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F2-04 THM Thermocouple module

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Hi Everybody, I am using a F2-04 THM thermocouple module in one of my applications where 4 J type Thermocouples are connected to it. I need to control the heaters depending on the Set point say 200 Deg Celsius. The temperature value in the PLC coming from the Thermocouples is not constant at all.The temperature goes on fluctuating continuously.It does not stop its fluctuation. But when I connect these Thermocouples to a Temperature Controller the Temperature display is stable and not fluctuating.It increases and decreases gradually but the Value in PLC is continuouly fluctuating. The fluctuation in the PLC is +/- 20 deg Celsius. Can anyone tell me what has to be done to get the Correct readings in the PLC without fluctuation. I have checked all the Jumpers are set correctly. The program is OK .

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Are you using grounded or ungrounded thermocouples?

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It can be normal mode noise from electrical hash makers like flourescent lights, VFD's; or induced from AC wiring. It can be common mode noise with a very low limit common mode limit of 5V. Direct from the Automation Direct F2-04THM manual, 7-8, 7-9 Ungrounded Thermocouple Assembly An ungrounded thermocouple is electrically isolated fromthe protective case. If the case is electrically grounded it provides a low-impedance path for electrical noise to travel. The ungrounded thermocouple provides a more stable and accurate measurement in a noisy environment. Use shielded thermocouples whenever possible tominimize the presence of noise on the thermocouple wire. Ground the shield wire at one end only. For grounded thermocouples, connect the shield at the sensor end. For ungrounded thermocouples, connect the shield to the 0V (common) terminal. The DL205 base has a switching type power supply. As a result of switching noise, you may notice some instability in the analog input data if you use the base power supply. If this is unacceptable, you should try one of the following: 1. Use a separate linear power supply. 2. Connect the 24VDC common to the frame ground, which is the screw terminal marked "G" on the base. Unused temperature inputs should be shorted together and connected to common. WARNING: A thermocouple can become shorted to a high voltage potential. Because common terminals are internally connected together, whatever voltage potential exists on one thermocouple will exist on the other channels. Use shielded wiring and ground the shield at the transmitter source. Do not ground the shield at both the module and the source. Do not run the signal wiring next to large motors, high current switches, or transformers. This may cause noise problems. If the T/C signal is not noisy when it is hanging in the air (not connected to where it should be), then you have noise source or path from the device you want to connect to. I'd disconnect T-C's to see only one has little or no noise, then keep adding until I find the noisy one or two. Check the other stuff.

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Thank you for your detailed information. I am using grounded Thermocouples. There is no other power cable running along with the Thermocouple cables. What do you mean by " For grounded thermocouples, connect the shield at the sensor end"? Do you want me to connect the Shield to the negative terminal of the Thermocouple at the Thermocouple itself? If it is so i have done it at the PLC and not at the Sensor end. Do you think this will solve the issue?

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