Switching Configuration
Transistorized sinking circuits connect the electrical load of a sensor to the negative return rail of the power supply. Choosing an npn output allows the sensor to act as a low-side switch that completes the circuit for an external device. This design relies on a bipolar junction transistor where the collector is tied to the output terminal and the emitter is connected to the common ground.
Circuit Behavior
Conduction begins when the internal sensor logic drives a small current into the transistor base. In this active state, the npn output pulls the output line down to a low voltage close to ground, typically less than one volt. When the base current is removed, the transistor turns off and the output line rises to the voltage of the external pull-up resistor.
This transition creates a clear digital low signal for downstream controllers.
Signal Compatibility
Interfacing with programmable logic controllers requires matching the input card configuration to the sensor drive method. Sinking sensors with an npn output must connect to sourcing input modules that provide the necessary current source. This configuration prevents false triggering from ground loops because the load and the sensor share a common reference point.
Testing involves verifying the leakage current in the off state to ensure it falls below the off-state threshold of the controller.
Protection Mechanism
Current-limiting resistors and flyback diodes protect the switching transistor from over-current and inductive voltage spikes. If the output is short-circuited to the positive supply rail, the excessive current can cause thermal destruction within microseconds.
