# PNP or NPN by sight alone?



## MDShunk (Jan 7, 2007)

Is there any rule of thumb (wire tail colors?) to tell whether a sensor that is to be replaced is PNP or NPN by sight alone? I know I can hook it up and see whether it pulls to positive or negative, but often the old sensor is failed and that cannot be done. I know I can trace it out in the control cabinet to see whether the output goes to a sinking or sourcing I/O card, or a similar load that needs either the pos or neg from the sensor. But... any rules of thumb to avoid that much work? (assume that the labeling is worn off the old sensor, which is often the case). 

Seems like many factories have tried to standardize their equipment as all NPN or PNP, but I'm doing work at a place that has some of both... sometimes on the same machine!


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## BuzzKill (Oct 27, 2008)

Drrrrr!

There's my two cents.

Me no know nothing bout no PNP, NPN...PLC work? that's my retirement job after contracting: kick back and thumb through the laptop. and "monitor".


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## MDShunk (Jan 7, 2007)

BuzzKill said:


> Drrrrr!
> 
> There's my two cents.
> 
> Me no know nothing bout no PNP, NPN...PLC work? that's my retirement job after contracting: kick back and thumb through the laptop. and "monitor".


No sweat. One of these factory guys, like maybe Riveter, might have some trick up their sleeve.


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## Bkessler (Feb 14, 2007)

Well if know one knows I'll pull a rabbit from my hard hat for you, it's a good trick. Probably won't help you much though.


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## BuzzKill (Oct 27, 2008)

I guess the domestic ones might taste a little better?


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## Bkessler (Feb 14, 2007)

BuzzKill said:


> I guess the domestic ones might taste a little better?


Pet rabbit's are great, when we have the big I can eat them before the dog. Their part of my earthquake survival plan. That and lots of spam.


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## BuzzKill (Oct 27, 2008)

Bkessler said:


> Pet rabbit's are great, when we have the big I can eat them before the dog. Their part of my earthquake survival plan. That and lots of spam.


oh man funny but not so much


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## MDShunk (Jan 7, 2007)

Thursday night isn't even a traditional drinking night, so how did we get from sinking and sourcing sensors to eating pet rabbits so quickly? 

Nevermind. I'm not sure I want to know.


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## BuzzKill (Oct 27, 2008)

MDShunk said:


> Thursday night isn't even a traditional drinking night, so how did we get from sinking and sourcing sensors to eating pet rabbits so quickly?
> 
> Nevermind. I'm not sure I want to know.


traditional in who's eyes?
I create new traditions.


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## Bkessler (Feb 14, 2007)

MDShunk said:


> Thursday night isn't even a traditional drinking night, so how did we get from sinking and sourcing sensors to eating pet rabbits so quickly?
> 
> Nevermind. I'm not sure I want to know.


I have a problem with needing to get the last word in.:thumbup:


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## BuzzKill (Oct 27, 2008)

same here


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## drsparky (Nov 13, 2008)

In the transistor world there is no difference visually between a PNP and a NPN. The emitter will have a dot or a tab near that lead. The collector generally will be the case because 100% of the power will pass through it and they heat sink it. (assuming it is common collector) Whip out an ohm meter and check for diode action between the emitter and the base and collector and the base, that will tell you if it is PNP or NPN. Most transistor will be EBC or CBE, the base is almost always the center lead. 
Chuck


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## ElectricArcher (Feb 25, 2010)

What type of sensor are you talking about? Is the sensor is 24vdc?


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## Sawdust454 (Sep 26, 2008)

If you can see which wire is connected you can tell the configuration.
Typical color code:
Brown +v
Blue -v
White sinking
Black Sourcing


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## MDShunk (Jan 7, 2007)

Sawdust454 said:


> If you can see which wire is connected you can tell the configuration.
> Typical color code:
> Brown +v
> Blue -v
> ...


S'zackly what I was looking for. Thanks!!


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## ampman (Apr 2, 2009)

sawdust you should post more


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## Sawdust454 (Sep 26, 2008)

Thanks, but you guys are too good and too quick for me. It is rare that someone does not beat me to the answer.


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## MaintenanceGeek (May 18, 2010)

Do you know if you are using a shielded or unshielded prox? A shileded prox will have threads right to the end of the sensor. Unshielded will have a portion of the sensor plastic showing. If it is unshielded it can get false triggers if something is in close proximity to the side of it's head. 
Just thought I would let you know if the sensor started to act funny for ya. 
I use sensors every day. If anyone needs sensor advice let me know.
I work in industrial automation, there is so much to choose from and take into consideration. 
Hope this helps.


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