Why is there a need to produce NO and NC inductive proximity switch?

[gubavac111]

Why is there a need to produce NO and NC proximity switch? Why not just swap the wires??

So, basically, when buying an inductive proximity switch, you have to choose whether it's gonna be PNP or NPN and whether it's gonna be NO or NC (check the attached drawing).

Regardless of the type of the switch, the following is true (I'll assume 24V power supply):

You apply 24V between brown (+24V) and blue (0V).

You connect the load (usually relay coil) between brown and black in NPN transistor and between blue and black in PNP transistor.

All clear until now.


Now... Say you choose NO NPN proxy switch because it suits your purpose. And you apply power between brown (+24V) and blue (0V) and connect the load between brown and black. Proxy switch is not activated (no metal object detected).

So, the measured voltages right now are:

Between brown and blue: 24V
Between brown and black: 24V (because it is NO type)
Between blue and black : 0V

If the load is connected between brown and black, 24V is applied to the load.
If the load is connected between blue and black, 0V is applied to the load.

When the proxy switch is activated, the voltages are:

Between brown and blue: 24V
Between brown and black: 0V (because NO has just become closed)
Between blue and black :24V (because black is at the same potential as brown)

If the load is connected between brown and black, 0V is applied to the load.
If the load is connected between blue and black, 24V is applied to the load.


So, it seems NO NPN proximity switch should be able to function both as NO and NC. Am I mistaken. I guess I am. Otherwise they wouldn't manufacture NO and NC proxy switches. But I would like to know where is my mistake...

 

[Nigel Goodwin]

Fairly obviously you can use either, as long as you have access to and full control over what you're connecting it to - failing that, you simply choose the one you want, and it works, no bother. Otherwise you reprogram the unit it's feeding, or redesign it to use the one you happen to have.

 

[gophert]

Why is there a need to produce NO and NC proximity switch? Why not just swap the wires??

So, basically, when buying an inductive proximity switch, you have to choose whether it's gonna be PNP or NPN and whether it's gonna be NO or NC (check the attached drawing).

Regardless of the type of the switch, the following is true (I'll assume 24V power supply):

You apply 24V between brown (+24V) and blue (0V).

You connect the load (usually relay coil) between brown and black in NPN transistor and between blue and black in PNP transistor.

All clear until now.


Now... Say you choose NO NPN proxy switch because it suits your purpose. And you apply power between brown (+24V) and blue (0V) and connect the load between brown and black. Proxy switch is not activated (no metal object detected).

So, the measured voltages right now are:

Between brown and blue: 24V
Between brown and black: 24V (because it is NO type)
Between blue and black : 0V

If the load is connected between brown and black, 24V is applied to the load.
If the load is connected between blue and black, 0V is applied to the load.

When the proxy switch is activated, the voltages are:

Between brown and blue: 24V
Between brown and black: 0V (because NO has just become closed)
Between blue and black :24V (because black is at the same potential as brown)

If the load is connected between brown and black, 0V is applied to the load.
If the load is connected between blue and black, 24V is applied to the load.


So, it seems NO NPN proximity switch should be able to function both as NO and NC. Am I mistaken. I guess I am. Otherwise they wouldn't manufacture NO and NC proxy switches. But I would like to know where is my mistake...

It seems to be a question of definition. Note that there is a difference between no current flow and 0V. A PNP device will allow your load to be connected to ground and have a true electrical connection to ground while the other side (power) will either be floating (no current flow) or +24V (depending if the sensor switch is NO/NC and Active vs Inactive)

The NPN will allow your load to be connected to +24V and connect your load to ground (allowing current flow) until the proximity sensor senses something and opens and stops current flow, OR, if you have a Normally open version, the load will not be connected to ground until the sensor senses something and makes a connection to ground (allowing current flow).

 

[MaxHeadRoom78]

PNP and NPN versions can be N.O. or N.C.
N.C. is generally used in fail safe areas where loss of power to the device simulates an off condition to the subject equipment it is used in.
Machine Over travel etc.
For very low current/load applications, you can also get the two wire versions, the two conductors carry the supply as well as the switching action.
PNP/NPN are used where a higher current load is switched.
Max..

 

[rjenkinsgb]

Whichever version you are considering, there is only one actual "switch" device (transistor or FET) in the output circuit - either between output and positive (PNP) or between output and negative (NPN).

A PNP output will switch a load that has the other side grounded.

If you connect a grounded load to an NPN output type, the switch is across the load and power is never applied.

Likewise, an NPN will control a load with the other side connected to power, but a PNP will do nothing in that configuration.

There may be a very small current due to the built-in indicator LED and it's resistor, so it gives a voltage change with no load - but with a load, there is no power switching if the load connects to the wrong rail.

After deciding PNP or NPN, then you have to decide NO or NC.

 

[MaxHeadRoom78]

Here is a PDF on sink/source also, as it applies to PLC etc that may help.
Max.

 

[gary350]

Maybe some day you will work somewhere that uses industrial controllers = computer that uses proximity switches on equipment and you are in charge of programming the controllers on all the equipment in the factory then it will be very clear why there are both NO & NC. Programming a controller is several lines of commands might be 30 lines or 100 lines. Each line is made up of several NO & NC commands per line might be 1 or 2 or as many of 8 or 10. Example of some programs I did, machines have people that operate them there are lots of safeties, door switch for open and closed. Machine open and closed, electricity on & off. Many others. Depending where each proximity switch and what is does determines if it needs to be NO or NC. One thing lots of programmers don't think about is, what if electricity to the building goes off. You don't want anyone getting hurt or killed. You need fail safe switches in some places. I have been retired from programming for many years I wish I could show you a picture of a program but that is of little help without seeing the machine and knowing what it does and how it works. Go to Ebay buy a used controller that is no longer popular they are very cheep $30 instead of $2500. Buy some prox switches do some programs at home. There is a ton of $$$$$$$$ to be made programming industrial controllers in factories and city traffic lights. City traffic lights are challenging and fun. Learn to do programs for several of the popular controllers that will put you in high demand in the job market.

 

[gubavac111]

Thank you all guys for your replies, I understand now!

 

[gophert]

Just to beat a dead horse (since you say you understand)...
Here are the on and off versions of NPN in NC and NO sensor module designs (conceptually).

Note that the LED (if needed) and it's current limiting circuitry should be placed in parallel with the load in an actual implementation

 

[MaxHeadRoom78]

All inductive proximity versions I have encountered have had the internal LED indicator.
Incidentally they also come in non-ferrous detect versions.
Max.