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Sinking vs. Sourcing: PLC Inputs and Outputs Explained
Sourcing means a device supplies current to the circuit (it connects to +V). Sinking means a device receives current and passes it to 0V. Every DC input or output connection needs one of each: a sourcing device paired with a sinking device. Get the pair wrong and nothing switches.
The one rule to remember
Current flows (conventionally) from +V, through the circuit, to 0V. In any connection between two devices, one side provides that current and the other receives it:
- Sourcing device + sinking device = works.
- Sourcing + sourcing, or sinking + sinking = no current path, does not work.
PLC inputs
| Input type | Input common connects to | Field device must be | Sensor type |
|---|---|---|---|
| Sinking input | 0V | Sourcing | PNP |
| Sourcing input | +24 V | Sinking | NPN |
A sinking input has its common tied to 0V. The sensor provides +24 V to the input terminal when it switches on, and current flows into the input and out through the common. That is why PNP (sourcing) sensors pair with sinking inputs. See PNP vs. NPN sensors.
A sourcing input has its common tied to +24 V. The input provides the current, and an NPN (sinking) sensor completes the path to 0V.
Dry contacts such as limit switches, pushbuttons and relay contacts work with either: they just pass whatever current the circuit provides. Wire the switch to the opposite rail from the input common.
PLC outputs
| Output type | What it switches | Load connects between |
|---|---|---|
| Sourcing (PNP transistor) | +V to the load | Output terminal and 0V |
| Sinking (NPN transistor) | Load to 0V | +V and output terminal |
| Relay | A dry contact | Either way, AC or DC |
Transistor outputs are fast and long-lived but DC only. Relay outputs switch AC or DC and higher currents but are slower and wear over time. Many Unitronics controllers are offered in transistor, relay or mixed versions, for example the V130-33-R34 with 12 relay outputs.
Why the labels are confusing
Manufacturers label inputs two opposite ways:
- By what the input does: "sinking input" (accepts PNP devices).
- By the device it accepts: "PNP input" or "pnp (source)", which means the same thing.
So "sourcing" on one datasheet can describe the same input another calls "sinking." Ignore the words and look at the wiring diagram: where does the input common go, 0V or +V? Common to 0V needs PNP devices. Common to +V needs NPN devices.
Which should you use?
In North America and Europe, sinking inputs with PNP sensors and sourcing outputs are the usual choice. A short circuit from a wire to grounded metal then tends to turn a device off or blow a fuse rather than switch something on unexpectedly. Asian-built machines often use the opposite. On an existing machine, match what is already there so spares stay interchangeable.
Many controllers let you choose. On a number of Unitronics Vision and Jazz models, a group of inputs can be set to accept PNP or NPN devices by wiring or a jumper; check the installation guide for the exact model.
Quick troubleshooting
- Sensor LED on, PLC input off? Check that the input common is on the opposite rail from what the sensor switches.
- Measure between the input terminal and the input common. With the device ON you should read close to 24 V; if you read 0 V, the pairing is wrong or the 0V references are not shared.
- Output on but load not running? Confirm the load's other side goes to 0V for a sourcing output, or +V for a sinking output.
Frequently asked questions
Is PNP sinking or sourcing?
A PNP output is sourcing. It connects to a sinking input.
Can one PLC card mix PNP and NPN sensors?
Only if the inputs are in separate groups with separate commons. Inputs sharing one common must all be the same type.
Do AC inputs have sinking and sourcing?
No. The terms apply to DC circuits.
Need parts?
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