Thermocouple-preamplifier

Description: The MAX420 is operated at a gain of 191 to convert the 52 µV/°C output of the type J thermocouple to a 10 mV/°C signal. The -2.2 mV/°C temperature coefficient of the 2N3904 is added into the summing junction with a gain of 42.2 to provide cold-junction compensation. The ICL8069 is used to remove the offset caused by the 600 mV initial voltage of the 2N3904. Adjust the 10 kΩ trimpot for the proper reading with the 2N3904 and isothermal connection block at a temperature near the center of the circuit's operating range. Use the component values shown in parentheses when using a type K thermocouple.

The MAX420 operational amplifier is configured to achieve a high gain of 191, effectively amplifying the minute output from a type J thermocouple, which typically generates a signal of 52 µV/°C. This amplification results in a more manageable output of 10 mV/°C, suitable for further processing or display. To ensure accurate temperature readings, cold-junction compensation is implemented by incorporating the 2N3904 transistor, which has a temperature coefficient of -2.2 mV/°C. This compensation is realized through a summing junction where the output from the 2N3904 is amplified by a gain of 42.2, effectively adjusting for temperature variations at the connection point.

The ICL8069 operational amplifier is utilized to eliminate any offset voltage that may arise from the inherent 600 mV initial voltage present at the output of the 2N3904. This step is crucial for maintaining the accuracy of the temperature measurement by ensuring that the output reflects true temperature changes rather than being skewed by initial voltage offsets.

For calibration purposes, a 10 kΩ trimpot is included in the circuit design, allowing for fine-tuning of the output signal. This adjustment should be performed with the 2N3904 and the isothermal connection block at a temperature close to the midpoint of the circuit's operational range, ensuring that the readings remain accurate across the expected temperature spectrum. It is important to note that when employing a type K thermocouple, specific component values, as indicated in the design schematic, should be utilized to ensure optimal performance and accuracy in temperature measurement.The MAX420 is operated at a gain of 191 to convert the 52 p.VI o C output of the type J thermocouple to a 10 mV/°C signal. The -2.2 mV/°C tempco of the 2N3904 is added into the summing junction with a gain of 42.2 to provide cold-junction compensation.

The ICL8069 is used to remove the offset caused by the 600-m V initial voltage of the 2N3904. Adjust the 10-KO trimpot for the proper reading with the 2N3904 and isothermal connection block at a temperature near the center of the circuit" s operating range. Use the component values shown in parentheses when using a type K thermocouple.