NPN and PNP Is one safer?

[JeremyAdair87]

Thank you everyone

The reason we are considering switching standards in the plant are due to most of the plant being shifted offshore and a new high tech product is replacing them. All of the old machines are SLCs and old panel views. We also only use two maintenance men per shift. It is a modular assembly plant for belt tensioners. You learn one machine and you know the whole plant.

I have been keeping the plant standard but there is an opportunity to change.

Also, thank you. There was a guy that said, for 24v common you have to use the hot for your common when trouble shooting. It is obvious now but I was scratching my head on how to make sense of it while still using 0v.

Also there was another guy who said, they use sinking inputs and outputs.... You can't sink an output to a sink input. Can you? I don't think so.

 

[JesperMP]

"Sinking" and "sourcing" is a water analogy.
So no, there can be no sinking without sourcing, and vice-versa.

 

[JerryH]

It is really driven for safety reasons depending on if the control voltage is positive or negative grounded.

For a Positive grounded system you should use Sourcing Input Cards and Sinking Output Cards.

For a Negative grounded system you should use Sinking Input Cards and Sourcing Output Cards.

This is done for safety reasons to prevent any unexpected inputs from energizing if an input wired is shorted to ground and prevents any output device from energizing if an output wire is shorted to ground.

I suppose if your DC supply is floating you could go either way.

 

[JerryH]

I would look at what is in the plant currently and stick with that philosophy !

james

As I work in a plant that for the first 15 yrs we received equipment that was setup as Positive ground all of our input devices were sinking (using input cards that source) and our output devices were sourcing (using output cards that sink).

Now with our new specification we require Negative grounded systems so all of our input devices now are sourcing (using input cards that sink) and our output devices are now sinking (using output cards that source).

So, besides the chance of installing the wrong device (and has happened many times) we must also stock all of our parts now both ways, for sinking and sourcing, which doubles our spare stock. This effects both the square footage required for our spare parts area and the cost associated with keeping both types of devices on hand.

Hope this makes sense, just trying to show some of the effects from changing the plant standard.

Jerry

 

[DwSoFt]

One thing to Mention... The RTUs that I deal with are only NPN. Although I do prefer PNP on my plcs I don't have a choice with the RTUs.

 

[LadderLogic]

I personally would encourage everyone to avoid using the terms 'sourcing' and 'sinking' altogether because they have become so confusing - thanks not in small part to Allen-Bradley.

It is either PNP (that is, positive voltage is 'on') or NPN (that is, negative voltage is 'on'). Cannot be clearer.

 

[Sergei Troizky]

I personally would encourage everyone to avoid using the terms 'sourcing' and 'sinking' altogether because they have become so confusing - thanks not in small part to Allen-Bradley.

It is either PNP (that is, positive voltage is 'on') or NPN (that is, negative voltage is 'on'). Cannot be clearer.

Agree with that, and always do so myself.
However, NPN is "not negative voltage is 'on'" but "zero voltage is 'on'" in typical industrial 24VDC control.

 

[Bit_Bucket_07]

I personally would encourage everyone to avoid using the terms 'sourcing' and 'sinking' altogether because they have become so confusing - thanks not in small part to Allen-Bradley.

It is either PNP (that is, positive voltage is 'on') or NPN (that is, negative voltage is 'on'). Cannot be clearer.

I find the use of "NPN & PNP" confusing, since these mnemonics refer to bipolar transistors. I was taught to remember these as "****ing in pot" and "not ****ing in", but that reminder came from the same instructor who had us remember trig functions as "Oscar Has A Hairy Old A-hole".

Anyway, an NPN transistor is "turned on" by applying a "high" voltage to it's base, while a PNP transistor is "turned on" by applying a "low" voltage to it's base.

 

[LadderLogic]

Agree with that, and always do so myself.
However, NPN is "not negative voltage is 'on'" but "zero voltage is 'on'" in typical industrial 24VDC control.

This is correct, but I wanted to make a more general statement since one of the DC power legs in not always grounded.

 

[LadderLogic]

Anyway, an NPN transistor is "turned on" by applying a "high" voltage to it's base, while a PNP transistor is "turned on" by applying a "low" voltage to it's base.

That is also absolutely correct. However, us, controls grunts, think of a transistor as if it was a diode that can be switched on or off by some external action So an NPN sensor has a "diode" inside that lets the current flow from a more negative to a more positive potential (from "zero" to "DC plus")when switched on - and a PNP sensor lets the current flow from "plus" to "zero".

The internal details really matter less. This is far from a strict engineering or scientific description but most people easily understand it.

 

[proof]

So an NPN sensor has a "diode" inside that lets the current flow from a more negative to a more positive potential (from "zero" to "DC plus")when switched on - and a PNP sensor lets the current flow from "plus" to "zero".

The internal details really matter less. This is far from a strict engineering or scientific description but most people easily understand it.

In an electrical circuit current flows from a more positive potential to a more negative potential. Electron flow is from a negative to a positive.

 

[Bit_Bucket_07]

In an electrical circuit current flows from a more positive potential to a more negative potential. Electron flow is from a negative to a positive.

Yes, electron current flow in a circuit is from negative to positive, but electronics schematics still indicate "classical" current flow, which is from positive to negative. Still, electron current does flow from a "negative potential" to a "positive potential". Hole current, meanwhile, flows in the opposite direction within a transistor.