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#1
User is offline   andrea 

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Hi Guys
I would like to know what is the most common sensor you guys use "NPN or PNP " on a PLC . And why do you use it perfer over the other ,and what is the advantage to the sensor "PNP or NPN "
Thanks
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#2
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you can get Ideas by go to this link

http://www.plcs.net/...ters/dcin25.htm
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#3
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for a 3-wire DC sinking PLC input card, use a sensor with a sourcing
(PNP) output.


For a 3-wire DC sourcing PLC input card, use a sensor with
a sinking (NPN) output.
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#4
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View Postandrea, on Dec 28 2006, 01:44 AM, said:

Hi Guys
I would like to know what is the most common sensor you guys use "NPN or PNP " on a PLC . And why do you use it perfer over the other ,and what is the advantage to the sensor "PNP or NPN "
Thanks


(with the appropriate input cards)
PNP a voltage present is then a true input.
NPN is not popular in the industry I'm in because grounding an input is then a logic true and thats not good.


View Postrpraveenkum, on Dec 28 2006, 02:02 AM, said:

you can get Ideas by go to this link

http://www.plcs.net/...ters/dcin25.htm


The article says that NPN is most popular in N.A. and I don't believe that without some kind of data/proof to back it up.

This post has been edited by jstolaruk: 28 December 2006 - 05:46 AM

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#5
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View Postjstolaruk, on Dec 28 2006, 05:42 AM, said:

View Postrpraveenkum, on Dec 28 2006, 02:02 AM, said:

you can get Ideas by go to this link

http://www.plcs.net/...ters/dcin25.htm


The article says that NPN is most popular in N.A. and I don't believe that without some kind of data/proof to back it up.

I don't think it is either. It is an extremely rare occasion I use NPN. In fact I can't think o the last time it did
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#6
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The only time I use NPN... :-2 when they placed the wrong purchase order :-)
With Omron NPN and PNP units are sometimes only distinct with an -1 at the end of the type number....
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#7
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From what I see a PNP is the most common sensor . When a input card can take NPN or PNP .
Thanks
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#8
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I don't understand why anyone would use NPN inputs in any application! Whoever has, i'm interested to know why?
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#9
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My problem with NPN is that grounding signals the input or turns on the output. What happens when a piece of conduit gets smashed or a device fails? Many times the same thing, the wire gets shorted to ground which signals the input or output.

With PNP if the same failure happens the circuit will fault blowing a breaker or fuse. Now many argue this is damaging to equipment. But which is better, damaging equipment or injuring an operator because of a fault trigger?
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#10
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Both are used but in usually different parts of the world.
PNP is wastly more popular in western world (Europe, America...),
while NPN seam to be common in East (Japan). Sensors with PNP and NPN outputs are
actually the same, it's is just the output of the sensor that has oposite polarity.

Popular or common means lower price, better selection and availablity, matches
common PLC I/O and more local guys who know how to use them properly (or troubleshoot).

It's not big deal to just make them work but there are gotchas (including safety/
uncontrolled motion).

For example NPN makes it easy to interface non-isolated circuits of
different potential if the output is rated for that voltage and circuits are
sharing same negative rail (equivalent of TTL open collector) which is ok
inside enclosed system which is not dangerous (like that inkjet printer or
big multifunction copy machine).
Industrial equipment like hydraulic lift or conveyor motors and chains etc.
is something else (someone can get seriously hurt or die).

Imagine someone trying to reach part stuck in the machine, this happens all the time.
They trip the safety circuit by opening gate or pressing E-Stop and reach in.
But if the machine (or part of it) moves suddenly when the person is only inches away
from hazard and not expecting it, things can get scary.
This is an example of what is called uncontroled motion. This is what happens if
there is circuit failure (open, short or short to ground for example) in a
incorrectly designed system (mix or sink and source I/O for example).
This applies to both inputs and outputs.

To be fair, if the safety system was designed and used correctly,
this would not happen but then, just ask around how many people are knowledgable
about safety circuit design, know how to apply it correctly and can tell the difference
between categories.

That's why all circuits should be analysed for failures and designed properly,
not just use bandaid solutions to make things work. This applies to inputs and
outputs as well. Look for Jim Rowel's article in download section for examples.

Btw. what happened to Jim? Did he just disappear a year ago...?
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#11
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View PostTWControls, on Dec 28 2006, 09:37 AM, said:

My problem with NPN is that grounding signals the input or turns on the output. What happens when a piece of conduit gets smashed or a device fails? Many times the same thing, the wire gets shorted to ground which signals the input or output.


There is already a discusion on this topic in the general forum: http://forums.mrplc....wtopic=9429&hl=

If you ground the positive terminal on the 24V power supply then you will be switching -24V with NPN so the grounding issue is not a problem. :-1

This post has been edited by Wulfgar: 05 January 2007 - 01:01 PM

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#12
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Once you get used to working with them, both are functionally equivalent.

But, with a positive supply NPN (all "active low" types in fact) behaves better under supply transient conditions: power-up, power-down and brown-outs. This is because the typical positive supply adjusts/regulates the positive output relative to its reference (or "ground") and, under transient conditions, a PNP output may feed the still-adjusting, unstable positive rail (and any noise with it) as an input to the probably partially functioning PLC with indeterminate results.

So it is desirable to have resets, faults, alarms etc as active low inputs for a more robust system. Note that this does not limit the functioning program logic to process such an input as logically true or false.

In the end however, as Panic Mode correctly put it earlier:

Quote

That's why all circuits should be analysed for failures and designed properly,
not just use bandaid solutions to make things work. This applies to inputs and
outputs as well.

