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Old July 10th, 2012, 02:02 AM   #1
Timeismoney08
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NPN and PNP

Can anyone explain the difference in NPN and PNP in a short answer?
i have watched many videos and read articles but i still dont understand completely, and how it has to do with on photo-eyes
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Old July 10th, 2012, 02:24 AM   #2
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The ab guys will hate my explanation...

Think of pnp as current sourcing. You can drive a small micro relay from a photo eye. Connect the a1 to the photo eye output and connect the a2 to your 0v. So it provides the power (current sources).

Think of npn as current sinking. From your new npn photoeye you connect the output of the photo eye to the a2 of your relay an connect the a1 to the 24v. So it provides the path to 0v (current sinks).
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Old July 10th, 2012, 02:30 AM   #3
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Ok great, that's what I was thinking but none of the videos explained it in a real world situation. Thanks
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Old July 10th, 2012, 02:55 AM   #4
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Be careful as different manufacturers describe this different ways - AB for example.
I look at PNP as positive switching and NPN as zero switching (or near to zero). From my experience that is the most common explanation but read above.
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Old July 10th, 2012, 05:44 AM   #5
DamianInRochester
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Forget the terms sinking and sourcing. They have been butchered to the point of being arbitrary.

Maybe an easier way to look at it as Bob mentions

N for Negative Logic and P for Positive logic.

The key is to look at what the field device gives you in the ON state. Think of it as a wall switch in your house.

An NPN wall switch would give you common (connects you to white)
A PNP wall switch would give you HOT (connects you to the black)
In either case, when the switch is off you have no current path.

So the in the NPN analogy of wiring a 115vac light bulb, you would bring 115vac to the bulb all the time, and switch it the common.

With the PNP analogy, it would be just like it is normally done. Common is brought to the bulb all the time, and you switch 115vac to it instead.



Applying the same reasoning to a DC photoeye

NPN ON:0V
NPN OFF: N/C

PNP ON: +V
PNP OFF: N/C
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Old July 10th, 2012, 06:43 AM   #6
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If I may digress; I can picture a poor apprentice (me 8 years ago) with a dark on NPN photoeye whilst detecting an object. 22.4VDC on the output it must be PNP, i'll get one overnighted from somewhere! Darn, why doesn't that new sensor work! **** it was NPN. Dark on Light on WTF? lol good times.
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Old July 10th, 2012, 07:17 AM   #7
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Quote:
Originally Posted by bobb View Post
be careful as different manufacturers describe this different ways - ab for example.
I look at pnp as positive switching and npn as zero switching (or near to zero). From my experience that is the most common explanation but read above.
+1
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Old July 10th, 2012, 08:21 AM   #8
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Have you read this one?

http://www.patchn.com/index.php?opti...plc&Itemid=118
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Old July 10th, 2012, 08:29 AM   #9
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Quote:
Originally Posted by RussB View Post
Quote:
Originally Posted by BobB View Post
Be careful as different manufacturers describe this different ways - AB for example.
I look at PNP as positive switching and NPN as zero switching (or near to zero). From my experience that is the most common explanation but read above.
+1
+2

Exactly the way I think of it. "Sink" and "Source" are virtually meaningless any more, especially if you deal with multiple manufacturers' products.
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Old July 10th, 2012, 08:33 AM   #10
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Quote:
Originally Posted by HJTRBO View Post
If I may digress; I can picture a poor apprentice (me 8 years ago) with a dark on NPN photoeye whilst detecting an object. 22.4VDC on the output it must be PNP, i'll get one overnighted from somewhere! Darn, why doesn't that new sensor work! **** it was NPN. Dark on Light on WTF? lol good times.
BTDT. Well, almost. Trying to figure out why an input wasn't behaving the way I thought it should. It had 24V on the input terminal, but it was off. The "load" wire on the sensor also had 24V. Of course, I finally (after a VERY long time) noticed the big letters on the front of the SLC input module, "SOURCING", and realized that something was up. First time I had so much as heard of such a thing and it was late at night with a "hot" machine down. Only machine in the building with that arrangement. Remember it like it was yesterday. Lessons learned the hard way tend to stick around a while.
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Old July 10th, 2012, 09:40 AM   #11
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Quote:
Can anyone explain the difference in NPN and PNP in a short answer?
NPN and PNP refer to two types of transistors - the way the silicon wafers are arranged creates the two different types. Both types are bipolar-junction transistors. The N silicon layers are positively doped and the P layers are negatively doped.

NPN: is a bipolar junction transistor in which a P-type semiconductor is sandwitched between two N-type semiconductors. The electron current-flow is from the transistor Emitter to Collector (via Base in common- base transistor). Used as a Source, the NPN Emitter will be positive.

PNP: is a transistor in which the N layer is sandwiched between the two layers of P-type semiconductor. The anti-electron current flow is of electron "holes" from Emitter to Collector (via Base in case of common base transistor). Thus the PNP current flow is negative with respect to the positive NPN current flow. Used as a Source, the PNP Emitter will be negative.

All other references to NPN and PNP are loosely based on the transistor types used. Some PLC manufactureres try to define their PLC inputs and outputs in terms of the type of transistor used, but to be meaninful, the location of the transistor (Source or Load) must be specified.

Last edited by Lancie1; July 10th, 2012 at 10:02 AM.
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Old July 10th, 2012, 11:49 AM   #12
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the two posts which start at the link below have helped many technicians who just want to know "how to hook it up" and don't really care about the internal workings of the devices ...

http://www.plctalk.net/qanda/showthr...08&postcount=4

in a nutshell suitable for most sensor applications:

a Pnp device "switches Positive" ...

an Npn device "switches Negative" ...
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Old July 10th, 2012, 01:01 PM   #13
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yea when we get to talking about the internal workings is when i get lost. I've found this picture to help me the most.
but I thought it was bad to have a switch on the 0v side of the load
and I've only seen pnp/npn on proximity switches and photo eyes, guess i never noticed on an wall outlet.
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Old July 10th, 2012, 01:06 PM   #14
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Notice that this is really not about "NPN and PNP". Those are properly transistor types. What you are talking about are types of Sources and Loads. A Source does not have to be a transistor, and a Load does not have to be a transistor.

If you start thinking that the transistor type translates as the Source or Load type, then when you encounter a transformer Source or a relay Load, you are going to be screwed.
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Old July 10th, 2012, 01:28 PM   #15
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Quote:
yea when we get to talking about the internal workings is when i get lost.


many people do ...

Quote:
I've found this picture to help me the most.


many people do ...

Quote:
but I thought it was bad to have a switch on the 0v side of the load


many people would agree with you – because if the "switch side" of the load gets "grounded" somehow (by a chafed wire, etc.) then the load would be energized ...

but ...

in DC sensor circuits, finding the "switch" (the sensor) on the 0v side of the load is a very common occurrence ... for better or for worse, many DC sensor circuits are wired that way ...

Quote:
and I've only seen pnp/npn on proximity switches and photo eyes, guess i never noticed on an wall outlet.


I'm losing you on that one ... when we talk about NPN and PNP we're NOT talking about wall receptacles, etc. ... the posts that I linked to are ONLY concerned with DC circuits (like sensors and photo eyes) – NOT with AC wall receptacles, etc. ...
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