NPN (sinking): Brown → +24 V, Blue → 0 V, Black (output) → PLC input. PLC input common → +24 V. Output pulls to 0 V when active.
PNP (sourcing): Brown → +24 V, Blue → 0 V, Black (output) → PLC input. PLC input common → 0 V. Output drives to +24 V when active.
Match the sensor output type to the PLC input type: NPN sensor → sinking (NPN) PLC input; PNP sensor → sourcing (PNP) PLC input.
NPN vs PNP: The Core Difference
The terms NPN and PNP describe the transistor type used in the proximity sensor's output stage, which determines how the output switches relative to the power supply:
The same rules apply to the special-environment series: the high-pressure inductive proximity sensors (SGI-HP, rated up to 1000 bar for hydraulic cylinders) and the high-temperature inductive proximity sensors (SGI-HT, +120 °C / +150 °C) are both ordered with an NPN or PNP 3-wire output (2-wire versions are also available) and are wired to the PLC input card exactly as shown below. For sensors with a continuous signal instead of a switching output, see analog 4-20 mA vs 0-10 V proximity output.
| Property | NPN (Sinking) | PNP (Sourcing) |
|---|---|---|
| Output when active | Pulls output to 0 V (GND) | Drives output to +Vcc (+24 V) |
| Current flow | Current sinks INTO the sensor | Current sources OUT from the sensor |
| PLC input type | NPN / Sinking input (common = +24 V) | PNP / Sourcing input (common = 0 V) |
| Geographic standard | Japan, Asia, some NA | Europe, North America |
| Wire colours (IEC 60947-5-2) | BN=+Vcc, BU=0V, BK=output | BN=+Vcc, BU=0V, BK=output |
The wire colours are identical — the difference is only in how the output transistor switches, and therefore how the PLC input common must be connected. The output stage is independent of the sensing principle, so inductive, capacitive and photoelectric sensors are all offered as NPN or PNP; the sensing principles are compared in types of proximity sensors.
Wiring an NPN 3-Wire Proximity Sensor
An NPN (sinking) proximity sensor has an open-collector NPN transistor output. When the sensor detects an object, the transistor turns ON and the Black output wire is pulled DOWN to 0 V. The load (PLC input, relay coil, or indicator lamp) connects between the Black wire and +24 V.
- For NPN sensors, the PLC input COMMON must be wired to +24 V (not to 0 V).
- This is the single most common wiring mistake — connecting PLC common to 0 V with an NPN sensor results in no signal or inverted logic.
- Check your PLC manual: NPN inputs are also labelled "Sink" or "Current Sink" on the terminal block.
Wiring a PNP 3-Wire Proximity Sensor
A PNP (sourcing) proximity sensor has an open-collector PNP transistor output. When the sensor detects an object, the transistor turns ON and the Black output wire is driven UP to +24 V. The load connects between the Black wire and 0 V.
NO vs NC Output: Normally Open and Normally Closed
Both NPN and PNP sensors can have Normally Open (NO) or Normally Closed (NC) output logic. This is independent of the NPN/PNP transistor type:
| Output Type | No Object in Range | Object Detected | Typical Use |
|---|---|---|---|
| NO (Normally Open) | Output OFF | Output ON | Object detection, counting, position sensing |
| NC (Normally Closed) | Output ON | Output OFF | Safety circuits, wire-break detection (fail-safe) |
A 4-wire sensor provides both NO and NC outputs simultaneously on separate wires (Brown, Blue, Black, White), allowing both logic functions from one sensor without wiring two sensors. On connector versions the NC output sits on pin 2 (white) of the M12 plug; see M12 connector pinout for sensors.
Standard Wire Colour Code (IEC 60947-5-2)
| Wire | Colour | Function |
|---|---|---|
| 3-wire sensors | Brown (BN) | +Vcc (positive supply, typically +24 V DC) |
| Blue (BU) | 0 V (negative supply / ground) | |
| Black (BK) | Output signal (switching output) | |
| 4-wire sensors | Brown (BN) | +Vcc |
| Blue (BU) | 0 V | |
| Black (BK) | Output 1 (NO) | |
| White (WH) | Output 2 (NC) |
How do you wire a 3-wire proximity sensor?
Wire a 3-wire DC proximity sensor with three connections: brown (BN) to +24 V, blue (BU) to 0 V and black (BK) to the PLC input terminal. The only NPN/PNP difference is the PLC input common: tie it to +24 V for an NPN (sinking) sensor and to 0 V for a PNP (sourcing) sensor.
Brown = +V (10–30 V DC on SGI-GP 3-wire models), blue = 0 V, black = switching output; NPN switches black to 0 V, PNP switches black to +24 V.
