Hello, we have talked about NPN sensors and PNP sensors many times before, what is the difference between these two sensors?
PNP sensors, also known as "source input", the current is flowing from the terminal, that is, the high level is valid. Then its output voltage is 24V in 24V.
NPN sensor is also called "leakage input", the current is flowing in from the terminal, which is active low. Then its output voltage is 0V out of 24V.
In fact, NPN and PNP have their own benefits.
For PNP, the general European and American companies will use it, than in terms of its anti-interference ability is stronger, because its sensor output signal voltage is 24V24V voltage compared to 0V voltage is more difficult to be other power interference out. So the PNP sensor is more stable, the external power supply requirements are not high.
I once had a high-power servo motor interference sensor in the factory, after a variety of processing is still unable to completely eliminate the interference of the servo system, and finally the sensor by the NPN-type replacement for the PNP-type on a complete solution, so what are the advantages of NPN sensors?
NPN sensor output signal is 0V, 0V disadvantage is easy to be interfered with, the advantage is that the ground will not be shorted. So even if the NPN signal line for some reason contact with the ground line, will not lead to the previous level of switching power supply trip or damage, so the adaptability of a little stronger, can better protect the system.
In fact, the actual production still occurs because the signal line of a sensor connected to the ground line leads to power supply burnout or the previous switching power supply tripped the case, this time if the NPN sensor will not have a problem.
All for different situations we can choose the right sensor according to the actual needs.
So how to distinguish and use NPN and PNP?
First of all, the general sensor is marked on the top, NPN or PNP one day clear, of course, there are some dry junction of the sensor does not distinguish between NPN and PNP in addition to some NPN and PNP can be switched to each other's sensors, but this kind of sensors in general is more expensive.
In addition, we can follow the PLC wiring to distinguish between sensor types.When the common terminal is 0V, the PLC is connected to PNP, then the PLC can only receive signals from PNP sensors, and there is no response to NPN sensors, while for dry junction sensors, generally speaking, you only need to connect the common terminal of the sensor to 24V, and you can connect it to the PLC of PNP.
When the common terminal is 24V, the PLC connection is NPN, then the PLC can only receive the signal of the NPN sensor, there is no response to the PNP sensor, and for the junction sensor, generally speaking, only need to connect the sensor's common terminal to 0V, it can be connected to the PLC of the PNP.
So many students will ask, why NPN and PNP related to the common terminal of the PLC?
This is based on the working principle of PLC digital inputs, how does the PLC detect that there is an input signal using only one wire?
The answer is voltage difference.
PLC needs to detect the voltage difference to confirm the detection of the input signal, and the input signal is only one wire, then the other end of the voltage difference is the common terminal, which is for comparison with the input signal.
Therefore, the 0V common terminal of PNP needs the input signal to be 24V to be activated, and the 24V common terminal of NPN needs the input signal to be 0V to be activated Then in some special cases we need to use NPN and PNP sensors at the same time, and this is how we should do it? The easiest way is to use a relay transition.
But this method has some disadvantages, that is, some signals can not be transition, such as encoders, etc. There is also because the relay needs to absorb the time, which will lead to some delay in the PLC scanning input signals, which is not permitted for a particular situation.
Therefore, I recommend that you use the second method, that is, a PLC to use a different common terminal, for the Siemens S7 series PLC, all PLCs have the function of expansion modules, and each module has a separate common terminal, so you can expand the common terminal of the module into the desired mode, so as to realize the function of accepting NPN and PNP sensors at the same time.