Which statement describes NPN transistor behavior?

Which statement describes NPN transistor behavior?

• A. NPN provides an active high signal; PLC input voltage is high when activated.
• B. When the sensor is off, the voltage at the sensor's output terminal is high. When the transistor is activated, the output terminal voltage goes to zero. The transistor conducts current to the PLC just like a switch. Due to this NPN transistors must be used with ACTIVE LOW PLCs.
• C. NPN outputs source current to PLC at high state.
• D. NPN is never used with PLCs.

Answer: (B)

NPN transistors in PLC circuits are used as sinking switches. When the base is driven, the transistor conducts from the collector to the emitter (to ground), so it pulls the connected load and the PLC input toward 0 V. In a typical setup, the input line is pulled up to +24 V when the transistor is off, so the PLC input reads high. When the transistor turns on, it sinks current and the input voltage drops to near zero, giving a low signal. This is why the description says the output starts high and goes to zero when activated, and why NPN behavior is described as a switch that sinks current. The “active low” phrasing is about the PLC interpreting a low voltage as the active state in this sinking configuration. The other statements conflict with how NPNs operate: they don’t source current to the PLC at high state, they aren’t never used with PLCs, and they don’t inherently provide an active-high signal.

NPN transistors in PLC circuits are used as sinking switches. When the base is driven, the transistor conducts from the collector to the emitter (to ground), so it pulls the connected load and the PLC input toward 0 V. In a typical setup, the input line is pulled up to +24 V when the transistor is off, so the PLC input reads high. When the transistor turns on, it sinks current and the input voltage drops to near zero, giving a low signal. This is why the description says the output starts high and goes to zero when activated, and why NPN behavior is described as a switch that sinks current. The “active low” phrasing is about the PLC interpreting a low voltage as the active state in this sinking configuration. The other statements conflict with how NPNs operate: they don’t source current to the PLC at high state, they aren’t never used with PLCs, and they don’t inherently provide an active-high signal.