Come to think about it, that is why in the end we as professionals and our services are required.
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#13
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this is from rockwell about npn & PNP stuff


Selecting Sinking (NPN) vs. Sourcing (PNP) - I/O modules.

Question
Selecting Sinking (NPN) vs. Sourcing (PNP) - I/O modules

Answer
Transistors are the typical solid-state output devices for low voltage DC sensors.
Consisting of a crystalline chip(usually silicon) and three contacts, a transistor
amplifies or switches current electronically. Standard transistors come in two
types: NPN and PNP.

Selecting I/O modules
Remember opposites attract.

If the Input device is NPN(Sinking) select a (Sourcing) I/O module.
If the Input device is PNP(Sourcing) select a (Sinking) I/O module.

NPN
For an NPN transistor output,the load must be connected between
the sensor output and the positive(+)power connection. This is also
known as a "Sinking" output.

PNP
A PNP transistor output is considered a "Sourcing" output. The load
must be connected between the sensor output and the negative(-)
power connection.


Transistors exhibit very low leakage current(measured in A) and
relatively high switching current(typically 100mA)for easy
interface to most DC loads.Response times of sensors with
transistor output scan vary from 2ms to as fast as 30s.However,
NPN and PNP transistors are only capable of switching DC loads.



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Quote

If the Input device is NPN(Sinking) select a (Sourcing) I/O module.
If the Input device is PNP(Sourcing) select a (Sinking) I/O module.


All well and good for the AB terminolgy......

Omron use the terminolgy of NPN sensor to NPN input module....etc...

Quote

NPN
For an NPN transistor output,the load must be connected between
the sensor output and the positive(+)power connection. This is also
known as a "Sinking" output.

PNP
A PNP transistor output is considered a "Sourcing" output. The load
must be connected between the sensor output and the negative(-)
power connection.


This second quote is more relevent.....and generic in that it explains the connections required...
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View Postpanic mode, on Dec 28 2006, 11:36 AM, said:


Imagine someone trying to reach part stuck in the machine, this happens all the time.
They trip the safety circuit by opening gate or pressing E-Stop and reach in.
But if the machine (or part of it) moves suddenly when the person is only inches away
from hazard and not expecting it, things can get scary.
This is an example of what is called uncontroled motion. This is what happens if
there is circuit failure (open, short or short to ground for example) in a
incorrectly designed system (mix or sink and source I/O for example).
This applies to both inputs and outputs.

To be fair, if the safety system was designed and used correctly,
this would not happen but then, just ask around how many people are knowledgable
about safety circuit design, know how to apply it correctly and can tell the difference
between categories.

That's why all circuits should be analysed for failures and designed properly,
not just use bandaid solutions to make things work. This applies to inputs and
outputs as well. Look for Jim Rowel's article in download section for examples.

Btw. what happened to Jim? Did he just disappear a year ago...?


Uhh...there's an even bigger problem here. Using an E-Stop as a lockout device is specifically a huge safety infraction. An E-Stop by definition is intended to shut down a machine as quickly as possible to prevent possible injury (the equipment be danged). It is not intended as a lockout device and never, ever should be used as one.

Why not? An E-Stop clearly does not disable energy sources. Your starters are still engaged and can still power up. Any other energy sources (air, hydraulic, etc.) still have a load present and could kill somebody. All you've done is dropped control power to them.

That being said, I can't tell you how many times people have used E-Stops as stop buttons or worse yet, lockout devices.
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We are located in the Indianapolis area and a great deal of our customers are Japanese based, or suppliers for such. They spec NPN. Other customers are direct or Tier suppliers for the BIG 3 and they spec PNP. Most of our pharmaceutical and drug industry customers utilize european equipment which requires PNP. This seems to be the "line drawn in the sand" that I have discovered between NPN and PNP.

This post has been edited by 5150: 27 January 2007 - 06:39 PM

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#17
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View Postpaulengr, on Jan 19 2007, 06:49 AM, said:

An E-Stop clearly does not disable energy sources. Your starters are still engaged and can still power up. Any other energy sources (air, hydraulic, etc.) still have a load present and could kill somebody.


I dont know which industry and part of the world you are in.
Here in Canada neither customer nor safety inspector of any agency will accept such "safety".
It exactly should kill all motor contactors and air/hydraulic pressure, no matter whether it was Estop button or safety guard activated. And these things should not be able to come back On automatically.
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Quote

It exactly should kill all motor contactors and air/hydraulic pressure, no matter whether it was Estop button or safety guard activated. And these things should not be able to come back On automatically.



Correct.... but he said that it should not be used for a lock out device.... ie a lock and TAG placed on to it not to activate it..... hence the role of an isolator.... indeed the E-stop isolates the power via a SAFETY relay and the like.... but an isolator has provision to lock out the power entriely to the control system and be padlocked.... i would trust this system over a simple e-stop that can be reset and the system reset / start button can be activated..... I prefer to isolate the power to the machine entirely if working on it...
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#19
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I never suggested this was a good idea. I was criticizing poor design and lax approach for both safety
as well as non safe circuits (such as PLC I/O). Intent of the mentioned post was to produce
horror scenario by naming common bad practices. Each may be dangerous, but when combined,
these are breeding grounds for bad surprises and "accidents" waiting to happen...

I think after el. designers and users, the only party not mentioned is PHSR guys. I hope I'm wrong but
none of those guys I've met so far, seamed to be able to even read electrical drawings,
not to mention evaluating if used safety circuitry made any sense.
...just having right price and someone to sign it...?
:no: ... :no:
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