FAQ
TL;DR: Most 12 V car relays draw under 150 mA [Elektroda, Andy74, post #6520930] “control the voltage (24 V)” [Elektroda, Artik_b, post #5806235] Pick PNP when your control side expects +24 V, NPN when it expects 0 V; both switch relays if the coil current is within the sensor’s rating.
Why it matters: Correct polarity avoids false triggers, blown inputs, and hours of troubleshooting in mixed-origin machines.
Quick Facts
• Rated sensing distance for M12 inductive switches: 4 mm on steel [Elektroda, weld, post #5478197]
• Typical sensor load current: 100–200 mA; many car relays ≤ 150 mA [Elektroda, dj-brylant #6520385; Andy74 #6520930].
• EU PLC inputs are usually PNP (sourcing) while many Japanese PLCs are NPN (sinking) [Elektroda, pcichomski, post #6177252]
• NO outputs give logic 1 only when the target is present; NC provides fail-safe detection of cable breaks [Elektroda, Artik_b, post #5806235]
• Edge-case: sensors can fail open or short, so critical axes use redundant devices [Elektroda, wd40, post #5811642]
What’s the basic difference between NPN and PNP inductive sensors?
Both use a transistor switch. An NPN sensor sinks current to 0 V when active; a PNP sensor sources +24 V when active. The load must be wired to the opposite rail [Elektroda, CervantesJ., post #5478297]
Will the sensor type affect switching a 24 V DC relay?
No, provided the relay coil current is below the sensor’s output rating and you wire the polarity correctly [Elektroda, CervantesJ., post #5478297]
How do I wire a PNP NO sensor to a relay coil?
- Connect brown to +24 V.
- Place the relay coil between black (output) and 0 V.
- Add a reverse diode across the coil (band to +24 V) to suppress flyback [Elektroda, Andy74, post #6520930]
How do I wire an NPN NO sensor?
Swap the rails: brown to +24 V, coil between +24 V and black, diode reversed. The sensor shorts black to 0 V when metal is detected [Elektroda, CervantesJ., post #5478297]
Can I mix PNP and NPN sensors on the same PLC?
Yes if the PLC has separate COM terminals. Universal input cards like Omron ID212 let you set one module PNP and another NPN by tying COM to 0 V or +24 V [Elektroda, pcichomski, post #6179908]
What happens if I plug an NPN sensor into a PNP-only input?
Nothing dangerous—inputs are protected—but the signal will never reach the threshold, so the PLC stays off [Elektroda, pcichomski, post #6170917]
Why do European machines prefer PNP while Asian machines use NPN?
EU wiring standards treat 0 V as reference and switch the positive rail. Japanese practice evolved from 120 V systems where grounding the input felt safer [Elektroda, pcichomski, post #6165927]
Should I choose NO or NC contacts for limit switches?
NC is safer for end-stops because a broken wire looks like an active alarm; NO works for general presence detection [Elektroda, Artik_b, post #5806235]
What load current can typical inductive sensors handle?
Most 3-wire DC sensors supply 100–300 mA; one user cited a 200 mA model [Elektroda, dj-brylant, post #6520385] Always check the datasheet before wiring multiple coils.
How do I detect sensor failure during operation?
Add redundant devices or compare time-outs. A broken cable opens an NC loop; a short shows constant ON. "Complementary detectors are used in critical points" [Elektroda, wd40, post #5811642]
Can a 24 V sensor drive a 12 V car relay?
Yes if the coil draws ≤ 150 mA and the supply matches both devices; ensure the sensor’s minimum output voltage still energizes the 12 V coil [Elektroda, Andy74, post #6520930]
What’s an edge-case I should watch for?
If the sensor transistor shorts, an NO device may stay permanently ON, bypassing safety logic. Redundancy or a secondary mechanical switch catches this fault [Elektroda, Mellon31, post #5814814]
