NPN or PNP

Hi

In designing a new (start from scratch) assembly machine, is there an established or normally accepted protocol as to whether inputs and outputs should be PNP or NPN. We have seen both on various types of machines and were wondering if this choice is a matter of preference for the system dsigner or are there some "rules" that normally apply. I understand that some functions and systems might dictate the choice, but generally speaking, what are the preferences of some of the more experienced designers in this group.

Thank you for your help. It is appreciated.

Clark

who is your customer? do they care?

we have several customers that require NPN so we do a lot of systems that are NPN (much more than average) based but since this is rather exotic on this side of pond, that makes only some 20% of all the projects. in western world PLC controls are PNP, you get more products and cheaper if you stick to it (unless this is something you want to ship to Japan or so).

Do a search in this Forum for "PNP or NPN" and you will find all the information you need

We always try to use PNP (sourcing) outputs and NPN (sinking) inputs on our equipment. This has sort of become our company standard; since we're building equipment for our own use we can do whatever we want. I guess it just makes more sense to me that an activated switch, sensor, or PLC output will provide a positive signal rather than a path to common. That's just my preference of course.

panic mode said:

who is your customer? do they care?

we have several customers that require NPN so we do a lot of systems that are NPN (much more than average) based but since this is rather exotic on this side of pond, that makes only some 20% of all the projects. in western world PLC controls are PNP, you get more products and cheaper if you stick to it (unless this is something you want to ship to Japan or so).

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This has been the trend I have seen also. Unless it is made in Japan it is PNP sensors. Even European machines tend toward PNP more so than NPN.

Clay B. said:

This has been the trend I have seen also. Unless it is made in Japan it is PNP sensors. Even European machines tend toward PNP more so than NPN.

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I agree with this. I have worked for a Japanese company in the past and they always used NPN inputs. The biggest problem that I saw in the field was that, statistically, if feild wiring is going to fail, it will short to ground (Conduit) as opposed to shorting to power. If you use NPN inputs then the input will energize in the event of a short to ground and you could start conveyors, pumps etc.

If you utilized PNP inputs and the field wiring shorted to ground, the fuse would blow (assuming proper design) and the input would never energize because electricity will follow the path of least resistance to ground and bypass the input.

Of course, there are exceptions and it is possible that the field wiring could short to power, but it is more likely that a short will be to ground. With that said, NPN inputs do have their place for certain applications... Encoder inputs in some applications, but as a rule, I use PNP inputs.

Regards

Here is a good article that I passed to my company and we now spec PNP equipment.
http://www.mrplc.com/kb/index.php?page=index_v2&id=44&c=38

Bob

Bob, thanks for that link, there is a lot of good safety and design information in that link.

Sorry to step on your thread but we have a filler that has npn sensors for the door saftys,which I plan on changing. What I am wondering is, is their ANY reasons why you would use npn if you had a choice?
Thanks
Tim

tjoneill said:

What I am wondering is, is their ANY reasons why you would use npn if you had a choice?

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Yes. A good reason to use NPN sensors is to avoid confusion with lots of other equipment in your place with lots of NPN sensors on it. I am yet to see an NPN and a PNP sensor form the same manufacturer that would be visually distinct in some way, save for some obscure characters on the part number. The potential trouble of someone plugging a PNP prox where an NPN one is expected and vice versa - and then spending time troubleshooting this - is not worth the trouble of switching from one to another.

I think the reason NPN sensors are still so popular in Japan and US is that some time ago (15, maybe 20 years) they were cheaper and more readily available than PNP ones. It is different now but... well, look at all those people sticking to inches and pounds. Despite the obvious advantages of the metric system, long-time school indoctrination in it, automotive and electronic industries' standards and so on. Habits die hard, especially the bad ones

bob1371 said:

Here is a good article that I passed to my company and we now spec PNP equipment.
http://www.mrplc.com/kb/index.php?page=index_v2&id=44&c=38

Bob

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The link that Bob posted makes the point on safety issues

The main issue with safety using PNP or NPN relates realy to whether you earth the positive lead or negetive lead

The norm in industry now days would be to use the PNP and you will find the Negetive is earthed.

Now if the Positive is earthed which is not normal except maybe in cars then the reverse is safer namely NPN.

So in Bobs case a company policy states PNP,
I also hope it states,
ONLY EARTH THE NEGATIVE.

kolyur said:

We always try to use PNP (sourcing) outputs and NPN (sinking) inputs on our equipment. This has sort of become our company standard; since we're building equipment for our own use we can do whatever we want. I guess it just makes more sense to me that an activated switch, sensor, or PLC output will provide a positive signal rather than a path to common. That's just my preference of course.

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I agree wholeheartedly with the above statement.

PNP is easier to troubleshoot, back side of your switch to any metal part of machine will tell you if its working.

Course if you eath the active not the neutral it will not.

If you have mostly NPN on site use NPN