PNP or NPN output circuits

Could anyone tell me which is the better option or most widely used in Europe regarding Transistor output cards? I have used both in many of my control systems but I want to know if there is a more preferred one used in Europe. I have some machines from Japan which are NPN switching but I have a new one which is PNP switching

Everything I've seen here in my 26 year carreer is PNP, except for the few machines that were acquired in the States. But don't take my word for it because I've only been employed in two companies in those years.

Kind regards,

Thanks - but do you know of any standard or regulation such as needed for CE marking that makes the choice of PNP rather than NPN

I prefer PNP because a ground fault will not cause an unwanted input. its just my personal preferance and ive seen examples of both on machines made in many different countries

My observations:

1. i believe CE requires that at least one leg of a supply be grounded.

2. In a DC supply most choose to ground the most negative lead.

3. To avoid accidental ground short induced movement of an output the output switching device should be in the ungrounded lead.

4. Therefor a PNP type switch is most appropriate.

What Dave and Bernie said. Check out this link:

http://www.mrplc.com/kb/index.php?page=index_v2&id=44&c=38

Interestingly, I just received a spec from a new customer (Japanese-based automotive subcontractor), and they require NPN devices. Hmmm.

Paula

It depends on what might happen when a wire shorts to ground.
Will your input or device turn on and cause something unexpected to happen?
You want to wire in such a way as to avoid that.
For that reason, the wiring, fusing and grounding of the power supply involved must be considered as well.

EDIT: Oops, I see Bernie covered all that...

Shortening to ground occurs more often than shortening to +24. See bernie_carlton post - he said all. For example, most Siemens IO modules have PNP logic.

In what I have seen, Japan uses NPN. We have bought several pieces of equipment and various robots from Japanese companies and have had to specify PNP because their standard is NPN.

Steve

thanks

Thanks for all the info.- It has helped me loads. I'm going to spec. PNP from now on. thanks

The ONLY time I use NPN is:
1.) To drive a TTL output that requires more current. You can sink more than you can source from a transistor.
2.) AND it has to be within the same enclosure. (ie. the TTL output card and device being driven are in the same box. NO conduit or cabling between enclosures (boxes).

Otherwise I ALWAYS use PNP. Especially for inputs.

Oh ... and by the way ... EXCELLENT article Paula! I've bookmarked the site and printed the article to give to my maintenance techs.
- Tony

So.

Why so many companies in US and Japan standardize on NPN? There must be some reason, mustn't it?

The answer "because they are ignorant" doesn't count. For obvious reasons.

davefinic said:

I prefer PNP because a ground fault will not cause an unwanted input. its just my personal preferance and ive seen examples of both on machines made in many different countries

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I'm really new to this. Can you please explain how ground fault could cause unwanted input? PNP input is as far as I know so called sourcing input but don't really understand what ground fault means. Does that mean thaat PNP input is by fault connected to ground or...?

All depends to how you define active and passive state in your program. If input grounding means passive (disabling) state nothing bad happens. For PNP input this is like wire break (occurs even more often than grounding). But for NPN if input grounding means passive state in program, wire break will mean the active state! What will occur after this - mostly god and good programmer (but not in all cases) know. PNP has an the advantage: both fault situation (wire break and grounding) can be programmed as disabling signal level. But for NPN this cann't be achieved.

But main disadvantage of NPN is not input, but output. If output wire grounding fault occurs, PLC cann't do nothing with attached actuator - it will be activated in case of NPN output. Shortening to +24V occurs rarely. So, PNP output is more safety than NPN.

I don't know why Japans define NPN as a standard. May be they use another power grounding schemes than Europe. If they use the same schemes - I cann't understand this.

I prefer PNP because a ground fault will not cause an unwanted input.

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The question was about outputs not inputs but I agree with you about inputs being positive switch wholeheartedly.

Interestingly, I just received a spec from a new customer (Japanese-based automotive subcontractor), and they require NPN devices. Hmmm.

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Very common in Japan for some reason - however I must admit I normally use NPN outputs and do not ground the 0 volt leg of the switchmode power supplies I use for supplying DC. Have NEVER had a problem with NPN outputs and ungrounded DC power supply.

grounding of the power supply involved must be considered as well.

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Why? I have already stated I have NEVER had a problem with ungrouded 24VDC, PNP inputs and NPN outputs. Ungrounded DC power supplies are quite common in Ozz - as are ungrouded isolation type 240/24VAC transformer power supplies. Never ground them either.

In fact if the 24VDC power supply is not grounded, an NPN input is unlikely to turn on when grounded in the field as there is no common connection to ground from the power supply.

I still use PNP inputs in the unlikely event that one MAY turn on.

Why so many companies in US and Japan standardize on NPN? There must be some reason, mustn't it?

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One thing you may like to consider is that in "the old days" PNP transistors used to be at least twice the price of NPN transistors. Yes, I am going back quite a way. Old habits die hard?????