NPN and PNP



many people do ...



many people do ...



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 ...



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. ...






Sorry I meant to say wall switches. someone made a reference to it.
 
To digress a little it is my understanding that many moons ago NPN transistors were a lot cheaper than PNP transistors and that is why the Japanese used NPN so much - it is now pretty much a standard in Japan.
 
Another way of thinking about it is:

NPN = True Low (ON = low voltage)
PNP = True High (ON = High Voltage)

I guess nobody gets taught transistor theory these days.

They do, it just gets lost becuase the little buggers head off to the pub after night school and get trashed on $2 pot thursdays
 
Sorry I meant to say wall switches. someone made a reference to it.
Yes, a switch is a perfectly valid Source. Source, Sink, and Loads are really what you are asking about, even though you think your question only involves NPN and PNP transistors.
 
Maybe i need some education too? I think 9 and 10 are wrong?
What is wrong with Circuits 9 and 10? Many people do not reallize that there is a sinking device and a sourcing device in the same circuit. That problem is why it is difficult to decribe PLC Inputs and Outputs as Sinking or Sourcing. First, you have to make clear WHICH PART of the circuit you are talking about. Most descrptions do not and are therfore worthless.
 
Just seeing the way the transisors were shown threw me for a bit. E.g pnp collector wired to the 0v.
 
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Opposite but the same for circuit 10, a pnp device used as the sinking part. Pnp collector wired to 0V?
Why would you think that. In circuit 10, study it carefully. The + voltage is on the right side, goes through the PLC input, then to the emitter. Current flow is from right to left. I remind you of the old admonition: Do not AssUMe that you know anything for sure. On the other hand, the -Vs is wired to ground, so with reference to ground, it is indeed 0 volts, but if the power supply is say +/- 6 volts, the the real voltage at the NPN Collector is about -5.4 VDC relative to the voltage at NPN Emitter.

It is so unfortunate that technical people are being taught that if a circuit is grounded, then the voltage at all grounded points has to be "0"! It really depends on what you are measuring. Voltage relative to WHAT? yes if you measure from an earth ground to that point, of course it will be 0, but that only tells you the voltage to ground. What is the voltage relative to +Vs? It is -6 of course.
 
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It is so unfortunate that technical people are being taught that if a circuit is grounded, then the voltage at all grounded points has to be "0"! It really depends on what you are measuring. Voltage relative to WHAT? yes if you measure from an earth ground to that point, of course it will be 0, but that only tells you the voltage to ground. What is the voltage relative to +Vs? It is -6 of course.

Lol assume is correct. My bad. I was thinking the typical industrial 24vdc supply configuration. Anyhow another little lesson...
 

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