Sinking and Sourcing PLC Outputs Explained

Summary

This video provides a practical guide to PLC troubleshooting, emphasizing that most faults originate from field devices, wiring, and power supplies rather than the PLC program. It explains the concepts of sourcing and syncing current flow, the differences between solid state and dry relay output modules, and how to interpret status and fault LEDs for effective diagnostics. Viewers learn how solid state modules operate, how to safely measure voltages, and the importance of matching modules to load types. The video also highlights training resources to build hands-on skills, reduce downtime, and enhance team capability in industrial automation systems.

Takeaways

• 🔧 PLC troubleshooting should begin with field devices, wiring, and hardware, not the PLC program, as most faults originate there.
• ⚡ Syncing and sourcing describe the direction of conventional current flow: sourcing provides current, syncing receives current.
• 💡 Understanding which device is sourcing or syncing is crucial for correctly wiring and troubleshooting PLC outputs.
• 🔹 Solid state output modules use BJTs or FETs to switch loads electronically, requiring attention to current direction when matching with devices.
• 🔄 Dry relay output modules use mechanical contacts, can handle AC/DC loads, and can act as either sourcing or syncing outputs.
• 🔍 Status LEDs on modules help with troubleshooting: ST LEDs show command logic, FLT LEDs indicate faults like open loads or broken wires.
• 📏 Voltage readings on solid state modules can be misleading due to internal diagnostics; interpret carefully with knowledge of module type.
• ⚡ Sourcing modules connect the load to a supply voltage, while syncing modules connect the load to ground or power supply return.
• 🛠️ High-speed or high-current loads may require interposing relays or modules with built-in electronic fusing for safe operation.
• 📚 Familiarity with normal and fault conditions for each module type prevents misinterpretation of LEDs and voltages, reducing downtime.

Q & A

Why should PLC troubleshooting start with field devices and I/O modules rather than the PLC program?

Because 70–80% of faults originate from field devices, wiring, power supplies, and other related hardware, while PLC program faults are rare unless recent changes have been made.

What is the difference between sourcing and syncing in PLC outputs?

A sourcing device provides current to the load, while a syncing device receives current from the source. The direction of conventional current flow (positive to negative) determines which device is the source and which is the sync.

Why is conventional current flow used to describe sourcing and syncing instead of electron flow?

Conventional current terminology is used due to historical standards and tradition, established before the behavior of electrons was fully understood.

How does a solid-state output module differ from a dry relay module?

Solid-state modules use electronic components (BJTs or FETs) to switch voltage quickly and reliably, with current flow direction being important. Dry relay modules use mechanical contacts, can switch AC or DC regardless of polarity, and can act as either sourcing or syncing outputs.

What is the purpose of ST and FLT LEDs on the Allen Bradley 1756-OB16D module?

The ST LED indicates the PLC command logic status, showing whether the output is commanded on or off. The FLT LED indicates potential faults in the output circuit, such as open loads or broken wires.

How does the Allen Bradley 1756-V16E module operate differently from the 1756-OB16D?

The 1756-V16E is a syncing module with electronic fusing. It requires a 24V supply to power its internal electronics and syncs the load current to ground. The ST LED indicates command status, but the actual voltage depends on the load, not the module itself.

Why is measuring output voltage sometimes misleading on solid-state modules?

Because internal diagnostic electronics can show significant voltage on a high-impedance multimeter even when the solid-state switch is open or the load circuit is faulty, potentially leading to incorrect interpretations.

When is it necessary to use an interposing relay with solid-state output modules?

When the load draws more current than the module can safely handle or when the load does not require high-speed switching, interposing relays are used to protect the module and ensure reliable operation.

What are the key benefits of understanding syncing and sourcing for PLC troubleshooting?

Understanding syncing and sourcing helps identify the correct current flow, match modules with field devices, correctly interpret LED indicators and voltage readings, and avoid misdiagnosis during troubleshooting.

What learning resources are recommended for deeper understanding of PLC hardware and troubleshooting?

The video suggests RealPars Business for technical training, the PLC Hardware Fundamentals course for hands-on understanding of syncing and sourcing modules, and the Industrial Electrical Maintenance Course Series for learning about relays, multimeter use, and industrial systems.

How do fault indicators like FLT LEDs improve troubleshooting efficiency?

FLT LEDs help identify specific output faults, such as open loads or broken wires, without requiring extensive voltage measurements, making it faster and easier to isolate issues in the field.