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Worksheet: PNP and NPN sensors

Name:Class:Date:

Material: the free PNP and NPN sensor simulator at examrobotic.com (examrobotic.com/en/tools/pnp-npn-sensor-simulator). Time: about 40 minutes.

Aim: wire a 3-wire inductive sensor to a PLC input, understand the difference between PNP and NPN and between sinking and sourcing input cards, and measure to find faults.

1. Colours and wiring

  1. Write where each wire of a PNP sensor goes:
    Brown: ____________   Blue: ____________   Black: ____________
  2. With a PNP sensor detecting, what voltage is on the black wire relative to 0 V? And with an NPN?

2. Distance and materials

  1. With the PNP set-up, bring the part closer until I0.0 lights. Switch-on distance: ______ mm. Move it away until it goes off: ______ mm. Why are they different?
  2. Change the material to aluminium and then to plastic. What happens to the sensing distance? Why?

3. PNP, NPN and the card

  1. Without touching any wire, change the sensor to NPN. Why does it stop working? Which wire do you move to fix it?
  2. With the NPN working, switch the card to «PNP only». The voltages look right, but the input stays off. Why?
  3. Set the contact to NC. When is the input on now? Give an example of what it is useful for.

4. Find the fault

Press «Surprise fault» three times. For each one, write down what you measured and what you found.

Answers (for the teacher)

  1. Brown to +24 V, blue to 0 V and black to the PLC input (with PNP, the input card common goes to 0 V).
  2. PNP: about 24 V (the sensor sources positive to the input). NPN: about 0 V (the sensor sinks the input to 0 V).
  3. It switches off further away than it switches on: that is hysteresis, which stops the output flickering when the part is right at the limit.
  4. The distance falls with aluminium (reduction factor) and plastic is not detected: an inductive sensor only sees metal.
  5. The NPN switches the negative, and the input needs its common the other way round. It is fixed by moving the card common (1M) to +24 V.
  6. A PNP-only card needs current to flow into the input from the sensor; with an NPN the current would have to flow out of the input, and that card does not allow it.
  7. The input is on when there is no part. It is used, for example, to detect a missing part or so that a broken wire shows up as «no signal».