The present invention relates to a temperature control apparatus for detecting the temperature of a unit whose temperature varies over a wide range, such as a heat exchanger placed in a refrigeration cycle, by means of a thermistor to thereby execute temperature control of the unit.
There have recently been in wide use air conditioners for controlling room temperature and humidity and ventilation of the room to provide comfortable atmosphere for human beings in offices, homes, vehicles, and the like.
Many of air conditioners have both air cooling and air heating functions and, during its operation, the room temperature is detected and the detected room temperature is compared with a set temperature to thereby achieve an air-conditioning operation. In the meantime, the temperature of a heat exchanger installed indoors is detected to prevent the heat exchanger from freezing or overheating.
The thermistor (temperature detection device) used as the temperature sensor has its electrical resistance varying with changes in temperature. By connecting a thermistor and a resistor in series and applying a voltage across the series connection, the temperature according to the voltage at the junction point is determined.
There are manufactured various types of thermistors which differ from each other in resistance-temperature characteristic according to the temperature range within which they are used. For example, there is a type which varies its resistance value with changes in temperature over a relatively narrow temperature range of 100.degree. C. or so, a type which changes its resistance value with changes in temperature over a relatively narrow temperature range of 300.degree. C. or so, and, further, a type changing its resistance value over a relatively large temperature range from -20.degree. C. to +80.degree. C. or in a temperature range from 0.degree. C. to +100.degree. C..
Thermistors are different in price according to their features. Those which have a wide detectable temperature range or those of which resistance value greatly changes for a given change in temperature (having good sensitivity) are expensive.
A resistance-temperature characteristic of a thermistor whose resistance value changes over a range of changing temperature of 90.degree. C. is shown in FIG. 7 as an example. When this thermistor is connected as shown in FIG. 10, the point a in FIG. 10 exhibits a voltage-temperature characteristic as shown in FIG. 8. Since the voltage-temperature characteristic of the thermistor becomes a curve as shown in FIG. 8, the voltage value at the point a does not vary uniformly for a given change in temperature.
In the temperature control of air conditioning apparatus of the described type, an A/D converter has previously been used for converting the voltage detected at a point such as a in FIG. 10 into a temperature. Namely, a temperature is given a corresponding value, for example, out of 0 to 255 (1 byte) by the A/D converter and a value previously stored in the microcomputer is compared with the value output from the A/D converter. The temperature control has been executed in accordance with the result of the comparison.
In this case, the comparison of the set temperature and the temperature detected by the thermistor is performed through a comparison between step values, and hence no problem arises even if the characteristic of the thermistor are nonlinear (curvilinear) as described above. However, when the thermistor characteristic is not linear, the value output from the A/D converter does not vary uniformly with respect to a given change in actual temperature.
If, now, there is a relationship expressed by a linear expression between the value x (hereinafter called "step value") obtained by the A/D conversion and the temperature t detected by the thermistor (for example, x=t+.alpha.), the control will be simple. However, since the step value after the A/D conversion does not change proportionally with temperature in the nonlinear portion of the themistor's characteristic, it becomes necessary, in actually performing the control using the nonlinear portion, to compensate the step value stored within the microcomputer so that this step value will uniformly vary with changes in temperature. From this, problems arise that the processing in the microcomputer becomes complex and that a difference in resolution is produced in the conversion of the temperature to the step value between the resolution in the nonlinear portion and that in the linear portion.
Therefore, it has been known to connect the thermistor in parallel with a resistor R1 as shown in FIG. 11 and detect the voltage at the point b. By so doing, the voltage-temperature characteristic at the point b in FIG. 11 comes to have a linear portion c in its middle portion as shown in FIG. 9. Since the voltage is substantially proportional to the temperature in the portion c, the step value obtained through the A/D conversion also becomes proportional to the temperature, and hence the control circuit becomes simpler. Thus, the portion c has so far been used for temperature detection.
The temperature of the indoor heat exchanger of an air conditioning apparatus varies over a wide range of temperature from -10.degree. C. or so to +70.degree. C. or so when both air cooling and air heating are taken into consideration. If such a wide temperature range is to be detected using low priced thermistors not having a wide detectable temperature range (such as that having a temperature range of about 20.degree. C. as the portion c of FIG. 9), it becomes necessary to use two thermistors, one detecting in a range of about -5.degree. C. to +15.degree. C. in the air cooling domain and the other detecting in a range of about +40.degree. C. to +60.degree. C. in the air heating domain.
This is because, referring to FIG. 12, it is necessary to decrease the number of revolutions of the compressor to suppress its capacity when its temperature becomes lower than +2.degree. C. in the cooling cycle, or to stop the compressor when the temperature becomes lower than 0.degree. C. and remains in this state for two minutes because, then, there is danger lest the compressor be frozen, or in the heating cycle, to decrease the number of revolutions of the compressor to suppress its capacity when the temperature becomes higher than +55.degree. C., or to stop the compressor when its temperature becomes higher than +60.degree. C. taking it for an overheated state.
Incidentally, it has been in practice, when the temperature becomes higher than 5.degree. C. after the compressor was stopped or its capacity was suppressed for preventing the indoor heat exchanger from being frozen, to restore the operation of the compressor to the original state and, when the temperature becomes lower than +47.degree. C. after the compressor was stopped or its capacity was suppressed to protect it from overheating, to restore the operation of the compressor to the original state.
Thus, when a plurality of thermistors are used to cover the wide range of changing temperature of the object whose temperature is to be detected, not only a plurality of thermistors are required, but also the processes for setting up the thermistors and inspecting them increase, and therefore cost is increased. If a thermistor having a wide c portion of FIG. 9 is used, this also increases the cost because such a thermistor is very expensive.