NPN and PNP

The one thing that it seems like the confused among you are missing is that there is ALWAYS BOTH. In order for a circuit to work, there must be both a "sink" and there must be a "source". The "Source" is positive, and the "Sink" is negative. Which one is which will depend upon how it is wired. A "load" can be either, and a "switch" can be either. If the load is closer to the positive side, it is a source, if a switch is closer to the positive side, IT is the source!!
 
Harley, well said.

Now if we could only figure out Source, Load, and Sink have anything to do with NPN and PNP transistors. I say that they are only remotely related. NPN and PNP transistors can be a type of Load, or Sink, or they can be a type of Source.
 
I keep it simple in my simple little mind...

PNP = Positive
NPN = Negative

This is of course when speaking of sensor output only.
 
The confusion stems from the digital I/O types.
There are 4 possibilities:

Sinking digital output
Sourcing digital output
Sourcing digital input
Sinking digital input

A sourcing digital I/O provides the voltage needed for
the circuit.

A sinking digital I/O provides the ground needed for
the circuit.

PNP transistors can be used to source or sink current.
NPN transistors can be used to source or sink current.
Because you need both a voltage source and a ground in
order to create a complete circuit, you need to have a
sourcing input or output connected to a sinking output
or input. If you wish to connect a sourcing input to a
sourcing output, or a sinking input to a sinking output,
you would need to add an additional pull up or pull
down resistor.

_NPNPNP_.jpg
 
Argh thats the drawing that wasn't in my instrument book. Wish i'd seen that before opening my big fat mouth. A have added a new page.
 
PNP = Positive
NPN = Negative

This is of course when speaking of sensor output only.
All that is approximately 50% true. Look at JRB's circuits. There are two possibilites for each type of DC bipolar transistor. Then there are other types of outputs, such as triacs (AC-switching transistors, FETs, and plain old coil relays.

It is a big waste of time trying to find simple rule-of-thumb rules that work in all cases. This is the real world, not some imaginary world where everthing works out simple and easy.
 
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folks, this is all well and good – but we're getting further and further from what MOST "PLC" people are interested in when they ask about "sinking" and "sourcing" ... I know this because my students ask about "sinking" and "sourcing" in every class that I teach ...

from a PLC standpoint, here's what they're invariably interested in:

suppose I have a SINKING input module ... do I need a SINKING sensor (to match sinking and sinking) – or do I need a SOURCING sensor instead? ...

or on the other hand ...

suppose I have a SOURCING input module ... do I need a SOURCING sensor (to match sourcing and sourcing) – or do I need a SINKING sensor instead? ...

again, from a PLC standpoint, the quick and easy answer is this:

one device has to be SINKING – the other device has to be SOURCING ...

so in simplest terms the devices must "complement" each other – they must NOT "match" each other ...

now then ... we could say "Amen" and go home ... but somehow this topic is getting much more convoluted than it needs to be ... (nothing wrong with more knowledge – it's just that we're sort of confusing the beginners who have a perfectly valid "simple" question) ...

below is a figure adapted from an Allen-Bradley sensor catalog - page 8-87 (Adobe page 92) at the following link:

http://literature.rockwellautomation.com/idc/groups/literature/documents/ca/c116-ca502_-en-p.pdf

continued in next post ...

.

sink_source.PNG
 
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the fact of the matter is that YES, we can hook up PNP and NPN transistors in MANY different ways and get them to work ...

BUT ...

most technicians reading this thread are not dealing with TRANSISTORS ... instead they're dealing with DC SENSORS and some brands/models of sensors might be marked:

(1) "PNP" or "NPN" ...

while other brands/models of sensors might be marked:

(2) "SINKING" or "SOURCING" ...

the figure below shows some information culled from the manual for the 1746-IV16 and 1746-IB16 DC input modules (used as examples) ... here's a link for anyone interested:

http://literature.rockwellautomation.com/idc/groups/literature/documents/in/1746-in027_-en-p.pdf

the information is taken from pages 21 and 22 (Adobe pages 21 and 22) ...

so once again, YES, you certainly can hook up PNP and NPN transistors in MANY different ways and get them to work – as long as the LOADS they're driving are ISOLATED ...

BUT ... (and here's the crux of the matter) ...

from a PLC technician's point of view, in MANY cases the LOADS are actually PLC INPUT MODULES – and in MANY cases those modules do NOT (I repeat) do NOT have isolated circuits ...

