Value of changing from NPN sensor setup to PNP sensor setup

Join Date
Nov 2013
Location
Sacramento
Posts
8
Im installing a sensor setup that has only NPN sensors. The sensors count jars that pass by them. The sensors do not control any equipment. The environment is humid and prone to washing down. The sensors, three of them, go through a junction block that is prewired for m12 connections for NPN sensors. the junction box goes to an AB Compactlogix PLC. When there is no product passing the Jars the inputs on the PLC are all lit. When the product passes, the inputs are not lit. I have a small fuse that is on the 24v feed to the junction block.

Why did I choose this NPN setup as opposed to the PNP setup? Im new to PLC wiring so did not know all the ramifications of this setup vs a pnp setup. I read that pnp is more common but the npn setup was more safe for the plc should the sensor ground out.

Am I going to have some difficulties is the future with this setup? Is there some advantages to this setup vs a pnp setup?

Thanks!
 
I read that pnp is more common but the npn setup was more safe for the plc should the sensor ground out.!

Where did you read NPN is more "safe" for a PLC? And "safe" in what regard?

The main reason why PNP is chosen above NPN (or positive as opposed to negative logic) is because you don't want an inadvertently grounded wire to come on as if the sensor itself was on. The likelihood of a signal wire shorting to ground is infinitely higher than shorting to the positive of the PS.

I never use NPN unless a customer specifically forces the issue.
 
" pnp is more common but the npn setup was more safe for the plc should the sensor ground out"

I think this is just the opposite. The npn inptut would be true if you grounded the wire. With PNP, +voltage has to flow to the input to be true. A gounded out wire would result in false at the input module.
 
...The sensors do not control any equipment. ...the npn setup was more safe for the plc should the sensor ground out.
Correct. With a short to ground, you have an incorrect input, but don't blow any fuses.
Am I going to have some difficulties in the future with this setup?
No. Your setup is fine. The most important issue is washdown, and it sounds like you've dealt with that.
 
How is this "safer" for the PLC?
It's not. Just doesn't blow anything.
Why would blowing a fuse on a short be considered a "bad" thing?
Why is it a good thing? Is a blown fuse the only way to find a short? PNP demands a fuse on every input. Is this really necessary on a 24DC circuit at 6mA?
I see the point on critical inputs, and would prefer PNP in that case.

For this case, the OP has non-critical inputs, and has spec'd and possibly purchased parts.
Now he asks if he screwed up. My answer is absolutely not.
NPN most likely will be cheaper (less wiring, fuses...) and easier to wire. He's using M12 connectors, and primary concern is moisture. With sealed connectors and junction boxes, shorts are highly unlikely anyway.
 
It's not. Just doesn't blow anything.

Regardless of whether the switch is NPN or PNP, shorting the wire that goes to the input of the PLC should not harm the PLC input. The only way it would harm the PLC is if the PLCs power supply is used to power the prox. If the prox is a 3 wire prox, then you have both ends of the power supply going to the sensor anyhow, so you are just as likely to short either one of those wires.

Why is it a good thing?
There a many reasons why it is a good thing. I tend to put a fuse in for practically every 24VDC device (not all input point of course, but at least to every input card bank.). Why, because if a short occurs then someone doesn't have to spend a couple hours pulling wires off a terminal strip to try and isolate where the source of the short is. I cringe when I see designs where they have one main fuse (or none at all) on the power supply and everything on the panel and machine is tapped off that one branch. A blown fuse can help you isolate the problem quickly. It can also


Is a blown fuse the only way to find a short?
No, it is the quickest/easiest way if the circuit is designed properly.


PNP demands a fuse on every input. Is this really necessary on a 24DC circuit at 6mA?
Says who? I have never seen that done.


I see the point on critical inputs, and would prefer PNP in that case.

For this case, the OP has non-critical inputs, and has spec'd and possibly purchased parts.
Now he asks if he screwed up. My answer is absolutely not.
NPN most likely will be cheaper (less wiring, fuses...) and easier to wire. He's using M12 connectors, and primary concern is moisture. With sealed connectors and junction boxes, shorts are highly unlikely anyway.

I never contended that he "screwed up". I was questioning the "yes" he got in confirmation that it is somehow safer for the PLC. Whether NPN or PNP, just as easy to wire. Not seeing your point here.
 
As you have seen from the discussion so far America prefers PNP
I believe in Asia NPN is common so its a cost or a preference that decides

Now to my question can you tell us which Plc model you are using, what input card you are using,
as it is possible on some Plc's to change the wiring to accomodate which ever is your need at the time.

Also give us the part number of the sensors you are using
 
I was told years ago that NPN transistors used to be cheaper than PNP and that is why they were used. These days same price and many PLC input cards are bi-directional - I know the ones I use all are.
Yes, NPN can cause problems switching inputs on if there is a ground in the field but only if the DC supply is not floating - I never ground DC supplies and let them float therefore that argument is invalid.
NPN is common in Asia - do not know about the US.
 
All other things being the same NPN transistors also have a higher maximum switching frequency. Therefore you would see it a lot on encoder outputs, pulsers, etc. Primarily though it is more suitable for power electronics (PWM power supplies, etc.) It makes it easier to mix voltage levels as well, although most modern inputs also having switching thresholds from 5 to 32VDC. As has been pointed out, the vast majority of input cards can be used in both fashions based on what you supply on the common. Each point is normally just a back to back photo coupler that lights up regardless of the current direction.
 

Similar Topics

Hi All, I am currently at the beginning of a new project for writing a PLC program (using CCW) for an old sugar centrifugal. Most of our...
Replies
3
Views
1,965
Hello All I am new to the forum and also to the world of PLCs. I would like to have your expert advice, If I can change the floating point value...
Replies
15
Views
5,733
Hello What would be the best way to write a value into "Source B" of a GEQ SLC500 PLC instruction from Wonderware? I would like the operator to...
Replies
2
Views
1,764
I am new to this line of work so please excuse me if this is extremely simple. I have also Searched the forums and web for answeres and am getting...
Replies
1
Views
1,233
Hello, I am trying to make a sequence program by using MOV function to change the value of a register. So far I have always used SET and RSET for...
Replies
5
Views
2,154
Back
Top Bottom