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from the EE department...
NPN/PNP Sensors
Sensor technologies. topic
Posted by Chris Reilly on 2 December, 2004 - 7:05 pm
I have a optical sensor which I want to connect to a PLC input (It is my first PLC project). How can find out if my sensor is NPN or PNP? I can not obtain this information from the manufacturer because the machine I am modifying is quite old.

Thanks in advance,
Chris


Posted by S. Reid on 3 December, 2004 - 9:15 pm
1) Power the sensor from (usually) 24VDC.

2) Connect one end of a (say) 10kohm resistor to the sensor output.

3) Connect the other end of the resistor to either 24Vdc or 24 VDC common.

4) Actuate the sensor.

%) If swirching st the sensor output happens when the resistor is connected to 24 VDC, the sensor is NPN.

5) If the sensor output switches when the resistor is connected to 24 VDC common, the sensor is PNP.


Posted by Steve Myres on 3 December, 2004 - 9:47 pm
One way to find out would be to look at the load the sensor is currently driving. If it is a relay, for example, the sensor output will go to one coil terminal and the other coil terminal will go to one of the power
rails. If it is the low side, your sensor is PNP; if the other side of the relay is the low side, you sensor is NPN. If it is presently not connected, you will need to connect two resistors in series across the DC power rails. 3.3k, 1/4w should do if your supply is around 24VDC. Now
hook the sensor output to the point between the resistors and activate the sensor. If the sensor is PNP, the senssor output point between the resistors will go high, if it's NPN, it will go low.
--
Steve Myres, PE
Automation Solutions
(480) 813-1145


Posted by Ron on 3 December, 2004 - 9:51 pm
Greetings Chris,

I'll assume that you have a three-wire sensor which uses 24VDC...

hook the sensor's positive wire (usually brown) to +24VDC... hook the sensor's negative wire (usually blue) to DC return... hook the sensor's signal wire (usually black or white) to a 24VDC lamp bulb...

hook the other side of the lamp to +24VDC... actuate the sensor off and on several times... if the lamp goes off and on reliably, then you have an NPN type sensor... basic idea: the lamp already "HAS" a positive connection... it's "WAITING FOR" negative to come through the sensor... if the sensor "SWITCHES" negative, then it's an NPN device...

if the lamp does not go off and on reliably, then proceed with the next test...

now hook the other side of the lamp to the DC return... actuate the sensor off and on several times... if the lamp goes off and on reliably, then you have a PNP type sensor... basic idea: the lamp already "HAS" a negative connection... it's "WAITING FOR" positive to come through the sensor... if the sensor "SWITCHES" positive, then it's a PNP device...

if the lamp does not go off and on reliably, then the sensor is probably bad...

important... do NOT use a meter to test the sensor... most of these solid-state devices pass a certain amount of "leakage current" which will lead to false conclusions when tested with sensitive instruments... a small lamp bulb will usually provide a sufficient "load" to give an accurate test...

good luck,
Ron


Posted by Meir Saggie on 5 December, 2004 - 11:31 am
Quite easy to deduct (an educated guess) from the connection diagram: For a two terminal sensor: If the sensor switches (is connected) between the positive power rail and the PLC input - it is PNP If the sensor switches (is connected) between the negative power rail and the PLC input - it is NPN

At the end of the day, it doesn't really matter what type the sensor is - what you really need to know is how to connect it!.
Meir

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