PNP sensors vs dry contact sensor

Charbel · Sep 28, 2015

Dear,

I was wondering which one is better to use a PNP sensor or a dry contact sensor,
I was checking the inductive sensor from Schneider electric XS4p12PA340, it is a 3 wires PNP sensor,

to connect it, it needs sinking inputs (NPN) and they are available from many PLC manufacturers,

my question is, is it better to convert it so it has dry contact (by adding to it a relay module), so that it can be easier for connection and troubleshooting.

I really appreciate any thoughts on this.

regards,

charbel

widelto · Sep 28, 2015

I prefer NPN (PNP), relays chatter.

JesperMP · Sep 28, 2015

For sensors, 24VDC PNP is the norm today.
Dry contacts are normally used for interface signals between systems.

Charbel · Sep 28, 2015

dear,

thank you for your feedback, one final thought, what about having a 2 wire inductive proximity switch instead of having to put a relay module in between, for ease of connection and troubleshooting?

osmanmom · Sep 28, 2015

this relay use for what?

Charbel · Sep 28, 2015

Dear Osmanmom,

the question was if it is better to use a proximity swtich with PNP output or use a 2 wire proximity switch.

osmanmom · Sep 28, 2015

generally use PNP ,

the important thing the 'response time' that match with your application.

rdrast · Sep 28, 2015

Charbel said:
dear,

thank you for your feedback, one final thought, what about having a 2 wire inductive proximity switch instead of having to put a relay module in between, for ease of connection and troubleshooting?

A relay in-between what?
For a three wire sensor, the sense lead is usually directly connected to your PLC input, and the input module shares the same common as the sensor.

For a two wire sensor, you connect the output lead to the PLC input, and the input module shares the same common as the sensor power supply.

just the cowboy · Sep 28, 2015

time matters

If it needs to work fast or cycles often don't use a relay, it is only one more point of failure.
If it is for switchs that are used for trouble points and you can't see the lights on the prox switch, a relay with pilot light can help in troubleshooting ( look for all lights on or off ).

rpoet · Sep 28, 2015

Why not simply purchase an NPN version of that sensor? Driving a relay with your PNP sensor and switching your NPN PLC input is certainly possible, but why go to all the trouble when you can simply purchase the right sensor for the application?

-rpoet

Charbel · Sep 29, 2015

Dear all,

thank you a lot for your replies, the intent was to see (technically or better over long run) which one is better to use a proximity switch with PNP output or use a 2 wire proximity switch, the application is for valve position (either opened or closed).

now it is clear to me that better to choose a proximity switch with PNP output.

Thanks!

Charbel

PNP sensors vs dry contact sensor

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