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Old July 6th, 2004, 10:37 AM   #1
Don_Dubé
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Smile NPN and PNP Sensors

Hello guys,

I need more some clarification on wiring sensors.

From what I have read this is what I understand: Typically an NPN sensor has the collector wired to the input of the PLCs which makes it "Sinking'

A PNP Sensor has the Emitor wired to the input module which makes it Sourcing

My questions: Is it possible or even common to have a NPN Sourcing and PNP Sinking. It is my understanding that Terry uses a NPN Sourcing? Therefore, Should I be more specific when ordering sensors i.e. NPN (Sourcing or Sinking) I hope this does not make the matter more confusing!

Thanks

note: You can e-mail me directly at dubedias@yahoo.com
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Old July 6th, 2004, 10:46 AM   #2
Mylo
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Kindof missed the point, but long story short, very short is,,

PNP = Sourcing

NPN = Sinking


If you have a PNP sensor you source the voltage and you switch the POSITIVE side of the sensor, if 2 wire, or 3wire

If you have a NPN sensor you sink the voltage and you switch the NEGATIVE side of the sensor, same but different,

Nothing to do with emmitter or receiver side of sensors.

i.e a PNP sensor connected to a PLC will have the neative/0V wire commoned to the PLC, it will take a +24V signal and switch it back to the PLC as an input,

a NPN sensor will common the +24Vdc and sink the Negative 0V side back to the PLC as an input, (less detrimental on electronics (Common in Asian PLC's))

hope this helps, there has been some really good threads on this if you search,,

Cheers

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Old July 6th, 2004, 12:20 PM   #3
panic mode
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No, it's always collector that goes to load.
Take a look at the picture...

First sensor in this drawing (presented with the output transistor Q1)
is "sourcing" signal/power for load presented with R1 (can be PLC input).

Second sensor in this drawing (presented with the output transistor Q2)
is "sinking" signal/power for load presented with R2 (can be PLC input).

Those two are the standard. Now technically you can make sensors (circuits)
as presented with Q3 and Q4. Nobody does it because in such design
output transistors (Q3 and Q4) would not get saturated ("fully on")
without using additional power which would then result in a large voltage drop
on Q3 or Q4 and that is not good - because we are talking about DIGITAL I/O
where signals should be ON or OFF (and the cleaner the better).
The larger voltage drop on switching element (output transistor) the lower
voltage for the load (PLC input). For example the first two circuits
have very low voltage drop (probably less than 0.2V).
Attached Images
File Type: gif pnp-npn.gif (58.6 KB, 554 views)

Last edited by panic mode; July 6th, 2004 at 12:24 PM.
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Old July 6th, 2004, 01:08 PM   #4
rsdoran
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Maybe this will help: http://www.patchn.com/npnpnp.htm
A good explanation provided by Terry Woods.
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Old July 6th, 2004, 02:11 PM   #5
Don_Dubé
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Sensors

Thanks for all your reply but I think that Terry has a pretty good examples which explains this confusion.

In his example he uses a NPN sensor with the Emmiter wired to the PLC input module which makes it Sourcing.

So I guess an NPN sensor can be either Sinking or Sourcing depending if teh load is connected to the Collector or Emmiter

http://www.patchn.com/npnpnp.htm see figure 9
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Old July 6th, 2004, 04:19 PM   #6
panic mode
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Whatever works for you...

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Old July 6th, 2004, 05:13 PM   #7
Don_Dubé
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Question Confusion - Confusion

Thanks "Panic Mode" for your excellent explanation but you said that the load of the sensors is always connected to the Collector nor the Emmitter?

In Terry examples he uses an NPN sensors as the sourcing device i.e. Emmitter wire to the load.

I am not planning to make my own sensors and it is not whatever works for me but I just want to know if Sensor manufacturers make NPN Sensors either sinking and sourcing. It is my understanding that NPN -Sinking and PNP Sourcing is the most commun in North America?

HEY!!
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Old July 6th, 2004, 07:28 PM   #8
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One KISS way I finally came up with to get a 2nd shift maintenance supervisor to understand the difference between NPN & PNP (without confusing the hell out of his 4th grade education) was:


NPN
N Negative power in
P Positive grounding ("common" to the rest of us)
N Negative out provided to the PLC


PNP
P Positive power in
N Negative "ground"
P Positive out provided to the PLC

Unfortunately this only addressed one item with this supervisor.
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Old July 6th, 2004, 11:08 PM   #9
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npn-pnp

Greg,
I'm writing down your illustration for your supervisor and thanks, its great. I was kinda getting the impression from Don's first post that he may be getting confused that when using sinkings devices, sourcing input modules/cards are used and for sourcing devices, sinking modules/cards are used. Is that still the way things are done? Or have the manufacturers gone with naming them for the device type. Was it Mitsubishi that did/does that? Too many things that I forget when I don't use them everyday. Thanks again Greg, and btw, is that supervisors name Ab?

