Npn & Pnp Proximty Sensors

[magdyfayad]

Please , I have some questions for NPN & PNP proximty sensors and I want to answer them if possible :

 

1- I did not understand what happen if we connect the collector of PNP to the ground and the emitter of PNP to PLC input as PNP then will be sink ?? what is the voltage at PLC input when the PNP is active at this connection ??

 

also what happen if we connect the collector of NPN to 24 vdc and the emitter of NPN to PLC input as NPN then will be source ?? what is the voltage at PLC input when the NPN is active at this connection ??

 

I know these two connections are not the standard, as when I connect the emitter to PLC input - ( as PNP sink & NPN source ) as the transistor is not good as a switch at this connection , but I did not know why ?

 

2- Is the transistor is good as a switch in both cases when the transistor is on and when off for both NPN & PNP , or what ??

 

3- I know the standard connection is connect the collector for both NPN & PNP to PLC input, - ( as PNP source & NPN sink ) as the transistor at this connection is good as a switch but also I did not know why it is good as a switch than the another connection ??

 

4- I know PLC input is from 10 : 30 VDC to operate on , is the voltage on the emitter is less than 10 VDC if we connect the emitter to PLC input or what is the voltage of the emitter will be – if we connected the emitter to PLC input – at both cases if the transistor is on and if the transistor is off for both NPN & PNP ??

 

5- the proximity sensor PNP & NPN has three wires :

 

1- brown 24 vdc

2- blue ground

3- black the output

 

what happen if I reverse the connection between the brown and blue ??

also what happen if I reverse the connection between the brown and black ??

 

is the proximity damages for both cases or not operate or what ??

 

Thanks in advance

[marke]

Hello magdyfayad

 

Welcome to the forum.

 

There are two types on input circuit to a PLC, NPN and PNP.

The input is either "active high" or "active low"

 

If the input is active high, it needs a connection from an input to the positive terminal to indicate a closed switch on the input, and if the input is active low, it requires a connection from the input to the ground terminal of the plc.

 

To pull an input up to the positive supply, a PNP transistor is used, with the emitter to the plus, and the collector to the input.

To pull an input up to ground, a NPN transistor is used, with the emitter to ground, and the collector to the input.

 

1. It is possible to design circuits such that an NPN transistor could be used to pull the input up, but you would directly control the output voltage and this would be a problem if the circuit did not operate off the same supply as the PLC. The use of the PNP provides a switch output rather than a variable voltage output. - The internal circuit is connected to the base and emmitter of the output transistor, but not the collector, so it is much easier to always connect the collector to the output.

 

2. Both NPN and PNP transistors equally act as switches, but the PNP emitter is connected to plus, and the NPN emitter is connected to ground. Always connect the collector to the PLC input.

 

3. see above.

 

4. see above.

 

5. If you reverse the wires, you could do permanent damage to the sensor. Some sensors have reverse polarity protection, but not all.

 

Best regards,

 

[chaterpilar]

Excellent reply, Marke...

 

Mag, for more info follow this link...

 

 

http://www.mikroelektronika.co.yu/en/books...r3/chapter3.htm

 

 

Good luck

 

 

Chaterpilar