12-05-2011, 02:27 PM
Here is a circuit through which thespeed of a fan can be linearly controlledautomatically, dependingon the room temperature. The circuit ishighly efficient as it uses thyristors forpower control. Alternatively, the samecircuit can be used for automatic temperaturecontrolled AC power control.In this circuit, the temperature sensorused is an NTC thermistor, i.e. one havinga negative temperature coefficient. Thevalue of thermistor resistance at 25°C isabout 1 kilo-ohm.Op-amp A1 essentially works asI to V (current-to-voltage) converterand converts temperature variationsinto voltage variations. To amplifythe change in voltage due to change intemperature, instrumentation amplifierformed by op-amps A2, A3 and A4is used. Resistor R2 and zener diodeD1 combination is used for generatingreference voltage as we want to amplifyonly change in voltage due to thechange in temperature.Op-amp μA741 (IC2) works as acomparator. One input to the comparatoris the output from the instrumentationamplifier while the other inputis the stepped down, rectified andsuitably attenuated sample of AC voltage.This is a negative going pulsatingDC voltage. It will be observed thatwith increase in temperature, pin 2 ofIC2 goes more and more negative andhence the width of the positive goingoutput pulses (at pin 6) increases linearlywith the temperature. Thus IC2functions as a pulse width modulatorin this circuit. The output from thecomparator is coupled to an optocoupler,which in turn controls the ACpower delivered to fan (load).The circuit has a high sensitivity andthe output RMS voltage (across load) canbe varied from 120V to 230V (for a temp.range of 22°C to 36°C), and hence widevariations in speed are available. Alsonote that speed varies linearly and notin steps. Besides, since an optocoupler isused, the control circuit is fully isolatedfrom power circuit, thus providing addedsafety. Note that for any given temperaturethe speed of fan (i.e. voltage acrossload) can be adjusted to a desired valueby adjusting potmeters VR1 and VR2appropriately.Potmeter VR1 should he initially keptin its mid position to realise a gain of approximately40 from the instrumentationamplifier. It may be subsequently trimmedslightly to obtain linear variation of thefan speed.
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http://electronicsforuEFYLinux/circuit/January2010/AUTOMATIC%20TEMPERATURE%20CONTROLLED%20FAN.pdf
