Which statement describes PNP transistor behavior?

Which statement describes PNP transistor behavior?

• A. Provides an active low signal; PLC input voltage is high until sensor activated.
• B. Provides an active high signal; PLC input voltage is low until sensor activated.
• C. Requires an external resistor network.
• D. Only used with DC signals.

Answer: (B)

The essential idea is that a PNP transistor in a sensing setup sources current to the PLC input, which makes the input register a high (active) signal when the sensor is activated. When the sensor is not active, the transistor is off and no current flows into the input, so the PLC input sits at a low level. This behavior—active high on activation and low when idle—matches the statement that the signal is active high and the input voltage is low until the sensor is activated. In practice, PNP (source) configurations drive the input from the positive supply, contrasting with NPN (sink) configurations that pull the input toward ground. External resistor networks aren’t typically required because PLC input modules provide the necessary biasing, and while many systems use DC sensors, there are contexts where other input types exist as well; the key takeaway is that PNP sourcing produces a high signal when activated.

The essential idea is that a PNP transistor in a sensing setup sources current to the PLC input, which makes the input register a high (active) signal when the sensor is activated. When the sensor is not active, the transistor is off and no current flows into the input, so the PLC input sits at a low level. This behavior—active high on activation and low when idle—matches the statement that the signal is active high and the input voltage is low until the sensor is activated. In practice, PNP (source) configurations drive the input from the positive supply, contrasting with NPN (sink) configurations that pull the input toward ground. External resistor networks aren’t typically required because PLC input modules provide the necessary biasing, and while many systems use DC sensors, there are contexts where other input types exist as well; the key takeaway is that PNP sourcing produces a high signal when activated.