Automatic tempearture controlled fan

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 C ollege of Engineering, M unnar College of Engineering Munnar

3. INTRODUCTION Ordinary fan regulators Electronic fan regulators College of Engineering Munnar

4. Why automatic temperature controlled fan? *Automatic speed control *Low power loss *Reduce Current Consumption * Allows fan to ramp up or down smoothly to new speed *Increase Fan Life *A single temperature or all temperatures can control the fan speed . *Small size and economic College of Engineering Munnar

5. Triac Optocoupler Increase in temperature BLOCK DIAGRAM AC supply 230 V, 50Hz 230 V AC College of Engineering Munnar

6. Components Used *Thermistor *IC LM324 *IC µA741 *Resistors * Zener diodes * Optocoupler (MOC3011) * Triac (BT 136) College of Engineering Munnar

7. CIRCUIT DIAGRAM College of Engineering Munnar

8. LM324 * 4 OpAmps in a single IC College of Engineering Munnar

9. NEED OF OPTOCOUPLER * 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. 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 . College of Engineering Munnar

10. thermistor A   thermistor  is a type of  resistor   resistance  varies significantly with   temperature used as  current limiters , temperature  sensors , self-resetting overcurrent protectors, and self-regulating  heating elements College of Engineering Munnar

11. triac 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, electronicsforyou.com COST 250 rs College of Engineering Munnar

13. THANK YOU College of Engineering Munnar