Urgent Help Needed

[suhusumu]

Please help me understand the following circuit. Please explain the working of the following circuit soon. I want to know why the diodes and the transistor is used.

 

[ronsimpson]

The transistor amplifiers current. If the motor runs at 1 amp, this current is too much for the op-amp. The transistor might have a gain of 100 so the input of the transistor will be 0.01 amps to control the motor.
The diodes are used to protect the transistor. A motor can work as a generator and push power back into the transistor. (during braking)

 

[dougy83]

Might I suggest you use a more descriptive title in future?

I want to know why the diodes and the transistor is used.

ZD1 provides a stable reference voltage of 5.1V, TS1 produces a voltage proportional to the temperature. The transistor amplifies/buffers the current to the fan. D1 & D2 provide transient suppression of the back-EMF produced by the fan.

 

[suhusumu]

what is transient suppression of back EMF produced by fan? How does the fan produce an EMF?

 

[dougy83]

If the fan uses coils & a commutator, the voltage on the coils will spike every time the current through them stops. The diodes provide a path for the current of these spikes to flow through, instead of through the output transistor (which may damage it).

 

[suhusumu]

what if i use the circuit without the diodes??

 

[dougy83]

Maybe the transistor will die, maybe it won't

 

[suhusumu]

i have used a normal 12V DC fan motor, do they have commutator or coils???

 

[suhusumu]

one more thing, why are two of the diodes used???

 

[dougy83]

Most probably. The ones that don't are brushless dc fans (e.g. a PC fan). Do you not have any diodes? They're pretty cheap. Alternately you can use a resistor & capacitor (or even just a capacitor) as a snubber across the fan to try to suppress the glitches.

 

[dougy83]

There's 2 diodes to account for current spikes in any direction. The top one is for current feeding back out of the motor, the lower for current into the motor.

 

[suhusumu]

Thanks a lot. It was very helpful!

 

[suhusumu]

Thank you!

 

[dougy83]

You're welcome.