximity switches only detect metal objects. Place a metal workpiece in front of the inductive proximity switch and check whether the sensor generates an output signal. If triggered, the corresponding indicator lamp on the PLC input terminal will light up. Remove the metal workpiece and verify that the PLC input signal disappears.
Wiring: connect the brown wire to the 24 V DC "+" terminal of the PLC power supply, the blue wire to the PLC 0 V terminal, and the black wire to the corresponding PLC input terminal, as shown in the figure below. For photoelectric proximity switches, place black and white workpieces in front of the sensor and check the signal status of the corresponding PLC input terminal. This is the practical method I use to test proximity switches without a multimeter.
