Automatic Temperature Fan Speed Control

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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.…show more content…
At this instant in time no current is flowing in either direction. This is the point at which a dimmer is electronically synchronized to turn the power ON or OFF. By chopping the waveform at the zero-crossing point, smooth dimming can be achieved without the lamp flickering. This turning on and off of the power device occurs every time the mains crossing point is reached (half phase), 100 or 120 times per second. Typically light dimmers are manufactured using a Triac or Thyristor as the power control device. These electronic parts are semiconductors not dissimilar to transistors. A Thyristor is a Uni.-directional device and hence, because AC power flows in both directions, two are needed. A triac is a bidirectional device and therefore only one is needed. An electronic circuit determines the point in time at which they turn ON (conduct). The ON state continues until the next zero-crossing point, at which point the device turns itself OFF. The electronic circuit then provides a delay, which equates to the dimness of the lamp, before turning the control device back on. The slight capacitance of the load, filters the chopped waveform resulting in a smooth light output. Some controllers use a microprocessor control with the above timing function being handled by an analogue circuit. More sophisticated systems, called digital dimmers, operate the switching direct from microprocessor. This has the advantage of greater reliability, quieter operation, lower cost and smaller
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