DEVICE TO MEASURE TEMPERATURE

Abstract: This invention concerns a device to measure tempera­ ture, said device comprising a thermistor (RNTC) connected in series with at least two resistances (R1, R2) of which at least one (R1) can be commutated by means of a transistor (TR).

Description: This invention concerns a device to measure temperature. To be more exact, this invention concerns a device to read the temperature by means of a thermistor and to obtain data relating to said temperature in a digital manner. The invention has the aim of meeting requirements linked to a wide range of temperatures covered with the use of a minimum number of low-cost components. The invention is used advantageously in applications linked to household appliances such as ovens, washing machines, dishwashers and so on, but can be applied also to other apparatus such as drying machines, sterilizing machines, etc. The employment of thermistors of the NTC type, for example, as means for reading the temperature is known. By placing a resistance in series with the thermistor and feeding a constant voltage it is possible to obtain a signal of an analogue type, which, however, it may be necessary to use as a digital signal. To this end the signal is made to pass through an analogue-digital converter. Whenever it is wished to employ simple converters, for instance, of a type commercially known with the mark TL507 of Texas Instruments, in the case of wide ranges of temperature Description of the invention entitled: "DEVICE TO MEASURE TEMPERATURE" the amplitude covered by the signal (voltage) which is sent out by the thermistor and which depends on the temperature is highly excessive in relation to the receptive capacities of the converters themselves. Steps are therefore taken to vary the resistance -in series with the thermistor so as to bring the voltage/temperature curve into correspondence with the minimum value, or maximum value, of the measurement band which can be read. By doing this, the curve leaves the band, downwards or upwards respectively, at a temperature too far from the maximum or minimum value respectively of the range. Steps then may be taken to flatten the curve by envisaging a resistance in parallel with the thermistor, but such flattening is not enough to cover the ranges usually required by the applications in question. These ranges normally cover from 20-25 C up to 280-3000C or more, depending on the application. Therefore, to enable the whole range to be covered with the use of sijT#le, economical elements, the present author planned, tested and embodied the device to measure temperature according to the invention. According to the invention the natural voltage/temperature curve of a thermistor is displaced at least once so as to cover the whole measurement range. The curve may advantageously undergo a first displacement so as to enable the minimum, or maximum, value to be set and in this case will then undergo at least one other displacement during the course of the reading of the whole range. According to the invention the commutation of the curve from one position to the other is brought about with a computerized system which controls at least one transistor. According to the invention the voltage/temperature curve is thus split into two so as to have one transposed curve which covers, for instance, from 10-15 C up to 140-1500C and another curve which covers, for example, from 110-1200C up to 290 3000C. In this way the head and tail of the curves can be eliminated so as to get two curves, 25-1300C and 130-2800C respectively, which are certain enough, whereby those zones in which it is easiest for the values to be not perfect are eliminated. According to the invention the commutation of the second resistance can be obtained by means of a transistor, and the measurement takes place by selecting firstly one curve or the other and then by passing to the curve not selected earlier if the voltage of the output is not comprised in the band belonging to said first-selected curve. This invention is therefore embodied with a device to measure temperature, said device comprising a thermistor and being characterized by the fact that said thermistor is connected in series with at least two resistances of which at least one can be commutated by means of a transistor. Let us now see a preferred lay-out of the invention with the help of the attached tables, which have been given as non-restrictive examples. -In the tables we have the following: Fig.1 shows a probe connected in parallel according to the invention; Fig.2 shows the two temperature/voltage curves obtained according to the invention; Fig.3 shows a simplified diagram of the interpreting circuit. In Fig.1 RNTC is the thermistor employed as a temperature probe. R3 is a resistance of a suitable value and serves to modify the slope of the proper voltage/temperature curve of the thermistor according to the specific requirements. Said resistance R3 is placed in parallel with RNTC. Two resistances, R1 and R2 respectively, are placed in se ries with RNTC. R2 is always connected in series, whereas R7 is commutated by means of a transistor TR. VCC is the supply voltage, whereas VOUT is the analogue output for the analogue-digital converter. Fig.2 shows the effect of commutation of the resistance R1 by the transistor TR. Let us assume, for instance, that we#are starting with the lowest temperature, for example during the heating of an oven. First of all, for temperatures from 25e to 1300C, for instance, both the resistances R1 and R2 are placed in series with RNTC and the curve A is obtained. When the output voltage VOUT reaches the extreme value in the straight-line band, namely 0.25 VCC in our example, the transistor TR is commutated and the resistance in series with RNTC is P2 alone. As a result tbe voltage VOUT/temperature "t" curve is displaced to E and this enables the higher values, from 1300C to 280do for example, to be read. Fig.3 shows a simplified diagram of the circuit used in conjunction with the temperature probe according to the invention. The output voltage VOUT is converted into a digital signal by an analogue-digital converter, which in this example is a TL507 module of Texas Instruments. A microprocessor P interprets the values given by the analogue-digital converter and controls the commutation of the transistor TR between the two ranges of temperatures. The temperature values read are shown on an appropriate display with memory D, the display being controlled by the microprocessor P. We have described here a preferred embodiment of the invention, but variants are possible without departing from the scope of the invention itself. For instance, the TL507 module can be replaced with another having equivalent specifications. Further variants are possible for a person skilled in this field without departing thereby from the scope of the invention itself. INDEX A - voltage/temperature curve B - voltage/temperature curve D - display with memory P - microprocessor R1 - resistance R2 - resistance R3 - resistance RNTC - thermistor t - temperature TR - transistor TL507 - analngue-digital converter of Texas Instruments VCC - supply voltage VOUT - output voltage

Claims: CLAIMS 1 - Device to measure temperature which comprises a thermistor (RNTC) and is characterized by the fact that said thermistor (RNTC) is connected in series with at least two resistances (R1, R2) of which at least one (R1) can be commutated by means of a transistor (TR). 2 - Device to measure temperature as in Claim 1, characterized by the fact that the analogue output signal (VOUT) is converted into a digital signal by an analogue-digital converter and is interpreted by a microprocessor (P) which controls the commutation of the transistor (TR). 3 - Device to measure temperature as in Claims 1 and 2, characterized by the fact that the values of temperature measured are shown on a display with a memory (D) controlled by the microprocessor (P). 4 - Device to measure temperature as in Claim 1 and in one or the other of the Claims thereafter, and substantially as described, claimed and shown and for the purposes allowed.

Inventors: ROTOLONI, PIETRO
Applicants: ZANUSSI ZELTRON INST
Application year: 1983
Main classification: G01K1
IPC codes: G01R15, G01K7, G01K1