NPN & PNP

magdyfayad

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Join Date
Aug 2007
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cairo
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I know the difference betwenn the connection of PNP & NPN sensors to PLC AS :



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BUT , I want to know what applications must we use PNP sensors and what applications must we use NPN sensors ??

and what are the advatges of each than the another ??

thanks
 
Not many plc manufacturers use npn anymore.

Some japanese manufacturers used it in the old days & there are a couple of reasons banded about as to why.
1. Compatability with some sensors
2. Using npn (sink) then any noise spikes are generally +ve so will not trigger a false input logical "1" in the plc (theory sounds good).
To be honest, I have worked in this industry for over 25 years & in the last 15 or so have never used npn input cards.
 
thanks parky

but generallly for PLCs or the traditional control system ( relay-contactor), which industry application we must use PNP ?? and which industry application use NPN ??


I know the famous used is for PNP , but when do we must use NPN for the traditional control system ?? , and what is the advatages of each than the another ??

thanks again
 
There is no must use npn or pnp it is strictly a choice.

I can see no reason why any industry cannot use either.
although I bet someone on here will know of one?.
I have worked in all areas from military, food, engineering etc. & I have not come across any that have stipulated npn or pnp for a specific reason other than compatability with existing equipment.
The only time recently I have used npn sensors connected to a plc was in an emergency when the only one availlable of the required dimemsions was npn so I used a resistor tied to +ve line & other end to sensor output as a load, & inverted the logic in the plc, not perfect but it got us out of a mess until a new sensor arrived.
 
thanks parky

do you mean if we connect the NPN sensor that when the sensor is active it will be false logic and when the sensor is inactive it will be true logic ??

thanks
 
Last edited:
There is a note here about Japanese using NPN years ago. They still do. NPN inputs are, for some reason, preferred generally in Japan.

I would suggest you only use PNP inputs - positive switching in.

Outputs are no real problem - NPN outputs are fine and I use them all the time.

The PLCs I usually use have a bidirectional input setup whereby NPN or PNP input is only determined by the common connection. Your choice - positive common is NPN input - negative common is PNP input.
 
the questions now :

1- is there an application must use NPN and a an application must use PNP or all the application are the same may be use NPN or PNP ??

2- What are the reasons which japaneese PLCs used NPN ??

3- is the logic gate connected to the NPN sensor will be false if the NPN sensor is active and the logic gate connected to the NPN sensor will be true if the NPN sensor is inactive ?? or the logic gate connected to the NPN sensor is equals to connected to the PNP sensor as when the PNP sensor is active the logic gate will be true and when the PNP sensor is inactive the logic gate will be false ???

thanks
 
In my experience, European-built equipment is 99.9% PNP. The only NPN gear I've met has been on machines of American or Japanese origin. Having said that, some application-specific components (Japanese servo PLC cards, American PC I/O cards) are only available in NPN flavour.
 
My personal preference is for PNP sensors as a short to ground on the signal line on a PNP will blow a fuse and a short to ground on a NPN will turn the input on.

Just my 2 cents
John
 
yes , European-built equipment is 99.9% PNP , but what are the applications and the defined reasons make us use NPN ??

Also , is the logic gate connected to the NPN sensor will be false if the NPN sensor is active and the logic gate connected to the NPN sensor will be true if the NPN sensor is inactive ?? or the logic gate connected to the NPN sensor is equals to connected to the PNP sensor as when the PNP sensor is active the logic gate will be true and when the PNP sensor is inactive the logic gate will be false ???

thanks
 
magdyfayad said:
Also , is the logic gate connected to the NPN sensor will be false if the NPN sensor is active and the logic gate connected to the NPN sensor will be true if the NPN sensor is inactive ?? or the logic gate connected to the NPN sensor is equals to connected to the PNP sensor as when the PNP sensor is active the logic gate will be true and when the PNP sensor is inactive the logic gate will be false ???

Normally an active NPN sensor on a PLC input is TRUE, the same as when a PNP sensor is active. Most PC I/O cards with NPN inputs show HIGH when the sensor is not made, although this is sometimes configurable. If you are interfacing at logic gate level then inputs are normally NPN, float HIGH and are energised LOW.
 
To early in the morning for me to say much but here goes.

A sensor is the same as any switch i.e. it can be N.O. or N.C.; which means if it is an input the signal wuld be there until the sensor energizes or would only be there when the sensor energizes.

As mentioned the reason most do not use NPN is because a short to ground could activate the device. The reason for this is the DC common is usually bonded to ground. I guess if the DC voltage was isolated from ground it would not matter.
http://www.patchn.com/npnpnp.htm
 
In an older thread on this subject, someone said one of the formats was prefered or mandatory for equipment going to Japan. I don't have time to search now.
 
this has been discussed many times already. there is no particular technical reason to use PNP vs. NPN or the other way around. it's just polarity so it's relative term (in fact there are plenty of devices that can convert signal from PNP to NPN and the other way around).

things do get another meaning when making industrial machinery vs. school project - because of safety. as long as you don't just try to make it work (bring signal into or out of PLC) but also think about uncontrolled motion (or whatever other hazard it may be such as automatic start, failure to stop etc.) you are wellcome to use any. the basic rule is to ensure that any SIGNAL potential is DIFFERENT from ground potential. in NPN system that would be -V, in positive it's +V, but always potential different from ground.

consider first image where NPN output is driving load. notice that emitter of NPN transistor is shown as grounded. imagine that load is something that causes motion (solenoid valve of some actuator or start command of VFD drive etc.). if you have failure such as short to ground BETWEEN the output and load, load will be energized (not by PLC but by that accidental short) which means you will get motion when not expected (and possibly hurt someone). this is classic example of poor design although electrically circuit can operate if everything is ok (problem is when something bad happens - and this is part of good design). it can be corrected by grounding positive end of the power supply instead of negative (this is hard to digest as many people still freak out when some mentions grounding of DC side of the power supply). same goes with inputs where short to ground could cause accidental start etc. or where both inputs and outputs tie to same PSU rail (negative for example). this (both inputs and output devices tie to same potential) is quite common in low power electronics (not enough energy to hurt someone) but this is NOT good enough for industrial shop flor.

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.
 

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