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Replacing an NPN Sensor with a Dual-Output Sensor: Ensuring Consistent Output Behavior

Article No: KA-09897
Version: 1.0
Subject to change without notice
Related Products
WLG4S-3N1332 RETRO REFL PHOTOEL.SW.
WLG4SP-1G311130ZZZZ2ZZZZZZZZZ1

Table of Contents

Replacing an NPN Sensor with a Dual-Output Sensor: Ensuring Consistent Output Behavior

 
In industrial automation, replacing a sensor requires careful consideration to ensure that the new device maintains the same functionality as its predecessor. This article explores the replacement of the WLG4S-3N1332, an NPN photoelectric sensor from SICK, with the WLG4SP-1G311130ZZZ, a dual-output (PNP/NPN) sensor from the same manufacturer, focusing on matching the original sensor's output behavior.
The Original Sensor: WLG4S-3N1332
The WLG4S-3N1332 is part of SICK’s W4 series of photoelectric sensors. It features an NPN output with a non-negated Q signal. In this context, "non-negated Q" means the output directly reflects the sensor’s detection state:
  • Active (detecting an object): The output is low (sinking current to ground).
  • Inactive (no detection): The output is high (open or pulled up by an external resistor).
This behavior is typical for NPN sensors in applications requiring a straightforward "active low" output logic.
The Replacement Sensor: WLG4SP-1G311130ZZZ
The WLG4SP-1G311130ZZZ is a more versatile sensor, offering both PNP and NPN output options. However, its output is specified with respect to the PNP configuration, where it is described as negated. For a PNP output, "negated" implies:
  • Active (detecting an object): The output is low (not sourcing current).
  • Inactive (no detection): The output is high (sourcing current).
This is the inverse of standard PNP behavior, where the output is typically high when active. However, the key detail for this replacement scenario is that in NPN mode, the output behaves like the original sensor. That is, when configured as an NPN output, the WLG4SP-1G311130ZZZ provides a non-negated Q signal, matching the WLG4S-3N1332:
  • Active: Output low.
  • Inactive: Output high.
 
Replacement Guidance
To replace the WLG4S-3N1332 with the WLG4SP-1G311130ZZZ while preserving the original system’s logic, engineers should:
  1. Configure the sensor for NPN operation. This typically involves wiring the output to the load, with the load connected to the positive supply voltage. When the sensor detects an object, it sinks current, pulling the output low—consistent with the original sensor’s behavior.
  2. Avoid PNP mode for this application. Using the sensor in PNP mode would result in a negated output (active low, inactive high), inverting the logic compared to the original NPN sensor.
 
Key Considerations
  • Wiring: Ensure the output is connected appropriately for NPN operation, as the sensor’s dual-output capability may depend on how it is wired (e.g., to a pull-up resistor for NPN).
  • PNP Mode Warning: If PNP operation is inadvertently used, the negated output will require adjustments to the system logic or additional circuitry to match the original behavior.
Conclusion
The WLG4SP-1G311130ZZZ, when used in NPN mode, serves as a seamless replacement for the WLG4S-3N1332, replicating its non-negated Q output behavior. This ensures continuity in applications where the original sensor’s logic is critical, making it a practical upgrade option for engineers seeking flexibility without sacrificing compatibility.
This article provides a clear, concise guide for application engineers, focusing on the output behavior and practical steps for replacement.
Keywords:
npn, pnp, output
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