Automatic tempearture controlled fan


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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
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.

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Automatic tempearture controlled fan

  1. 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. 2. Mini project by Students of College of Engineering, Munnar College of Engineering Munnar
  3. 3. INTRODUCTION Ordinary fan regulators College of Engineering Munnar Electronic fan regulators
  4. 4. Why automatic temperature controlled fan? College of Engineering Munnar
  5. 5. BLOCK DIAGRAM AC supply 230 V, 50Hz Increase in temperature Bridge Rectifier High Gain Amplifier 230 V AC College of Engineering Munnar Thermistor Triac Comparator Optocoupler
  6. 6. Components Used *Thermistor *IC LM324 *IC µA741 *Resistors *Zener diodes *Optocoupler (MOC3011) *Triac(BT 136) College of Engineering Munnar
  7. 7. CIRCUIT DIAGRAM College of Engineering Munnar
  8. 8. LM324 * 4 OpAmps in a single IC College of Engineering Munnar
  9. 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. College of Engineering Munnar 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. 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 College of Engineering Munnar
  11. 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. 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 COST 250 rs College of Engineering Munnar
  13. 13. College of Engineering Munnar