- The supply wiring (brown and blue) is the same for NPN and PNP sensors.
- The PLC input common decides which output type works: +24 V common for NPN, 0 V common for PNP.
- The output code in the SEGMENsensor part number tells you the type: NA / NB = NPN, PA / PB = PNP.
How it works
- Brown and blue power the oscillator and switching electronics inside the sensor.
- When a metal target enters the sensing distance, the output transistor turns on.
- The black wire then sinks (NPN) or sources (PNP) the input current, and the PLC input changes state.
Step-by-step: wiring a 3-wire sensor to a PLC
| Step | Action | What to check |
|---|---|---|
| 1 | Switch off and isolate the 24 V DC supply. | No voltage between the supply terminals. |
| 2 | Read the output code on the sensor label or part number (NA / NB = NPN, PA / PB = PNP) and the PLC input type (sinking or sourcing). | NPN sensor with a sinking input; PNP sensor with a sourcing input. |
| 3 | Connect brown (BN) to +24 V. | Supply within the rated range: 10–30 V DC for standard SGI-GP 3-wire models. |
| 4 | Connect blue (BU) to 0 V. | Same 0 V as the PLC input card. |
| 5 | Connect black (BK) to the PLC input terminal. | One sensor per input terminal. |
| 6 | Wire the PLC input common: +24 V for NPN, 0 V for PNP. | Common polarity matches the sensor output type. |
| 7 | Power up and move a steel target into the sensing distance (for example 4 mm for the SGI-GP12S-040 flush M12 model). | The sensor LED and the PLC input LED change state together. |
2-wire vs 3-wire proximity sensors: which should you choose?
Choose a 3-wire sensor for new PLC designs, because separate supply and output wires make NPN or PNP selection explicit and keep the input signal clean. Choose a DC 2-wire sensor mainly when replacing a 2-wire limit switch on existing cabling. The 2-wire sensor is wired in series with the load and draws its own supply through it.
| Property | 3-wire DC (NPN / PNP) | 2-wire DC |
|---|---|---|
| Wires | Brown +V, blue 0 V, black output | Two wires in series with the load (brown and blue) |
| SEGMENsensor supply (SGI-GP) | 10–30 V DC (V36: 5–36 V DC, V55: 10–55 V DC) | 10–60 V DC |
| Output codes | NA / NB / NC (NPN), PA / PB / PC (PNP) | DA (NO) / DB (NC) |
| Off state | Output transistor off; the input sees no current from the sensor | A small leakage current keeps the sensor powered; check it stays below the PLC input's off-current limit |
| On state | Output follows the supply rail | A residual voltage drop across the sensor; check the PLC input still reads ON |
| Typical use | New machines, PLC input cards, fast counting | Retrofits of 2-wire switches, reduced cabling |
For more on the sensing principle behind both versions, see how an inductive proximity sensor works; to decide between metal-only and non-metal detection, see inductive vs capacitive proximity sensors. Typical housings for these wiring cases are the M12 flush inductive proximity sensor and the M18 inductive proximity sensor.
Download the one-page datasheet (PDF)
The SEGMENsensor inductive proximity sensor datasheet lists the published housing sizes, sensing distances, output options, supply voltage and ordering information. No registration required.
Which SEGMENsensor Proximity Sensor Fits Your NPN or PNP Wiring?
SEGMENsensor inductive proximity sensors carry the output type in the part number, so each wiring case above maps to one order code. In the published part-number tables, NA is NPN NO, NB is NPN NC, NC is NPN NO + NC, PA is PNP NO, PB is PNP NC, PC is PNP NO + NC, and DA / DB are DC 2-wire NO / NC. The M12 examples below are taken from those tables.
