Thermocouple module with wire resistance compensation

摘要

An interface circuit (27) is provided for an Input/Output (I/O) module (24) in an industrial controller (12) to compensate for voltage generated by a bias current in a thermocouple (26). During a calibration routine (100), the interface circuit (27) supplies two known bias currents to a thermocouple (26) and measures a voltage generated across the thermocouple (26) as a result of each bias current. The measured voltages and known current values are used to determine the resistance value of the thermocouple leads (23). Using two known bias currents provides for an accurate measurement of the resistance value of the thermocouple leads (23) when the thermocouple (26) is generating a voltage corresponding to the measured temperature. Either the I/O module (24) or the industrial controller (12) may determine a voltage resulting from the bias current applied to the thermocouple (26) during operation as a function of the measured resistance and compensate the voltage measured at the thermocouple leads (23) to accurately determine the measured temperature.