Automatic Temperature Fan Speed Control

INTRODUCTION: This is a standard circuit which can be used to adjust the brightness of mains lights and the speed of AC motors. It uses a triac, diac and has a radio-frequency interference (RFI) noise suppression circuit built into it as well. The circuit controls the average power to a load through the triac by phase control. The AC supply is applied to the load for only a controlled fraction of each cycle. The triac is held in an OFF condition for a portion of its cycle then is triggered ON at a time determined by the circuit. Each time the triac is turned on, the load current changes very quickly - a few micro seconds – from zero to a value determined by the lamp resistance and the value of the mains voltage at that instant in time. This transition generates Radio Frequency Interference. It is greatest when the triac is triggered at 90 degree and least when it is triggered at close to zero or 180 degree of the mains AC waveform. L-C suppression network is thus used to suppress these electrical noises.

HISTORY : Light dimming is based on adjusting the voltage which gets to the lamp. Light dimming has been possible for many decades by using adjustable power resistors and adjustable transformers. Those methods have been used in movie theatres, stages and other public places. The problem of those light controlling methods have been that they are big, expensive, have poor efficiency and they are hard to control from remote location. The power electronics have proceeded quickly since 1960. Between 1960-1970 thyristors and triacs came to market. Using those components it was quite easy to make small and inexpensive light dimmers which have good efficiency. Electronics controlling also made possible to make them easily controllable from remote location. These types of electronic light dimmers became available after 1970 and are nowadays used in very many locations like homes, restaurants, conference rooms and in stage lighting

WHAT ISADIMMER? Various definitions

Bibliography: • Reference book: POWER ELECTRONICS-Dr. P.S. Bhimbra • Power electronics- M.H. Rashid • http://howstuffworks.com • http://doityourself.com • http://sprags.com Thermistors are thermally sensitive resistors whose prime function is to exhibit a large, predictable and precise change in electrical resistance when subjected to a corresponding change in body temperature. Negative Temperature Coefficient (NTC) thermistors exhibit a decrease in electrical resistance when subjected to an increase in body temperature and Positive Temperature Coefficient (PTC) thermistors exhibit an increase in electrical resistance when subjected to an increase in body temperature. U.S. Sensor produces thermistors capable of operating over the temperature range of -100 ° to over +600 ° Fahrenheit. Because of their very predictable characteristics and their excellent long term stability, thermistors are generally accepted to be the most advantageous sensor for many applications including temperature measurement and control. Since the negative temperature coefficient of silver sulphide was first observed by Michael Faraday in 1833, there has been a continual improvement in thermistor technology. The most important characteristic of a thermistor is, without question, its extremely high temperature coefficient of resistance. Modern thermistor technology results in the production of devices with extremely precise resistance versus temperature characteristics, making them the most advantageous sensor for a wide variety of applications. A thermistor 's change in electrical resistance due to a corresponding temperature change is evident whether the thermistor 's body temperature is changed as a result of conduction or radiation from the surrounding environment or due to "self heating" brought about by power dissipation within the device. When a thermistor is used in a circuit where the power dissipated within the device is not sufficient to cause "self heating", the thermistor 's body temperature will follow that of the environment. Thermistors are not "self heated" for use in applications such as temperature measurement, temperature control or temperature compensation. When a thermistor is used in a circuit where the power dissipated within the device is sufficient to cause "self heating", the thermistor 's body temperature will be dependent upon the thermal conductivity of its environment as well as its temperature. Thermistors are "self heated" for use in application such as liquid level detection, air flow detection and thermal conductivity measurement.