NPN or PNP

What is the most used input NPN or PNP for a PLC in the US?

TIA
John

For stuff originally built in the US, I would guess that PNP is more common (or at least >50%). From my personal experience, Europe seems to favor PNP as well, but when you move east (Japan, China, etc.), NPN dominates. Why? I dunno...

Why were old Volkswagen electrical systems common positive? Just to be different? There's probably some reasoning behind this... Any trivia buffs have an answer?

My stuff? PNP unless the customer specifies NPN

beerchug

-Eric

I can appreciate the simple nature of the question... "Which is the most used...".

But, it really doesn't matter. Both exist, and both need to be understood. Sometimes you find yourself in the situation where, while you might usually use type-A, you find that you only have type-B on hand.

The differences ain't that big a deal if you know the basic differences.

Go ahead, ask me!

Not Pointing IN

Terry Woods said:

Sometimes you find yourself in the situation where, while you might usually use type-A, you find that you only have type-B on hand.

Click to expand...

Which is exactly why I make sure to keep a stock of 4.7Kohm resistors on hand at all times!

-Eric

I kinda prefer PNP as a short will not put a signal into the input card but blow the fuse...


Thanks for the input...

John

Ah yes, but only if the negative side of the power supply is common to ground!

Remember, everything is relative!

beerchug

-Eric

Don't you normally connect the negative from the power supply to the ground buss?

When would you not do so?

TIA

Eric Nelson said:

Ah yes, but only if the negative side of the power supply is common to ground!

Remember, everything is relative!

beerchug

-Eric

Click to expand...

History Lesson

As I understand it, the first transistors developewd were NPN, by Bell Labs, and became the de facto standard in the US for years. When PNP came along, Europe realized the value of closing the high-side of the circuit instead of the low-side, but in the US, we stubbornly clung to the "good ol' American way".

I cannot think of this without recalling the fiasco of introducing the Metric system into US schools...altogether a superior method of measuring to Imperial Standard...but I actually knew a guy who thought it was a conspiracy to make him by twice as many tools to work on his car...


TM

Originally posted by jthornton
Don't you normally connect the negative from the power supply to the ground buss?

When would you not do so?

TIA

Click to expand...

When the customer requires that all control circuits are floating. I currently have a project in-house with this is a requirement. The only things tied to the ground buss are ground wires and shields.

Steve

Steve Etter said:

When the customer requires that all control circuits are floating. I currently have a project in-house with this is a requirement. The only things tied to the ground buss are ground wires and shields.

Steve

Click to expand...

I hope I don't ever have to put my hands in that control panel. What's their motivation for wanting to let the commons float around like that?

AK

To prevent the opportunity for noise to enter via a ground loop, I assume. They weren't particularly forthcoming about all the why's and wherefor's in their spec's. But they are insistent.

Steve

Try this one...

http://www.plcs.net/dcforum/DCForumID1/2645.html

In Figure-15, you can see the NPN and PNP field devices providing an input signal to the PLC.

If the field device shorts internally (from Emitter to Collector, either NPN or PNP), any fuse installed on the source or load side of the device will NOT blow!

This is because, in normal operation, when a solid-state switch turns ON, for all practical purposes, it IS a SHORT ! ("Short" does not necessarily mean Short-to-Ground!). In that case, the PLC input circuit provides the appropriate load to limit the current and that prevents the fuse from blowing - the PLC will always see the input as ON.

In a grounded system...
If the input to, or the output from, the field device is shorted to ground, then the fuse will blow and the PLC will never see an input signal - whether the device is NPN or PNP.

In an ungrounded system...
If the input to the field device is shorted to ground, then the fuse might blow. If it doesn't, then the PLC might see an input signal. It depends on the nature of the "short-to-ground".

In an ungrounded system...
If the output from the field device is shorted to ground, then the fuse might blow. If it doesn't, then the PLC might see an input signal. It depends on the nature of the "short-to-ground".

Statistically, I have to say that the ungrounded method does increase the reliability of the signal.

However, you will not know that you have a problem. Not to mention the grief involved in troubleshooting an ungrounded system.

Thats some good info

Thanks