One more post about NPN vs PNP

So, I will readily admit that I'm one of those dirty young "Electrical Engineers" that really are just programmers trying to absorb electrical engineering on the job. One of the things that I'm having a hard time wrapping my head around is why PNP vs NPN exists. It seems like our plant has problems fairly regularly with people getting the wrong type of device, and even the EEs that went to school for it seem to just get ****ed trying to talk about the two types.

To me, it seems like both types are doing the same job in a way that doesn't make much of a difference. I understand PHYSICALLY there is a difference because the flow direction is reversed, but I don't understand the practical reason for the two different options to be necessary, beyond the fact that there are sinking AND sourcing inputs and outputs.

Does anyone want to take a crack at explaining why they are both necessary? Is this just a case of two different standards brought together by globalization and enough people using both that both standards stuck? Or are there practical advantages to each type?

I'm pretty sure . ..

NPN versus PNP hark back to 'bipolar transistor' days.

And that had to do with transistor biasing - PNP was a shorthand for 'Positive to Negative to Positive' and if memory serves me correctly a PNP type transistor had to have the positive applied to the Collector. An NPN type transistor had to have a negative applied to its Collector.

(The 'Positive to Negative to Positive' had to do with the 'doping' - Pure silicon is an insulator, so they add one element to it to make it a 'P' type semiconductor then another element for the 'N' type base followed by some more 'P' type element to make the emitter.)

So, because the inputs use transistors and the outputs use transistors and, depending on the design, they might be PNP or they might be NPN one has to design the circuits and devices connecting to the inputs and outputs to match.

And, yes, there are advantages to using one or the other . . . or sometimes even both. An amplifier, for example, might want to amplify the positive and the negative side of the signal at the same time and to do that one would use both types in the circuit.

Or is it something else?
MadPoet.

Hopefully I have put an amplifier circuit here . . Yes.

http://www.plctalk.net/qanda/showthread.php?t=2648

This post is along the same line and provides some answers.

Regards,

Seems to be an Atlantic thing.

The machines we had from the states usually always had NPN, the machines from everywhere in the world, usually had PNP.

Who can say for sure what is the 'best', but why anyone would use NPN in modern times is beyond me.

I don't know if this is true as I don't work in electronics manufacturing, but a teacher of mine said a lot of it was due to component cost in circuit designs.

If you are making one unit who cares, but if you are distributing on a large scale, the pennies start to add up.

I don't think this would apply today, but perhaps to earlier designs.

NPN transistors were much cheaper than PNP in the early days - PNP were about 4-6 times the price so NPN was popular. Forget about NPN and PNP - have a look at the common and get the right device. PNP switches high (+ volts) NPN switches low (0 volts). Therefore the common for PNP in a PLC is 0 volts and the common for NPN in a PLC is + volts. The PLC manufacturers have not helped - having NPN and PNP inputs listed has added to the confusion.

the big thing to look at is the wiring diagram of the circuit, what the sensor is being wired into.

another thing that get you confused is this.
an ab 1746-ib16 is defined as a sinking input card, meaning you provide power to the input so it can sink it to ground. as pointes out, that would be a pnp device.

on the flip side, if my memory is correct, ge has a dc sinking input card. but they require a sinking device or a npn transistor, so their card is providing power.

we use pnp type sensors that provide power to the input because it requires less power.

the same is true for the outputs.
if I am incorrect, someone please correct me.

james

Why are there still so many pnp or npn sensors? In a normal plc Environment (0V/24V) it would be the best if the sensor output has eighter +24 or 0V, instead of a floating state. Is it only a historical reason, or are there some real benefits?

In one company i worked some years ago, we had machines from different manufactures. So we had 4 different inductive sensors:
- NPN normal open
- NPN normal closed
- PNP normal open
- PNP normal closed
One type of sensor (active high and active low, with a normal open and a normal closed output) could have been a replacement for all 4 types.

They make a proxy with 4 wires that can be any combination by different connections of the wires.
I always have a couple in the car.

The most common fault in electrical circuits is where a wire touches down to earth, using the wrong type NPN or PNP increases the likelihood of a dangerous condition. The top two drawings show what I would consider to be the correct use of PNP and NPN, the bottom two show what I would consider to be the incorrect use.

One thing I saw in the posts:

Using NPN means there is NO voltage applied to devices therefore the less likelihood of any form of shock

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which I am reluctant to comment on because the man who wrote it is no longer around to defend his words. But it is nonsense. If you look at the circuit in box 3, if anyone touches wire 2 they will still have almost 24V contact. There will be a slight reduction because the relay coil will have resistance, but compared with a human it will be a low resistance. The voltage is shared in proportion to the resistance, so there will still be almost 24V there.

Of course there is the argument that you shouldn't ground the supply, but that is an argument I have had elsewhere and is a bad reason not to earth the supply.

crawler009 said:

Why are there still so many pnp or npn sensors?

Click to expand...

Because people are used to do things a certain way and are not willing to change without some really good reason. Yes, Japan and US has been mostly NPN historically, Europe being PNP. When there is a large archive of the past projects, it is not easy to radically change.

LadderLogic said:

Because people are used to do things a certain way and are not willing to change without some really good reason. Yes, Japan and US has been mostly NPN historically, Europe being PNP. When there is a large archive of the past projects, it is not easy to radically change.

Click to expand...

I'm actually a bit thrown by this idea that the US has strong historical preference for NPN devices. I've been in the game for for over twenty-five years and I never remember seeing an AutoMate, Modicon, or PLC-2 with NPN cards. The only time I see them on ControlLogix (or even Mitsubishi or Fuji) is if they're on equipment from Asia. There's two AB distributors within thirty miles of me, I checked their local stock and neither have NPN cards in stock, both have a variety of PNP cards.
When the Yaskawa Sigma-V servopaks came out in America they were NPN only and got such a backlash they were quickly changed to accept either.

I remember this one from a long time ago: what is the difference if safety is involved.

http://www.omron.com.au/service_support/FAQ/FAQ02285/index.asp

AutoMax said:

The only time I see them on ControlLogix

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Or the ubiquitous Slick-Five-Hundred. Or a GE (formerly GE-Fanuc). Or... I have seen a lot of them.