PnP-NpN question

Hello,

By Mistake a bunch of Baluff switches got ordered pnp instead of npn. I have seen alot of talk about using a relay to reverse them, but no specifics, how would I do this? Or can I just reverse the positive and negative (neutral?). These are 3 wire proximity switches, and are being run into a Siemens Simatic plc, 24v power supply feeding them, so nothing too serious.

Thanks alot in advance!

Sean

err... can't you rewire the connection in siemens plc?

I've used these before because they fit on the end of a quick connect in the field and take up no panel room.


http://www.simcotech.com/smartplug.htm


Cost is around $50 each. Inexpensive compared to panel space, relay, wiring, and print changes.

Mr.SEAN,

PNP sensors have positive output and you can wire it directly to Sourcing PLC Input Module. NPN sensor have negative output and you can wire it directly to Sinking PLC Input. In your case you purchased PNP but your PLC input module is Sinking I/O.

To used the PNP sensor you have. you need a relay. Positive output of PNP sensor will be connected to relay coil. Relay NO contact will be connected to negative and it will go to the PLC Sinking Input module.

Option No.2 if you dont want to used relay then replaced your Sourcing Input module into Sinking Input module.

You can use a "pull-down" resistor and reverse the logic in the PLC. Start with a 10K 1/4W and experiment with the values till you get it to work. Connect the resistor from the PLC input to common and wire the sensor normally as a npn.

I would talk to my sales rep. first to see if they would exchange them for the correct one. As long as they are unopened and haven't been hooked up I would think that it should not be a problem.

danpreps said:

To used the PNP sensor you have. you need a relay. Positive output of PNP sensor will be connected to relay coil. Relay NO contact will be connected to negative and it will go to the PLC Sinking Input module.

Click to expand...

Would this relay work if you have the opposite situation?. Let's say we have an NPN with sourcing inputs?

Thanks alot for your input guys, will try a few things and report back. I would return them myself, but I lack the scissors necessary to cut through the red tape surrounding everything we do here, including something simple as returning a part, if ya know what I mean.

Think in terms of the output of NPN pulling toward neutral (Ground) when it is On or True and undefined when it is Off or False. For PNP, it's output will pull toward the Positive Supply when it is On or True and float or undefined when Off or False.

It is often desirable to use a "Pull Up" to define an NPN when it is in it's undefined mode (Off) and a "Pull Down" to define a PNP when it is undefined.

Care is required to do this successfully over the long term because these resistors add to the current load on the sensor when it is On. Also, think about what happens when the sensor is in the On state for long periods of time. The resistor is tugging against it and both must be properly sized to deal with the power dissipation. If the resistor's value is too low, it pulls more current than necessary and it may cook itself or the sensor.

I suggest that you use nearly the highest value of resistance that will get the proper voltage transistions. Measure the current and then do the math. Then make the resistor power rating is at least twice as high as the calculated result.


Bob A.

Or once again, use a $50 smart plug that I referenced earlier and then forget about it. No calculations, no print changes, easier for maintenance, assured they work as intended without having to get a bunch of resistors.


In the long run, you won't regret it.

Awesome tip Bruce! I just had someone run into this yesterday. Got in a rush and ordered the wrong switch. We just ordered 2 of these for such an occasion.
Can't believe I've never seen them before.

danpreps said:

Mr.SEAN,

PNP sensors have positive output and you can wire it directly to Sourcing PLC Input Module. NPN sensor have negative output and you can wire it directly to Sinking PLC Input. In your case you purchased PNP but your PLC input module is Sinking I/O.

To used the PNP sensor you have. you need a relay. Positive output of PNP sensor will be connected to relay coil. Relay NO contact will be connected to negative and it will go to the PLC Sinking Input module.

Option No.2 if you dont want to used relay then replaced your Sourcing Input module into Sinking Input module.

Click to expand...

.

That's not correct. Or at least, not consistent across manufacturers and their use of sinking and sourcing. You have to think in terms of the object device. As an example, a Sourcing Sensor sends + Voltage. But a Sinking Input receives + Voltage. So, you would not wire a PNP sensor to a Sourcing Input module as you suggest. Think of them in pairs. A sourcing Sensor (which uses a PnP transistor) needs a sinking input (which uses the complementary nPn transistor).

That smartplug is pretty incredible, I will be looking into having that in the future just for situations like this. I did get the sensor working with a relay for now, and hopefully the number I give purchasing will be the number they use next time, but its always good to have a backup plan.

Thank you all for your suggestions.