NPN & PNP

[Joe_WaZoo]

The semiconductor industry seems to prefer the NPN type over PNP. At least that is what I ran into many times with Applied Materials when with TURCK we were promoting their DeviceNet stations and Applied would only try them if they had NPN style inputs.

Joe_WaZoo

 

[rsdoran]

Y'all sure do like to complicate things.

NPN or PNP just relates to how it should be wired for proper operation. If it is a sensor input device then it output could be normally open, normally closed, or configurable.

The thing is to be aware of what you need and for the specific operation then obtain the necessary type. Take a look at these and look at the different types, and configurations.
http://www.sick.com/gus/products/product_catalogs/industrial_sensors/induktive/acdc2wire/en.html

You do have to know how the signal will work within the plc.

 

[ToolGuyFred]

As for NPN input LED indicators being on when sensor is not energized - i've never seen this and I'm using both PNP and NPN a lot and variety of platforms. When diagrams of internal circuitry are available, they show that LED is in series with input. Maybe ToolGuyFred can explain which PLC he meant.

N/O sensor NPN energised => NPN PLC input = LED ON
N/O sensor PNP energised => PNP PLC input = LED ON

The only time I have seen an N/O NPN sensor de-energised switch an input on is on a PC I/O card - PC in this instance meaning personal computer, NOT programmable controller. Apologies for the confusion.

 

[BobB]

a short to ground on a NPN will turn the input on.

Not quite true all the time. If one uses a switchmode power supply for the 24VDC and does not ground the negative there is no ground reference to turn on the input. If the negative is grounded the input will definately turn on. I never ground negatives from switchmode power supplies anyway and have never had a problem.

I am lead to believe that the NPN thing goes back to when PNP transistors were 3-4 times the price of NPN transistors in the early days. Perhaps someone else may have heard the same thing.

 

[magdyfayad]

can any one help me and give me the internal electerical circuit digram for PNP proximity switch and the internal electerical circuit digram for NPN proximity switch



NOTE : I know the external electerical circuit digram for both NPN & PNP , but i want to know the internal electerical circuit digram for botht types of proximity switch and when the proximity switch sense for the metal what happened internally is a current pass to the base of the transistor or what ??

thanks

 

[BobB]

You will have to get that from your chosen supplier. There are many different types - driven - open collector etc. Many suppliers have different circuits due to the different types of NPN and PNP outputs. Talk to Turck or someone and ask them for their sircuit.

 

[rsdoran]

can any one help me and give me the internal electerical circuit digram for PNP proximity switch and the internal electerical circuit digram for NPN proximity switch



NOTE : I know the external electerical circuit digram for both NPN & PNP , but i want to know the internal electerical circuit digram for botht types of proximity switch and when the proximity switch sense for the metal what happened internally is a current pass to the base of the transistor or what ??

thanks

Why would you need this?

 

[keithkyll]

Simple explaination:
A coil is on the face of the prox. It's part of a tuned oscillator circuit. Putting a piece of metal in the coil's field upsets the tuning and causes it to draw more current. A current detector IC outputs a signal to the base of a transistor. High will turn on an NPN, low will turn on a PNP.

If you want to design your own, here's more info than you'll ever want from the University of Texas at Austin.

 

[panic mode]

Not quite true all the time. If one uses a switchmode power supply for the 24VDC and does not ground the negative there is no ground reference to turn on the input. If the negative is grounded the input will definately turn on. I never ground negatives from switchmode power supplies anyway and have never had a problem.

I am lead to believe that the NPN thing goes back to when PNP transistors were 3-4 times the price of NPN transistors in the early days. Perhaps someone else may have heard the same thing.

maybe you didn't read my comment, i was refering to circuit in first post. leaving power supply output floating only buys you some time (first failure may go undetected) but this does not mean that system is safe and this one failure will be the only one equipment will experience and that following failures will be harmless.

 

[magdyfayad]

tanks all

 

[Thomas]

What about the obvious?

Hardware compatibility is the answer for specific implementations.

Homework compatibility might be tha answer to the question.

 

[rsdoran]

I will try.

The terms they use these days make things so confusing, so I am not going to get into sinking and sourcing.

These are the symbols for NPN and PNP, note NPN is NOT POINTING IN to the base:

This is why I made the earlier statement that NPN and NPN gives the relationship for proper wiring. Power supply and biasing is positive for NPN and negative for PNP.

What does that mean? With an NPN device, properly wired, the emitter will be connected to 0v so the circuit would be something like this:

As you can see in this ciruit when the device is OFF you would see V+ at the collector/load, think of the load as the plc input and when it is ON there would be almost 0V at the collector; therefore at a PLC input when the NPN device is ON you would get almost 0V. This could possibly confuse some electricians which may be another reason it is not as prefered.


With a PNP device the Emitter will be connected to V+ to offer a ckt like this:

This means that when the PNP device is OFF there will be 0V at the collector/ plc input, just think of the LOAD as a plc input. When the PNP device is ON there will be V+ on the collector i.e. the plc input.

What happens if a PNP device were replaced with a NPN or vice versa, this depends BUT in many cases all that happens is it works in the opposite manner. This can get complicated depending on the devices involved but where the PNP device would be in an OFF condition (not triggered) the NPN would be in an ON condition (not triggered).

I hope I explained that properly.

When using 3 wire sensors the Black is always the output.

The best way to learn this is get some sensors and just play with them i.e. get involved and you will understand.

 

[BITS N BYTES]

Nice explanation from rsdoran.

I've always used:-

NPN = Active Low = Input ON when connected to 0V. PLC Common connected to +V.
PNP = Active High = Input ON when connected to +V [these days usually +24VDC]. PLC Common connected to 0V.

This approach never failed me.

Mt 2 cents.

 

[magdyfayad]

thanks rsdoran

 

[testsubject]

The practice in Japan may harken back to to good old days. As far as semiconductor devices go, the NPN device is cheaper to make and operates faster than the PNP device. 30 years ago, this would have been a big deciding point over which device to use. Currently, this is not such a big deal. Even though it still take more transistors to make a PNP device, the cost is the same to buy them. Once the price was not an issue anymore, I would guess that everyone started using PNP since positive logic makes more sense than negative logic to the layman (applying a voltage to turn on an input instead of grounding it to turn it on).