NPN vs PNP Sensors — Which Output Type Do You Need?
When sourcing sensors in India, one question comes up on almost every enquiry: _NPN or PNP?_ Get it wrong and your sensor won't signal the PLC correctly — or worse, it will appear to work but give erratic outputs.
The one-sentence answer
NPN sensors sink current (the output pulls to 0 V when active). PNP sensors source current (the output pulls to the supply voltage when active).
If your PLC input card is sourcing type (common on Siemens, Schneider, Allen-Bradley), use PNP.
If your PLC input card is sinking type (common on older Mitsubishi, Omron, Fanuc), use NPN.
Why this matters
Most Indian factory floors run a mix of PLCs bought over 10–15 years. You might have a Siemens S7-1200 on the new line, a Mitsubishi FX3U on the older conveyor, and a local-brand panel somewhere in the corner. These all expect different sensor output types.
Connecting a PNP sensor to a sinking-input PLC card will usually do one of three things:
1. Nothing — the input never registers
2. Damage the input card (rarer but happens)
3. Seem to work but give phantom signals at startup
Checking your PLC input card
Look at the PLC input module label or datasheet for one of these phrases:
| What you see | Output type needed |
|---|---|
| "Sourcing input", "Source input", "PNP input" | PNP sensor |
| "Sinking input", "Sink input", "NPN input" | NPN sensor |
| "24 V common" | Usually PNP |
| "0 V common" | Usually NPN |
The switchable shortcut
If you are unsure, or if you supply sensors to multiple customers with different PLCs, choose a sensor with NPN/PNP switchable output (also labelled "complementary output" or "IO-Link configurable"). Most RIKO and AECO sensors in our range offer this.
Switchable sensors cost marginally more — typically ₹100–200 premium on a ₹1,000–2,000 sensor — but eliminate the guesswork entirely. For stocking purposes, one SKU covers both wiring scenarios.
3-wire vs 4-wire
Proximity and photoelectric sensors come in 3-wire and 4-wire versions:
- 3-wire: Brown (+24V), Blue (0V), Black (output). Standard for most applications.
- 4-wire: Brown (+24V), Blue (0V), Black (output 1), White (output 2 — either NC, or the complementary output).
Capacitive sensors often come with a 4-wire connection to provide both NO and NC outputs simultaneously.
Practical wiring guide
NPN sensor → sinking input PLC
Sensor Brown → +24V DC
Sensor Blue → 0V (COM)
Sensor Black → PLC Input terminal
PLC COM → 0V
PNP sensor → sourcing input PLC
Sensor Brown → +24V DC
Sensor Blue → 0V (COM)
Sensor Black → PLC Input terminal
PLC COM → +24V
Summary table
| Parameter | NPN | PNP |
|---|---|---|
| Output active state | Pulls to 0 V | Pulls to +24 V |
| PLC input type | Sinking | Sourcing |
| Common PLC brands | Mitsubishi, Fanuc, older Omron | Siemens, Schneider, Allen-Bradley |
| India market share | ~30% | ~70% |
| Our recommendation | If you know your PLC is sinking-input | Default choice — covers most modern PLCs |
For most new installations in India, PNP is the safer default. Siemens and Schneider dominate new panel builds, and both expect sourcing sensors.
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_Need help choosing between NPN and PNP for your specific application? Contact our technical team or WhatsApp us with your PLC model number — we will confirm the right output type in minutes._