What is an RTD?
RTD (Resistance Temperature Detector) is a temperature sensor that works based on the principle that the resistance of a metal increases with temperature.
Example:
A common RTD is Pt100 - it has 100 ohms resistance at 0°C.
? Why do we use an RTD module in a PLC system?
You cannot directly connect an RTD sensor to a PLC because:
RTD gives resistance as output, but PLC analog inputs usually read voltage (0-10V) or current (4-20mA).
RTD signals are small and need special handling.
That's why we use an RTD Input Module with the PLC.
How does the RTD Module work?
1. The RTD module sends a small, constant current into the RTD sensor.
2. Based on the temperature, the RTD sensor produces a voltage drop.
3. Using Ohm's Law (V = IR), the module calculates the resistance.
4. The module converts the resistance into a temperature value.
5. This temperature is then sent to the PLC, either as an analog signal or a digital value.
▲ Types of RTD Wiring
Type
Accuracy
Use
2-wire
Least accurate
Simple applications
3-wire
Industrial standard
Most commonly used
4-wire
Most accurate
Labs or critical systems
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