Electronic circuit for driving a thermocouple element, temperature sensing device, and method for observing a leakage resistance of the thermocouple element
Various embodiments include an electronic circuit for driving a thermocouple element comprising: contact electrodes for connecting cold-end electrodes of the thermocouple element; and a measurement circuit to measure a thermo-voltage generated by the thermocouple element. One of the electrodes is connected to a supply voltage and a ground potential over a voltage divider having resistors with pre-defined resistance values such that a common mode voltage between the contact electrode and the ground potential is in a pre-defined range. If a leakage resistance effective between the contact electrode and the ground potential is greater than a pre-defined threshold, a processor compares the common mode voltage to the pre-defined range and generates a calibration value for the measurement circuitry if the common mode voltage is outside the range.
1. An electronic circuit for driving a thermocouple element, the electronic circuit comprising:
contact electrodes for connecting cold-end electrodes of the thermocouple element;
a measurement circuit to measure a thermo-voltage generated by the thermocouple element between the contact electrodes as a function of a temperature difference between the cold-end electrodes and a hot-end side of the thermocouple element; and
a processor;
wherein one of the contact electrodes is connected to a supply voltage and to a ground potential of the electronic circuit over a voltage divider having resistors with pre-defined resistance values such that a common mode voltage between the contact electrode and the ground potential is in a pre-defined range;
if a leakage resistance effective between the contact electrode and the ground potential is greater than a pre-defined threshold, the processor compares the common mode voltage to the pre-defined range and generates a calibration value for the measurement circuitry if the common mode voltage is outside said pre-defined range.
2. An electronic circuit according to claim 1 , wherein the calibration value depends on a model describing a functional relationship between the leakage resistance and the common mode voltage.
3. An electronic circuit according to claim 1 , wherein the contact electrode is connected to a measurement electrode of the measurement circuit over a switch configured to alternately connect the measurement electrode to the contact electrode and to the ground potential; and
further comprising a control circuit configured to actuate the switch so the measurement circuit alternately measures the thermo-voltage and the common mode voltage.
4. An electronic circuit according to claim 1 , further comprising a connection switch connected to the contact electrodes to provide a short circuit between the contact electrodes when the connection switch is closed.
5. An electronic circuit according to claim 1 , wherein the processor is configured to generate a warning signal if it detects a low leakage resistance value.
6. An electronic circuit according to claim 1 , further comprising a ground contact electrode for connecting a sheath of a mineral-insulated cable of the thermocouple element to the ground potential.
7. A temperature sensing device comprising:
a thermocouple element;
contact electrodes for connecting cold-end electrodes of the thermocouple element;
a measurement circuit to measure a thermo-voltage generated by the thermocouple element between the contact electrodes as a function of a temperature difference between the cold-end electrodes and a hot-end side of the thermocouple element;
wherein one of the contact electrodes is connected to a supply voltage and to a ground potential of the electronic circuit over a voltage divider having resistors with pre-defined resistance values such that a common mode voltage between the contact electrode and the ground potential is in a pre-defined range, if a leakage resistance effective between the contact electrode and the ground potential is greater than a pre-defined threshold; and
a processor to compare the common mode voltage to the pre-defined range and to generate a calibration value for the measurement circuitry if the common mode voltage is outside said pre-defined range.
8. A method for determining a temperature related signal using a thermocouple element and observing a leakage resistance of the thermocouple element, the method comprising:
measuring a thermo-voltage generated by the thermocouple element between contact electrodes as a function of a temperature difference between the cold-end electrodes and a hot-end side of the thermocouple element;
wherein one of the contact electrodes is connected to a supply voltage and a ground potential of an electronic circuit over a voltage divider having resistors with pre-defined resistance values;
generating a common mode voltage with the voltage divider between the contact electrode and the ground potential, wherein the common mode voltage is in a pre-defined range;
if a leakage resistance effective between the contact electrode and the ground potential exceeds a pre-defined threshold, comparing the common mode voltage to the pre-defined range; and
if the common mode voltage is outside said pre-defined range, generating a calibration value for calibrating the measurement circuit.
9. An electronic circuit according to claim 1 , wherein the calibration value depends on a model describing a functional relationship between the leakage resistance and the thermo-voltage.