SO ...

to someone who doesn't already understand the concepts involved, it sort-of-kind-of sounds like some of you guys are saying that you can hook up "any old" type of sensor to "any old" type of module ... those of us who've been around for a while know that is definitely NOT the case – but the beginners who are reading this thread could easily come away with the WRONG conclusions ...

just to show how "perplexing" this simple topic can become, here's a story from "once upon a time" when I worked as a tech rep for an Allen-Bradley distributor ...

at one point in time the company computers were set up so that whenever a customer would order either one of these SINKING or SOURCING type input modules, the computer wouldn't allow the sales clerk to complete the order – until Gary or Kit or I got involved ... basically we had to call the customer and make SURE that he was ordering the type of module that he really needed ... this is one of those situations (life is full of them) where you have a 50-50 chance of getting it right – but it comes out WRONG at least 90% of the time ... the main idea was that when the wrong-type module that the customer had specified wouldn't work – they'd want to return it ... we couldn't sell it again – the seal had been broken and wires had been connected to it ... that's the reason for the extra "check and make sure" this is REALLY what the customer needs to work correctly with his other components ...

party on ...

.

sink_source_modules.jpg
 
most technicians reading this thread are not dealing with TRANSISTORS ... instead they're dealing with DC SENSORS and some brands/models of sensors might be marked:
Exactly. The problem is that the original question was about NPN and PNP, with NO mention of Sinking or Sourcing. To answer the actual question asked, you would have to talk ONLY about thse types of transistors. The level of ignorance is so great that there is no common ground to build on.

to someone who doesn't already understand the concepts involved, it sort-of-kind-of sounds like some of you guys are saying that you can hook up "any old" type of sensor to "any old" type of module
No, I am saying exactly the opposite, and also there is my idea that these things are not as simple as the average technician has been lead to believe.
 
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I think what has gotten lost in all of this is the concept of "open collector" and WHY PNP transistors are predominantly used in positive logic and NPN are used predominantly in negative logic. And that has to do with the gate logic levels, noise immunity, and interfacing different logic levels together without modifiying the inside circuitry.
 
Sinking and Soucing are NOT all about NPN and PNP!

What has been lost is the concept of Sinking and Sourcing. Transistors are only part of it. I saved this post from Terry Woods way back in 2002. Notice that NPN and PNP do not even show up at all in Terry's great picture.

Sinking-Sourcing 2002 Terry Woods.JPG
 
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What has been lost is the concept of Sinking and Sourcing. Transistors are only part of it. I saved this post from Terry Woods way back in 2002. Notice that NPN and PNP do not even show up at all in Terry's great picture.


I disagree Lancie. Look at the title of the post. It is NPN and PNP, not Sinking & Sourcing. Read post #1. He never once mentioned the word Sinking or Sourcing!


Why an NPN transistor is normally used for "sinking" to low and a PNP transistor is normally used for "sourcing" from high is because interface circuitry is normally done in "open collector" fashion. That is what it has to do with photoeyes, since the transistor output of a photoeye will normally be "Open Collector".

It was everyone else that started going crazy with the sinking/sourcing debate.
 
Look at the title of the post. It is NPN and PNP, not Sinking & Sourcing. Read post #1. He never once mentioned the word Sinking or Sourcing!
Yes, that is correct. If you notice my first explanation in Post #11, it does NOT mention Sinking or Sourcing either! It was some others that brought that into the discussion, thinking correctly that was the REAL subject. The question was so poorly and ignorantly worded. It assumed that "NPN and PNP" are the ONLY methods needed or used for the other topic.
 
As far as how NPN and PNP relates to sinking and sourcing in devices used with PLC's, I like this description best.
The big problem with that description is that it is biased toward the one that Numatics likes. It is for a particular type of transistor usage, defining Sinking and Sourcing in terms of NPN and PNP. For me that is just silly. Others agree:
However, we must be cognizant that not all manufacturers follow the same definition of the relationship between Sinking and Sourcing I/O devices. The following excerpt is from Allen Bradley’s Discrete Input and Output Modules manual Pt. #1746-2.35.


“Sinking/Sourcing describes a current signal flow relationship between field input and output devices in a control system and their power supply.

Sourcing I/O modules supply (or source) current to sinking field devices. Sinking I/O modules receive (or sink) current from sourcing field devices.”
 
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