How does a multimeter differentiate between PNP sensors and NPN type sensors?

There are various types of sensors in the automated production line. They judge the production status in the automated production line and detect the progress of the production process in real time. Finally, the detected signals are sent to industrial control equipment such as PLCs and frequency converters. The processors in these equipments are processed by logical operations and then output to drive various execution devices such as motors, solenoid valves, and pneumatic devices.

There are various types of sensors in automated production lines. Common ones include inductive proximity sensors, which can detect metal objects; capacitive proximity sensors, which can control the on and off of switches; Hall proximity switches, which It can detect magnetic substances, and there are also photoelectric sensors that can detect materials of different materials.

In some proximity sensors, such as the inductive proximity switch we talked about earlier, it can be divided into four types: NPN normally open type, NPN normally closed type, PNP normally open type and PNP normally closed type. When we use it, we can use a multimeter to distinguish between PNP sensors and NPN sensors.

Its operation method is relatively simple, that is, after applying a suitable voltage to the sensor, when an object to be detected arrives, it can be distinguished by detecting the voltage of the signal output terminal close to the sensor. In fact, it is to measure their output voltage. Flat high and low.

When the proximity switch is turned on, if we use a multimeter to measure the output level of the signal output terminal as high level, this output is a PNP sensor. When wiring, the load must be connected between the signal output terminal and the negative pole of the power supply. During the period, PNP type sensors can be divided into two types: positive logic moving type and positive logic moving type. The schematic diagram is shown in the figure below.

Similarly, when the proximity switch is turned on and the signal output terminal of the sensor measured with a multimeter is low, it can be determined to be an NPN type sensor. It is also divided into two types: positive and negative logic moving-on type and negative logic moving-off type. Its schematic diagram is shown in the figure below.