heat exchanger for detecting temperature of refrigerant flowing therein. When the heat exchanger operating as an evaporator, dew is deposited each on the surface of the heat exchanger and on the surface of the thermistor. The thermistor is usually molded with a plastic materials as a protective seal. Nevertheless, the dew on the thermistor is penetrates inside the thermistor through the protective seal and reaches to the electrodes the thermistor. The electrodes of the thermistor are formed with silver or metals containing silver, which often causes undesirable effects known as migration of silver. When the migration is occurred, whiskers of silver in the electrodes gradually grow and extend from one electrode to the other electrode. The whiskers then short-circuit the electrodes during the operation of the thermistor. Theshort-circuit state is partial and tentative in the beginning of the migration, The short state is resolved as long as the whiskers is thin enough to be melt by the short circuit current. The conductivity between electrodes are varied by the development of the whiskers, which results in shifting the temperature coefficient of resistance of the thermistor. It means that the thermistor indicates higher temperature than the actual temperature. The thermistor finally becomes malfunction because of the whiskers, which now causes the fully short-circuit state between the electrodes.
In a conventional air conditioning apparatus, pulses having a pulse width of 50 msec is periodically (20 sec) applied to the thermistor in place of a fixed D.C. potential for avoiding or delaying the migration.
However, there is a problem because the