Here is a circuit through which the speed of a fan can be linearly controlled automatically, depending on the room temperature. The circuit is highly efficient as it uses thyristors for power control. Alternatively, the same circuit can be used for automatic temperature controlled AC power control. In this circuit, the temperature sensor used is an NTC thermistor, i.e. one having a negative temperature coefficient. The value of thermistor resistance at 25°C is about 1 kilo-ohm. Op-amp A1 essentially works as I to V (current-to-voltage) converter and converts temperature variations into voltage variations. To amplify the change in voltage due to change in temperature, instrumentation amplifier formed by op-amps A2, A3 and A4 is used. Resistor R2 and zener diode D1 combination is used for generating reference voltage as we want to amplify only change in voltage due to the change in temperature. Op-amp μA741 (IC2) works as a comparator. One input to the comparator is the output from the instrumentation amplifier while the other input is the stepped down, rectified and suitably attenuated sample of AC voltage. This is a negative going pulsating DC voltage. It will be observed that with increase in temperature, pin 2 of IC2 goes more and more negative and hence the width of the positive going output pulses (at pin 6) increases linearly with the temperature. Thus IC2 functions as a pulse width modulator in this circuit. The output from the comparator is coupled to an optocoupler, which in turn controls the AC power delivered to fan (load). The circuit has a high sensitivity and the output RMS voltage (across load) can be varied from 120V to 230V (for a temp. range of 22°C to 36°C), and hence wide variations in speed are available. Also note that speed varies linearly and not in steps. Besides, since an optocoupler is used, the control circuit is fully isolated from power circuit, thus providing added safety. Note that for any given temperature the speed of fan (i.e. voltage across load) can be adjusted to a desired value by adjusting potmeters VR1 and VR2 appropriately. Potmeter VR1 should he initially kept in its mid position to realise a gain of approximately 40 from the instrumentation amplifier. It may be subsequently trimmed slightly to obtain linear variation of the fan speed.

1. Mini project presentation
AUTOMATIC TEMPERATURE
CONTROLLED FAN
Abhilash.K.M(02)
Abin Baby(03)
Ajith Kumar R(04)
Bineesh Babu P(10)
College of Engineering Munnar

2. Mini project by
Students of
College of Engineering, Munnar
College of Engineering Munnar

3. INTRODUCTION
Ordinary fan regulators
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Electronic fan regulators

4. Why automatic temperature
controlled fan?
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5. BLOCK DIAGRAM
AC supply
230 V, 50Hz
Increase in
temperature
Bridge Rectifier
High Gain
Amplifier
230 V
AC
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Thermistor
Triac
Comparator
Optocoupler

6. Components Used
*Thermistor
*IC LM324
*IC µA741
*Resistors
*Zener diodes
*Optocoupler (MOC3011)
*Triac(BT 136)
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7. CIRCUIT DIAGRAM
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8. LM324
* 4 OpAmps in a
single IC
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9. * Basically an opto-isolator, designed to
transfer electrical signals by utilizing light
waves to provide coupling with electrical
isolation between its input and output.
*The main purpose of an opto-isolator is
to prevent high voltages or rapidly
changing voltages on one side of the
circuit from damaging components or
distorting transmissions on the other side.
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Schematic diagram of an
opto-isolator showing
source of light (LED) on
the left, dielectric barrier
in the center, and sensor
(phototransistor) on the
right.

10. • A thermistor is a type of resistor
• resistance varies significantly
with temperature
• used as current
limiters, temperature sensors, self-resettin
g overcurrent protectors, and
self-regulating heating elements
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11. • speed controls for electric fans
• TRIACs are bidirectional and so current can
flow through them in either direction.
•
TRIACs can be triggered by either a positive or a
negative current applied to its gate electrode.
• Once triggered, the device continues to conduct
until the current drops below a certain
threshold, called the holding current.
College of Engineering Munnar

12. OUR PROGRESS
We have designed and wired the circuit on the bread board level, and the output
is obtained upto 2 stages.
REFERENCE
Wikipedia, electronicsforyo
u.com
COST
250 rs
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13. College of Engineering Munnar