In instrumentation wiring, what does it mean when a device is described as sourcing versus sinking?

• A. There is no practical difference.
• B. Sourcing devices supply current to the field device.
• C. Sourcing devices supply current to the field device; sinking devices draw current to return; important for wiring and power rails.
• D. Sinking devices supply current to the field device.

Answer: (C)

Understanding where current flows in relation to a device and its power supply is what “source” versus “sink” describes. A sourcing device pushes or provides current to the field device from the positive supply, and the current returns to the power source through the return path. A sinking device, on the other hand, pulls current through the field device toward the return path, effectively drawing current to the common/negative side.

This distinction matters for wiring and power rails because it tells you which side of the loop a device sits on and how the loop is completed. In a 24 V loop, a sourcing device would be on the supply side feeding current into the load, while a sinking device would be on the return side drawing current through the load toward ground. Matching the device type to the wiring ensures the current path is correct and within the device’s ratings, preventing no-current operation or damage.

So, a sourcing device supplies current to the field device; a sinking device draws current to the return.