NPN transistor output sensors are described as which type of current devices?

• A. Sinking
• B. Sourcing
• C. Floating
• D. Bidirectional

Answer: (A)

NPN transistor output sensors pull current toward ground when they activate, so they are current-sinking devices. In a typical setup, the load is connected to a supply (like +24 V), and the sensor output connects the load to ground through the NPN transistor. When the sensor is on, the transistor conducts, completing the circuit and allowing current to flow from the supply, through the load, into the sensor, and down to ground. That pulling-down action is what we mean by sinking.

Sourcing would be the opposite behavior, where the device provides current from the supply to the load (using a PNP output). Floating means the output is not connected to anything, and bidirectional implies current can flow in both directions, which isn’t how a single-output NPN sensor operates.