Differences Between NPN and PNP Sensors and Their Applications in Safety Light Curtains
NPN and PNP sensors are the two main types of switching sensors, and their core operating principles are based on the saturation and cutoff states of transistors. The opposite polarity of their output signals is particularly important in applications such as safety light curtains.
Differences Between NPN and PNP Sensors
Current Direction and Voltage Polarity
◾ NPN Sensors:
The base (B) to emitter (E) current (IB) controls the collector (C) to emitter current (IC). Typically, the collector voltage is higher than the base and emitter voltages (VC > VB > VE). When the output is low, the common terminal of the NPN sensor connects to the 24V power supply. This means the input signal is at the same potential (24V) without a voltage difference. Only when the input signal is 0V will a voltage difference occur, allowing the PLC to detect the input.
◾ PNP Sensors:
Current flows from the emitter (E) to the base (B). However, in semiconductor theory, this means external current is injected into the base. This controls the reverse current (IC) from the collector to the emitter. The emitter voltage is usually higher than the base and collector. When the output is high, the common terminal of the PNP sensor connects to the 0V of the 24V power supply, meaning a voltage difference is detectable only when the input signal reaches approximately 24V.
Type of Charge Carriers
◾ NPN Transistors: Majority carriers are positive holes.
◾ PNP Transistors: Majority carriers are negative electrons.
Output State
◾ NPN Sensors: Outputs low-level (0).
◾ PNP Sensors: Outputs high-level (1).
Applications in Safety Light Curtains
Safety light curtains are photoelectric protection devices designed to detect and prevent people or objects from entering hazardous areas. The choice of NPN or PNP sensors depends on the electrical system configuration and safety requirements.
Applications of NPN Sensors in Safety Light Curtains
When a safety light curtain detects a hazardous object:
◾ NPN Sensors Output Low-Level Signals, indicating the safety status is broken.
1️⃣ Normally Open (NPN-NO): Outputs are disconnected (high impedance or floating) when there’s no signal, and outputs low-level signals when triggered. Suitable for scenarios requiring low-level signals to activate subsequent control circuits.
2️⃣ Normally Closed (NPN-NC): Outputs low-level signals when inactive and disconnects when triggered. Suitable for maintaining safety with a low-level signal; when an object is detected, it cuts power or triggers other safety measures.
Applications of PNP Sensors in Safety Light Curtains
When a safety light curtain detects a hazardous object:
◾ PNP Sensors Output High-Level Signals, indicating the safety status is broken.
1️⃣ Normally Open (PNP-NO): Outputs are disconnected when there’s no signal and outputs high-level signals when triggered. Suitable for scenarios requiring high-level signals to activate subsequent control circuits.
2️⃣ Normally Closed (PNP-NC): Outputs high-level signals when inactive and disconnects when triggered. Suitable for maintaining safety with high-level signals; when an object is detected, it cuts power or triggers other safety measures.
Selection Recommendations
When selecting NPN or PNP sensors, consider the following factors:
◾ Electrical System Configuration: Ensure the output signals of the sensor match the input requirements of the electrical system.
◾ Safety Requirements: Choose the appropriate sensor type (normally open or normally closed) based on specific application safety needs.
◾ Environmental Adaptability: Assess the sensor’s anti-interference capability, stability, and ability to operate in harsh environments.
