Sinking Sourcing – SS Automation and Controls

Overview

Sinking and sourcing is one of the most confusing topics in industrial wiring — and one of the most important to get right. Wire it wrong and you get phantom inputs, outputs that won't turn on, or worse, damaged modules. This guide explains it in plain language with practical wiring examples.

The Core Concept

The Simple Version

Every DC circuit needs a path: current flows from +24V, through the device, and returns to 0V (ground). The question is which side the PLC I/O module provides.

Sourcing = The module provides the positive (+24V) side. Current flows OUT of the module.

Sinking = The module provides the ground (0V) return path. Current flows INTO the module.

The Water Analogy

Sourcing output: The PLC is the faucet (source). Water flows out of the PLC, through the device, and drains to ground (0V).

Sinking output: The PLC is the drain (sink). Water flows from the supply (+24V), through the device, and the PLC pulls it down to ground.

The device gets energized either way — the water still flows through the pipe. The difference is which end the PLC controls.

Inputs: Sinking vs Sourcing

Sourcing Input Module

The input module sources current to the field device:

  PLC Input Module          Field Device (Switch/Sensor)
  ┌──────────┐              ┌──────────┐
  │          ├──(+24V out)──┤          │
  │ SOURCING │              │  Switch  │
  │  INPUT   │              │          │
  │          ├──────────────┤          ├──── 0V (Common)
  └──────────┘              └──────────┘

Current path: Module → through device → to 0V

The module provides +24V, the field device switches the path to ground. Field device connects between the input terminal and 0V.

Sinking Input Module

The input module sinks current from the field device:

  +24V Supply ──────────────┐
                             │
  PLC Input Module          Field Device (Switch/Sensor)
  ┌──────────┐              ┌──────────┐
  │          ├──────────────┤          │
  │ SINKING  │              │  Switch  │
  │  INPUT   │              │          │
  │          ├──(to 0V)     └──────────┘
  └──────────┘

Current path: +24V → through device → into module → to 0V

The field device switches +24V into the module. Field device connects between +24V and the input terminal.

Outputs: Sinking vs Sourcing

Sourcing Output (Most Common)

The output module switches +24V to the load. When the output turns ON, it connects +24V to the load terminal. The load's other wire goes to 0V.

+24V ──── PLC Output ────(+24V switched)──── Load ──── 0V

Sinking Output

The output module switches the ground path. When the output turns ON, it connects the load terminal to 0V. The load's other wire goes to +24V.

+24V ──── Load ──── PLC Output ────(switched to 0V)

Matching Sensors to Inputs

Sensor Type	Sensor Provides	Needs Input Type
PNP (sourcing)	+24V when active	Sinking input
NPN (sinking)	0V path when active	Sourcing input
2-wire	Either	Either (check spec)

The golden rule: The sensor and the input module must be opposite types. One sources, the other sinks. If both source or both sink, the circuit doesn't work.

How to Identify What You Have

Check the Module Spec Sheet

Allen-Bradley examples: 1769-IQ16 = 16-point sinking input (most common). 5069-IB16F = 16-point input, sinking/sourcing configurable.

Check the Wiring Diagram

Look at the common terminal on the module. Common labeled "COM" or "0V" = likely sinking output or sourcing input. Common labeled "+V" or "24V" = likely sourcing output or sinking input.

The LED Test

If you have an unknown module with a sensor connected: disconnect the sensor wire, touch it briefly to +24V — if the LED turns on, it's a sinking input. Touch it to 0V — if the LED turns on, it's a sourcing input. Use caution and verify voltage levels first.

Common Mistakes

Mistake #1: NPN Sensor on a Sinking Input

Both the sensor and the input are trying to provide the ground path. No current flows. The input never turns on.

Fix

Use a sourcing input, or replace the sensor with PNP.

Mistake #2: Phantom/Ghost Inputs

Inputs turning on when they shouldn't — especially when nearby outputs energize.

Fix

Usually induced voltage on unshielded signal wires running alongside power cables. Use shielded cable, separate signal and power runs, and add input filters if available.

Mistake #3: Forgetting the Common

A sourcing output module with no 0V common wired = nothing works, even though the PLC shows the output ON.

Fix

Always wire the common terminal. Check if the module has one common for all points or grouped commons.

Mistake #4: Mixing Types on the Same Module

If the module has a single common terminal for all points, all points must be the same type.

Fix

Check the module's spec sheet. If you need both types, use separate modules.

Quick Reference

Scenario	Use This
PNP sensors (most modern sensors)	Sinking input module
NPN sensors (older / Asian market)	Sourcing input module
Driving relay coils	Sourcing output (most common)
Driving indicator lights	Sourcing output
Connecting to another PLC's input	Match to that input's type
Not sure / new install	PNP sensors + sinking inputs (industry standard)

Pro Tips

Standardize on PNP + sinking inputs — This is the most common configuration in North America and Europe. +24V on the signal wire = sensor active, which simplifies troubleshooting.
Label your commons — At the terminal blocks, clearly label which commons are +24V and which are 0V.
Use different wire colors — +24V = red, 0V = blue, signal = white. Whatever your standard, be consistent across the entire machine.
Check before you connect — A multimeter takes 10 seconds. Connecting an NPN sensor to the wrong input type won't damage anything, but connecting a sourcing output to +24V instead of 0V on the load side can.

Still Confused?

Sinking and sourcing trips up even experienced electricians. If you're designing a new system or troubleshooting phantom signals, we can help sort it out fast.

Get Wiring Help Call (615) 854-2420

Sinking vs Sourcing: I/O Wiring Explained