Are you interested in learning how to wire a thermocouple to a PLC analog input module and eliminate the challenges associated with temperature readings? Look no further! In this article, we will guide you through the process, ensuring precise measurements without any unwanted voltages.
Contents
Understanding the Basics of a Thermocouple
A thermocouple consists of two dissimilar metal wires joined together at one end, forming the Measurement Junction. When heat is applied to the Measurement Junction, a voltage is produced across the open end. Different combinations of metal wire pairs have been assigned letters to indicate their Type, such as Type K, composed of alloys Chromel and Alumel. Thermocouple Tables provide voltage values at specific temperature points for these combinations.
Overcoming Challenges in Temperature Measurement
Although measuring the open-end voltage seems straightforward, it is not without challenges. The voltages produced are small, and the change in voltage per degree Celsius is minuscule. Additionally, connecting a measuring device creates new junctions and unwanted voltages.
Wiring a Thermocouple to a PLC Analog Input Module
To ensure accurate temperature readings, most PLC vendors offer specialized Thermocouple modules. These modules include Cold Junction Compensation, which eliminates the impact of new junctions. Let’s take a closer look at a typical Thermocouple module, such as the Siemens 6ES7 531-7PF00-0AB0, capable of connecting eight thermocouples.
Understanding Cold Junction Compensation
Different options for Cold Junction Compensation exist due to the varying locations of the cold junction, depending on the application and thermocouple placement. Whenever a thermocouple wire connects to a different material, a new junction is created, further emphasizing the need for compensation.
Wiring Scenarios
In one scenario, where the thermocouple is directly connected to the module, the cold junction is at the module’s terminals. The module itself includes an internal sensor for compensation. The Thermocouple Type and cold junction source are configured through the Siemens STEP 7 software program.
In another scenario, where the thermocouple is located at a distance from the PLC, a thermocouple extension wire can be used. Standard copper wires would create new junctions and unwanted voltages, but with thermocouple extension wires made of the same material as the thermocouple, this issue is eliminated. Using thermocouple termination blocks further ensures no new junctions are created.
Finding the Right Wiring Option
The best resource for determining the appropriate wiring option is your module’s manual. The manual provides detailed compensation options and wiring diagrams for different scenarios. If you want to delve deeper into the subject, you can also explore other informative videos on thermocouples, temperature sensors, and temperature transmitters.
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In conclusion, wiring a thermocouple to a PLC analog input module requires careful consideration of Cold Junction Compensation and the elimination of unwanted voltages. By understanding the principles behind thermocouples and utilizing specialized modules, you can achieve accurate temperature readings, paving the way for your success in the field of PLC programming.
