How to Connect Different Types of Temperature Sensors to a PLC

Temperature plays a critical role in industrial automation, influencing process stability, product quality, and equipment safety. However, many engineers — especially beginners — often find it difficult to differentiate sensor types and choose the correct PLC wiring method. Improper wiring or module selection can result in inaccurate readings or system faults.

This guide summarizes the main temperature sensor types and explains how to correctly interface them with PLC systems.

1. Understanding Temperature Sensor Output Types

Before wiring any temperature sensor to a PLC, you must know the signal type. Signal type determines the required PLC input module and wiring method.

1.1 Analog Signals

Analog sensors output continuous values and are widely used in automation.

Common forms:

• Current (4–20 mA)
• Voltage (0–5 V or 0–10 V)

These connect to the PLC’s standard Analog Input (AI) module.

1.2 Resistance Signals (RTD, Thermistor)

RTDs (such as Pt100) and thermistors output a change in resistance, not voltage.

PLC requirement:

• A dedicated RTD module, or
• A signal transmitter to convert resistance into a standard 4–20 mA current loop.

1.3 Millivolt Signals (Thermocouples)

Thermocouples output very small voltage signals and require:

• A thermocouple input module
• Proper compensation cable
• Cold Junction Compensation (CJC)

1.4 Digital Output (Temperature Switches)

These sensors output ON or OFF states rather than actual temperature.

Types:

• PNP (sourcing output)
• NPN (sinking output)

They connect to the PLC’s digital input module.

1.5 Bus / Fieldbus Signals

Smart sensors transmit temperature digitally using protocols such as:

• Modbus
• PROFINET
• PROFIBUS
• EtherNet/IP
• IO-Link

These connect to the PLC’s communication port or network module.

1.6 Wireless Signals

Wireless temperature sensors use:

• Wi-Fi
• LoRa
• Zigbee
• BLE
• NB-IoT

A wireless gateway is needed to convert the wireless data into a PLC-supported protocol.

2. How to Connect Different Temperature Sensors to a PLC

2.1 Standard Analog Sensors (4–20 mA / 0–10 V)

4–20 mA

• Two-wire loop
• One wire to AI+, one to AI–
• Power and signal share the same loop
• Best for long-distance transmission

0–10 V

• Three-wire system
• Requires separate power supply
• One wire for +V, one for GND, one for Vout to PLC

2.2 RTD Sensors (Pt100, Pt1000)

RTD connection requirements:

• Use a PLC RTD module or
• Install an RTD transmitter to output 4–20 mA

Wiring options:

• Two-wire: simplest but least accurate
• Three-wire: most common, compensates for lead resistance
• Four-wire: highest accuracy

2.3 Thermocouples

Connection practices:

• Must use compensation cable matching the thermocouple type
• Requires thermocouple input module
• Ensure proper cold junction compensation

2.4 Digital Temperature Switches

Connection steps:

• Output connects to PLC DI
• Follow correct wiring based on sensor type
• PNP: output provides +24V
• NPN: output switches to 0V

Always confirm sensor output with PLC input module type.

2.5 Fieldbus Temperature Sensors

Integration steps:

• Connect the sensor to the PLC’s communication port or bus coupler
• Configure the network in PLC software
• Map temperature data to PLC memory addresses
• Use a protocol gateway if PLC and sensor protocols differ

2.6 Wireless Temperature Sensors

PLCs cannot directly decode Wi-Fi, LoRa, or Zigbee signals. Connect via:

• Wireless IoT gateway
• Industrial protocol converter

The gateway converts wireless data into Modbus TCP, MQTT, or another PLC-supported format.

3. Multi-Sensor Integration in Industrial Systems

As temperature monitoring points increase, traditional point-to-point wiring becomes inefficient. Modern automation systems use the following methods:

3.1 Distributed I/O (Remote I/O)

Remote I/O modules:

• Installed close to field sensors
• Collect signals locally
• Send aggregated data to PLC via Modbus TCP, PROFINET, or EtherNet/IP
• Reduce wiring complexitySupport mixed signal types

3.2 Wireless Network Deployment

Useful for:

• Moving machinery
• Large warehouses
• Tank farms
• Long-distance or hard-to-install locations

Industrial wireless APs or bridges provide reliable temperature data backhaul for PLC.

3.3 Centralized Network Management

Managed industrial switches provide:

• VLAN segmentation for sensor networks
• QoS prioritization for alarm data
• Port monitoring for quick troubleshooting
• Topology visualization tools for maintenance

3.4 PLC-to-SCADA or Cloud Integration

Temperature data collected by PLCs may need to be uploaded to:

• SCADA
• MES
• Cloud platforms

Industrial gateways convert PLC data to the required protocols for upper-level systems, supporting Ethernet, cellular, and Wi-Fi uplinks.

Conclusion

Choosing the correct temperature sensor and matching PLC input module is essential for reliable measurement. Whether using analog, RTD, thermocouple, fieldbus, or wireless sensors, proper wiring and module selection ensure data accuracy and system stability.

Modern factories often involve many distributed temperature points. Using remote I/O, wireless solutions, industrial switches, and protocol gateways provides a flexible, scalable, and reliable architecture for industrial temperature monitoring.

Source: https://www.come-star.com/blog/how-to-connect-temperature-sensor-to-plc/