Sinking and sourcing, settled
Two words that describe which way current flows, and a reliable way to stop guessing which one you need.
Short answer
Sourcing supplies current, sinking accepts it. A sourcing output pushes current into a sinking input, and a sinking output pulls current from a sourcing input. The pairing rule is the only thing you have to remember: a sourcing device must connect to a sinking one, never to another sourcing device.
Sourcing supplies current. Sinking accepts it. Everything else follows from that sentence, including which sensor works with which card.
The pairing rule
A sourcing device must connect to a sinking device. A sourcing sensor pushes current, so it needs an input that will accept it. A sinking sensor pulls current, so it needs an input that will supply it.
Two sourcing devices connected together do nothing at all, because nothing completes the circuit.
Which is which
PNP is sourcing. NPN is sinking. That is the pairing most people memorise, and it is enough for the majority of sensor selections.
A useful check with no jargon in it: with the sensor active, does the signal wire sit near 24 V or near 0 V? Near 24 V means it is sourcing. Near 0 V means it is sinking.
Why both exist
Convention, mostly regional. PNP dominates in Europe and on Siemens equipment; NPN is more common on Japanese machinery. Neither is safer or faster.
There is one practical argument for PNP: with a sinking input card, a short to ground on the signal wire looks exactly like a switched-on sensor. With sourcing, it looks like nothing.
When they do not match
An interposing relay solves it for a handful of points. For more than a few, change the card. Wiring around a mismatch with pull-up resistors works on a bench and fails in a cabinet at 40 degrees.
Common questions
- Is PNP sinking or sourcing?
- A PNP sensor is sourcing: it supplies positive current to the input when it activates. An NPN sensor is sinking: it pulls the input down to common. PNP is the European convention and dominates on Siemens systems; NPN is more common on Japanese equipment.
- Can I connect a sourcing sensor to a sourcing input?
- No. Both would be trying to supply current and neither would accept it, so the circuit never completes and the input never turns on. A sourcing device must pair with a sinking one. If a sensor and a card do not match, you need an interposing relay or a different card.
Keep reading
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Two standards, two scales, and one machine. Which one applies, how they map to each other, and why the answer is usually ISO 13849.
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Five architectures, what a single fault does to each, and the practical wiring that goes with them.
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Safety relay or safety PLC: how to decide
One is a wiring decision, the other is a programming one. The count of safety functions, not the size of the machine, is what settles it.