| Wiring case in this article | Output code | SEGMENsensor example (M12) | Sensing distance | Supply | Series page |
|---|---|---|---|---|---|
| NPN 3-wire, NO, to a sinking PLC input (common +24 V) | NA | SGI-GP12S-040NA-Q4 (flush, M12 connector) | 4 mm | 10–30 V DC | M12 flush SGI-GP |
| PNP 3-wire, NO, to a sourcing PLC input (common 0 V) | PA | SGI-GP12S-040PA-Q4 (flush, M12 connector) | 4 mm | 10–30 V DC | M12 flush SGI-GP |
| NC output for fail-safe or wire-break detection | NB / PB | SGI-GP12S-040NB-Q4 / SGI-GP12S-040PB-Q4 | 4 mm | 10–30 V DC | M12 flush SGI-GP |
| 4-wire sensor with NO and NC outputs | NC / PC | SGI-GP12S-040NC-Q4 / SGI-GP12S-040PC-Q4 | 4 mm | 10–30 V DC | M12 flush SGI-GP |
| DC 2-wire sensor in series with the PLC input | DA / DB | SGI-GP12S-040DA-Q4 / SGI-GP12S-040DB-Q4 | 4 mm | 10–60 V DC | SGI-GP inductive series |
| Same NPN or PNP wiring, sensor in pressurized hydraulic oil (500 bar, M12) | NA / PA | SGI-HP12S-015NA-C2-L43 / SGI-HP12S-015PA-Q4-L69 | 1.5 mm | 10–30 V DC | SGI-HP high-pressure |
| Same NPN or PNP wiring, ambient −25 to +150 °C | NA / PA | SGI-HT12S-020NA-C2-T150 / SGI-HT12S-020PA-C2-T150 | 2 mm | 10–30 V DC | SGI-HT high-temperature |
SGI-GP 3-wire models are also listed for 5–36 V DC (V36 code) and 10–55 V DC (V55 code), and the same output codes run across 6.5 mm, M8, M12, M18 and M30 housings. To get a quotation, send the PLC input type (sinking or sourcing), NO or NC logic, housing size and flush or non-flush mounting, sensing distance, supply voltage, cable or connector, operating temperature or pressure, and estimated annual volume. Qualification samples of 1–5 pcs ship in 5–7 working days; production lead time and MOQ are confirmed with the quotation.
Frequently Asked Questions
An NPN sensor pulls the output DOWN to 0 V when active (current sinking). A PNP sensor drives the output UP to +24 V when active (current sourcing). NPN is standard in Japan and Asia; PNP is standard in Europe and North America. Match the sensor output type to the PLC input type: NPN sensor → PLC NPN sinking input; PNP sensor → PLC PNP sourcing input.
Connect Brown to +24 V, Blue to 0 V, Black (output) to the PLC input terminal. Connect the PLC input's common terminal to +24 V (not 0 V — this is the key difference for NPN sinking inputs). When the sensor detects an object, the Black wire pulls to 0 V, which the PLC reads as an active input.
Connect Brown to +24 V, Blue to 0 V, Black (output) to the PLC input terminal. Connect the PLC input's common terminal to 0 V. When the sensor detects an object, the Black wire is driven to +24 V, which the PLC reads as a HIGH active input.
NO (Normally Open) means the output is OFF when no object is detected, and switches ON when an object is in range. NC (Normally Closed) is the opposite: ON with no object, OFF when an object is detected. NO is used for object detection and counting; NC is used for safety circuits where a broken wire should trigger an alarm (fail-safe wiring).
Not directly. Use a PNP-to-NPN converter relay module, replace the sensor with the correct NPN output type, or use a sensor with selectable NPN/PNP output. Always match sensor output type to PLC input type for correct operation.
Per IEC 60947-5-2: Brown (BN) = +Vcc (+24 V), Blue (BU) = 0 V ground, Black (BK) = switching output. For 4-wire sensors: Brown = +Vcc, Blue = 0 V, Black = output 1 (NO), White = output 2 (NC). This colour code is used by SEGMENsensor and most European and Asian manufacturers.
Standard 3-wire DC proximity sensors drive cable runs up to 200–300 metres with shielded cable (0.34 mm² or larger). For longer runs or noisy environments near VFDs, use shielded cable with the shield grounded at one end. Verify that voltage drop on the Brown (+Vcc) wire does not drop below the sensor's minimum supply voltage for runs over 100 m.
Read the two letters after the sensing distance in the part number: NA is NPN NO and PA is PNP NO. An M12 flush sensor with 4 mm sensing distance and an M12 connector is SGI-GP12S-040NA-Q4 for a sinking (NPN) PLC input or SGI-GP12S-040PA-Q4 for a sourcing (PNP) PLC input. NB and PB are the NC versions; all four run on 10–30 V DC.
A 3-wire sensor has separate supply wires (brown +V, blue 0 V) and a black output, so it is ordered as NPN or PNP to suit the PLC input. A 2-wire sensor is wired in series with the load and draws its supply through it, so its off-state leakage current and on-state voltage drop must suit the PLC input. SEGMENsensor SGI-GP 3-wire models run on 10–30 V DC and DC 2-wire models on 10–60 V DC.
With the PLC input connected and power on, move a steel target into the sensing distance. The sensor LED and the PLC input LED should change state together. With a multimeter, a PNP output measured to blue reads close to the supply voltage when the target is present; an NPN output measured to blue reads close to 0 V when the target is present.
The most common cause is a mismatch between the sensor output and the PLC input common: an NPN sensor needs the common at +24 V and a PNP sensor needs it at 0 V. Check the output code on the label (NA / NB = NPN, PA / PB = PNP), the common wiring and that the black wire lands on the correct input terminal.