Hi all...
To start wid, am just a beginner and i cant figure out a neat and well defined explanation for the circuit.. Pls guys help me in giving an explanation so tat even a non-electronic student also can understand... I specifically want to know how the relay circuit works from the time the dc voltage s given and after the op-amp compares the voltages and gives signal to the base of the transistor and the signal s obtained across the collector terminal... tat s wen the relay opens and how...
Pls help guys......

 

Why don't you start out by reading the page where the schematic came from:
http://www.uoguelph.ca/~antoon/circ/auto-fan.html

 

Say them that the transistor acts a switch. When the voltage on its base it greater than 0.7 it closes, otherwise it is open. When the switch (transistor is off) is open the no current flows and thus the relay is off. When the switch is closed (transistor is on) current flows and the relay is on.
Another easy way is by resistance terms. Transistor off-high resistance-relay off, transistor on-low resistance-relay on.

That is how students can understand it easily even if the operation of the transistor is not actually like that.

 

Thanks yaar... But i need the relay operation... tat s how the relay s positioned (i mean the contacts of the relay) before the 12 v s applied and after it s applied and then after the transistor s turned on....

 

When no voltage is applied to the relay coil a spring holds one of the contacts (moving contact) away from the other. When voltage is applied across the relay's coil a magnetic field is created and it pulls the moving contact until it touches the other contact.

Have a look here:

http://www.howstuffworks.com/relay.htm

 

Wont tat 12v dc voltage v apply to the circuit hav any effect on the relay..? s tat coz of the diode tat s present..? S tat so tat only the voltage at the collector terminal turns on the relay..?

 

The diode is there to protect the rest of the circuit from the back EMF the coil of the relay when it is turned off.
When the transistor is off the bottom side of the relay coil is at 12V. Because the top side is also at 12V there is no voltage drop across the relay coil and thus is off.
When the transistor is on the bottom of the relay coil is almost at zero volts. Because the top side is at 12V the voltage across the relay coil is almost 12V. Thus the relay turns on.

 

Thanx a lot....

 

You are welcome