NPN and PNP sensors are widely used in industrial automation for detecting objects, positions, or other parameters. However, situations may arise where the output type of a sensor (NPN or PNP) does not match the PLC input requirements. This guide will explain how to convert an NPN sensor to behave like a PNP sensor, and vice versa, before integrating them into a PLC.
Understanding NPN and PNP Sensors
NPN Sensors:
- Known as “sinking” sensors.
- The output provides a connection to ground when the sensor is activated.
- The load is connected between the sensor’s output and the positive power supply.
- The output signal is low-level active.
PNP Sensors:
- Known as “sourcing” sensors.
- The output provides a connection to the positive power supply when activated.
- The load is connected between the sensor’s output and the ground.
- The output signal is high-level active.
Conversion Techniques
1. Converting NPN to PNP
If you have an NPN sensor but need a PNP signal for your PLC, you can achieve the conversion in the following ways:
Method 1: Using a Relay
- Connections:
- Connect the output of the NPN sensor to one end of the relay coil.
- Connect the other end of the relay coil to the positive power supply.
- Use a flyback diode across the relay coil to prevent voltage spikes.
- Use the relay’s normally open (NO) contacts to generate a PNP signal. One side of the contact should connect to the positive power supply, and the other side to the PLC input.
- Operation:
- When the NPN sensor is triggered, it activates the relay coil.
- The relay closes its NO contact, allowing current to flow from the power supply to the PLC input, mimicking a PNP signal.
- Pros and Cons:
- Advantage: Simple to implement.
- Disadvantage: Relays are mechanical and may introduce delay or wear over time.
Method 2: Using an NPN to PNP Conversion Module
- Setup:
- Many manufacturers offer pre-built signal conversion modules specifically designed to convert NPN outputs to PNP.
- Connect the NPN output to the input of the module, and the module’s output will provide a PNP signal.
- Pros and Cons:
- Advantage: Compact, reliable, and fast response time.
- Disadvantage: May increase cost slightly.
2. Converting PNP to NPN
For systems where a PNP sensor needs to mimic an NPN output, the following methods can be applied:
Method 1: Using a Relay
- Connections:
- Connect the output of the PNP sensor to one end of the relay coil.
- Connect the other end of the relay coil to the ground.
- Use a flyback diode across the relay coil to protect against voltage spikes.
- Use the relay’s normally open (NO) contact to generate an NPN signal. One side of the contact connects to the ground, and the other side to the PLC input.
- Operation:
- When the PNP sensor is triggered, it activates the relay coil.
- The relay closes its NO contact, allowing current to flow to ground, simulating an NPN signal.
- Pros and Cons:
- Advantage: Simple and widely understood.
- Disadvantage: Mechanical components may degrade over time.
Method 2: Using a PNP to NPN Conversion Module
- Setup:
- Use a pre-built PNP-to-NPN converter module.
- Connect the PNP output to the module’s input, and the module’s output will provide an NPN signal.
- Pros and Cons:
- Advantage: Reliable and fast.
- Disadvantage: May add to system cost.
Connecting the Sensor to a PLC
After converting the sensor signal, ensure the wiring matches the PLC’s input configuration:
NPN Signal to PLC:
- The PLC’s input common (COM) terminal should be connected to the positive power supply.
- The sensor’s output is connected to the PLC input terminal.
PNP Signal to PLC:
- The PLC’s input common (COM) terminal should be connected to ground.
- The sensor’s output is connected to the PLC input terminal.
Key Considerations
Voltage Compatibility:
- Ensure that the sensor, relay, conversion module, and PLC share the same voltage rating to prevent damage.
Signal Integrity:
- Use shielded cables to minimize electrical noise, especially in environments with high electromagnetic interference.
Speed Requirements:
- Relays may introduce delays due to their mechanical nature. For high-speed applications, solid-state conversion modules are preferred.
Load Current:
- Ensure the relay or module can handle the sensor’s output current and the PLC’s input requirements.
Conclusion
Converting between NPN and PNP sensors is a common requirement in automation projects, especially when integrating existing sensors into new systems. By using relays or dedicated conversion modules, the conversion process is straightforward and effective. Always consider the specific needs of your application, including cost, speed, and reliability, when selecting a conversion method.
For seamless PLC integration, follow wiring best practices and ensure all components in the system are compatible. This approach will ensure robust and reliable operation in your automation system.