BACKGROUND Temperature sensors are widely used in instrumentation and controls for many diverse applications involving temperature sensing and control.

Silicon p-n junctions (diodes) provide an attractive, low-cost solution for applications that span a temperature range from about-50°C to +175°C.

Under a constant-current bias the voltage-drop across the diode decreases linearly by approximately 2.2 mV/°C. However, the voltage drop across the diode as well as its rate of change with temperature, varies a little from diode to diode. This results in the need to"calibrate"each individual sensor. The calibration is performed by suitably adjusting the"gain"and"offset"of the circuit in which the device is used. The calibration needs to be repeated whenever a sensor is changed. This procedure involves the use of special constant-temperature baths or thermo-wells that are not easily portable. This limitation makes it difficult or infeasible to implement such a calibration when a sensor is to be changed in the field at the point of use, and generally the entire instrument has to be returned to the factory for implementing this requirement.

The object of this invention is to overcome the above limitation by incorporating the calibration data within the body of the sensor at the time of its manufacture including special circuit for supplying the said calibration data in a pre-determined sequence data rate to the electronic digital control unit.

To achieve the said objective this invention provides a temperature sensing device with built in calibration data for an electronic digital control unit comprising: -a p-n junction temperature sensor for sensing the temperature, -a non-volatile memory connected across the said sensor through a rectifier means for storing the calibration data of the sensor, the said non-volatile memory includes a special circuit for supplying the said calibration data in a pre-determined sequence and data rates, -the arrangement between the said p-n junction temperature sensor, rectifying means is such that when the polarity of the common line with respect to the sensor output calibration data output line is in one direction, the p-n junction temperature sensor is enabled and the rectifying means disables the non-volatile memory and when the polarity of the common line with respect to the sensor output/ calibration data output line reverses, the p-n junction temperature sensor is disabled and the rectifying means enables the non-volatile memory to supply the calibration data by the said special circuit to the required electronic digital control unit.

An isolating means is connected in the output of the said non-volatile memory for its protection. The said isolating means is a diode.

The said rectifying means is a diode with a capacitor connected across the non-volatile memory for providing a constant DC supply to the non- volatile memory. The said non-volatile memory is EEPROM (electrically erasable programmable read only memory).

The calibration data stored in the EEPROM is obtained from the p-n junction temperature sensor at the time of its manufacture and is supplied to the electronic digital control unit at the time of use.

The invention will now be described with reference to the accompanying drawings.

Fig. 1 shows a temperature sensing device with built in calibration data in SENSE mode for an electronic digital control unit, according to this invention.

Fig. 2 shows a temperature sensing device with built in calibration data in READ mode for an electronic digital control unit, according to this invention.

Referring to the figures, item (Dl) shows a temperature sensing p-n junction (diode) connected across two wires labeled Common line (1) and sensor output/calibration data output line (2). The electronic digital control unit (E) is connected between the common line (1) and sensor output/ calibration data output line (2). EEPROM (N) contains the calibration data (CD) of the p-n junction temperature sensor and special internal circuitry for supplying the said calibration data (CD) of its contents in a pre-defined sequence and serial data rate after power is supplied to it through rectifying means (D2, Cl). The said rectifier means consists of a rectifying diode (D2) and a capacitor (Cl) connected across the EEPROM (N) for providing a constant DC supply to the said EEPROM. An isolating diode (D3) is connected to the output of the EEPROM (N) for providing protection to it when the polarity of the electronic digital control unit (E) reverses.

The temperature sensing device operates in two modes, the SENSE mode and the READ mode. In the SENSE mode, as shown in figure 1, the p-n junction temperature sensor (Dl) is enabled by the polarity of common line (1) with respect to the line (2) and provides sensor output to the electronic digital control unit, while the rectifying means (D2, Cl) disables the EEPROM (N). In the READ mode, as shown in figure 2, the p-n junction temperature sensor (Dl) is disabled by the reversal of the said polarity and the rectifying means (D2, Cl) enables the EEPROM (N) to supply calibration

data (CD) through the special circuit (SC) to the electronic digital control unit.

WORKING: An electronic digital control unit (E) is connected to the temperature sensing device between the common line (1) and the sensor output calibration data output line (2). The said two lines (1 & 2) carry bias current from the electronic digital control unit (E) to enable p-n junction temperature sensor (D1) when the common line (1) is polarized positively with respect to the"sensor output/calibration data output line (2) (SENSE mode) to provide sensor output to the electronic digital control unit (E) and disable EEPROM On the other hand, when the Common line (1) is polarized negatively with respect to the sensor output/calibration data output line (2) by the digital control unit (READ mode) the two lines (1 & 2) carry the operating supply current to the EEPROM (N) through the rectifying means (D2, Cl), as well as the calibration data (CD) supplied by it through the special circuit (SC) to the electronic digital control unit (E). Rectifying diode (D2) isolates the EEPROM (N) electrically from the circuit during the SENSE mode and at the same time provides a rectified constant DC supply with the help of capacitor (C1) to supply power to the EEPROM (N) in the READ mode.

Isolating diode (D3) provides protection for the data output terminal of EEPROM (N) in both the modes. EEPROM (N) contains special circuit (SC) for supplying the said calibration data (CD) in a predetermined sequence and serial data rate after power is supplied to it through rectifying means (D2, Cl).

ADVANTAGES: + Any p-n junction temperature sensor of digital control unit can be replaced with the temperature sensing device with built in calibration data of this invention in the field.

+ The device according to this invention makes it possible to incorporate this facility without the addition of any extra lines, which results in a significant cost saving in many applications.