Bob
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Old July 7th, 2004, 03:48 AM   #10
REZAR
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Lightbulb frequently?

guyz, what is the frequently sensor u had used( standart), npn or pnp?
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Old July 7th, 2004, 04:01 AM   #11
Eric Nelson
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Re: frequently?

Quote:
Originally posted by REZAR
guyz, what is the frequently sensor u had used( standart), npn or pnp?
You will probably find THIS thread interesting reading, REZAR...

beerchug

-Eric
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Old July 7th, 2004, 04:24 AM   #12
REZAR
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Thumbs up

good info endeed!
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Old July 7th, 2004, 05:55 AM   #13
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For better or worse I always use sinking plc inputs and sourcing outputs. That way I only have to remember 2 things at once, which is about all my mind can correctly process. And I have much higher than a 4th grade education.
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Old July 7th, 2004, 08:33 AM   #14
Don_Dubé
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Sensors

Thanks all!

NPN = Sinking
PNP = Sourcing

And if I understand this correctly that is because sensor manufacturers always have the load connected on the Collecter side (connected to the input modules) to make sure that the transistor is fully saturated - a term that I haven't used and heard in years!! but always good to review - Thanks Panic Mode! I guess what confused me is that Terry uses the Emitter has the load but this is just an examples.

This is a basic questions but how do I use a transistor in switching mode i.e. having it fully saturated. Not that I really need to know this now but...ya never know!

All I remember is: Ic = Ib * Gain. With this all memorized I was able to pass my 4th grade!

Don
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Old July 9th, 2004, 07:04 PM   #15
Terry Woods
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Don_Dube'... (picture the apostrophe after the "e" as an accent over the "e")

The purpose of my drawings was to show the relationships between field devices and the PLC input or output circuits, and the relative terms (Sinking or Sourcing) as they applied.

While it is true today that sensor manufacturers have settled on a particular configuration (NPN = Sinking and PNP = Sourcing), that wasn't always the case.

In the early days there was not such a variety of sensors as there is now. And manufacturers had not selected a "standard". Quite often a developer would have to design his own sensors. Some of us didn't wait for the sensor to be designed, produced, and distributed by a "legitimate vendor"; we couldn't wait... so we designed our own.

Almost all of today's sensors use principles that have been known since the 19th century. The only difference between then and now was the transistor.

The transistor can be used as an amplifier or a switch.

In terms of a "switch", all that a developer needed to do was design a sensor circuit that would provide enough current to "tickle" the input circuit for as long as the condition existed (or NOT existed... Normally Open vs. Normally Closed.).

Some PLC's provided Sinking Input circuits. Some PLC's provided Sourcing Input circuits. Some PLC's (as shown in my drawings) didn't care... Sinking or Sourcing was fine. However, it was usually the case (although not always) that all inputs on a particular card be configured as the same type - Sinking or Sourcing (this was before the "brick" type PLC). There was a time when inputs did not have a "common" bus.

So...

During that era, a developer might very well build his own sensors. All the developer needed to do was get some damned current flowing into the input circuit in the PLC.

The same is true today.

If you look at the design of any sensor you will see that anyone experienced in transistor-circuit design can reproduce the same effect using off-the-shelf components. Would that be a reasonable approach in these days? The answer is a resounding "NO!" Back then... the answer was, if the required sensor did not exist or was too expensive, a resounding "YES!"

There are transistor switching circuits that have "both" a Collector-Load AND an Emitter-Load. Since, in a transistor-circuit, I(C) = I(E) (practically speaking)... the PLC input circuit could represent the Collector-Load or the Emitter-Load.

The point is, while it is common practice (by vendor standard) to connect the COLLECTOR to the load (PLC input), it is NOT an electrical requirement!

"Efector" has a two-wire, AC/DC sensor that can be Sinking or Sourcing... just by switching the leads. Hmmm...?


Don_Dube'... (picture the apostrophe after the "e" as an accent over the "e")

You asked about "saturation" in a switching circuit. I'll respond to that later.
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