Answer / carl_ellis

Often identification data is printed on proximity (or prox) sensor's label but it is not uncommon for the label to damaged, missing or unreadable.

Many prox sensors are DC powered, 3 wire devices that conform to a standard wiring color code:

brown is (+) supply voltage, typically 24Vdc
blue is (-) supply voltage common or return
black is the sensor's switched output

To test a 3 wire sensor to determine whether it is a PNP or NPN type you need a DC power supply and a DC voltmeter.

Set the meter to DC voltage to a range greater than the supply voltage (typically 24Vdc), like 50/100/200/600V

The black meter probe should be in the 'common' meter socket
The red meter probe should be in the DC 'volts' socket

Connect the sensor's brown wire to the power supply (+)
Connect the sensor's blue wire to the power supply (-).

Turn the power supply on.

Ensure that the prox sensor is not triggered by presence to an object.

Connect the black meter probe to the blue sensor wire.
Connect the red meter probe to the black switched signal output sensor wire.

If the meter is connected to a PNP device, the meter will read 0V.
If the meter is connected to an NPN device, the meter will read 24Vdc.

Force the sensor to change state/output by detecting an object.
- If it is an inductive proximity switch, place a piece of metal in front of the sensor.
- If it is a photoelectric sensor, block the photoelectric beam.
- For an ultrasonic sensor or a capacitive sensor, you can just use your hand to make the sensor output. Some prox sensors use an LED that lights up when the sensor detects an object.

With an object present, a PNP device's output will change and the voltmeter readout will read 10V to 30V. This is also known as a "sourcing" device.

With an object present, an NPN device's output will change and the voltmeter readout will be 0V. This is also known as "sinking."

Another test to check for PNP:
Connect the power supply (+) to brown, (-) to blue.
Connect the red meter probe to the brown (+) sensor wire.
Connect the black meter probe to the sensor's black signal output wire.

When sensing no object, the signal output will be 10V to 30V.

When sensing an object, the signal output will be 0V.

Another test to check for NPN:
Connect the power supply (+) to brown, (-) to blue.
Connect the red meter probe to the brown (+) sensor wire.
Connect the black meter probe to the sensor's black signal output wire.

When sensing no object, the signal output will be 0V.

When sensing an object, the signal output will be 10V to 30V.

If the average power during generation is zero, why do the analog meters have + and - ?

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