Automatic Temperature Fan Speed Control

Topics: Alternating current, Resistor, Lighting Pages: 12 (3897 words) Published: July 25, 2011
 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.

 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

 Various definitions for a dimmer are as follows: 
1. In lighting, the electrical device (technically known as a potentiometer) that regulates the current passing through the bulb filaments and, thereby, the amount of light emitted from the lighting instruments. 2. Electronic controls that allow stage lighting to fade up or down slowly, as opposed to being on or off only. 3. A control that regulates light levels.

4. Device which provides adjustable voltage to a lighting fixture to control light output. It can also be a term to refer to a mechanical device, such as a shutter, that controls output.

Dimmer Types :
 Older lighting controls used either resistors or large variable transformers called auto-transformers; these units had hand wheels mounted on top that needed to be turned in order to adjust the level of output. Needless to say these devices were very inefficient, large and expensive. Modern Triac or Thyristor based dimmers are either ON or OFF and hence dissipate very little heat. A typical voltage drop from these units when the power is transferred to the load is only a few volts with most of this being lost through the electrical noise suppression part of the circuit. The power devices have 3 connections

a) Power in
b) Power out
c) Gate.
The units conduct when a low power control signal is provided to the gate

   A light dimmer works by essentially chopping parts out of the AC voltage. This allows only parts of the waveform to pass to the lamp. The brightness of the lamp is determined by the power transferred to it, so the more the waveform is chopped, the more it dims. Mains power is comprised of an alternating current that flows in one direction and then in the other, along the cable, at the rate of 50 or 60 cycles per second (known as Hertz). The value 50 or60Hz is dependent on the countries power system. The current alternates back and forth changing direction at the zero point. If we were to look at this waveform it...
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