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auto fan project

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falseeto

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Hi, for a school project i am planning to make a circuit that will turn a computer fan on when the temperature reaches a certain level, or control of the speed of the fan in accordance to the temperate if possible.

I have done some research into it and have found 2 possible circuit designs:

**broken link removed**

or

**broken link removed**

I plan to connect it to a seperate power supply (not my computers PSU) to minimize risk. Are either of these circuits what i'm looking for? Which would be more practical?

Any advice or help greatly appreciated, thanks
 
the former is a variable speed solution that has the habit of stalling the fan at low speeds. the latter is an ON/OFF solution that will run your fan at full speed.

to minimize risk, I recommend running the circuit off your computer's power supply instead of introducing outside sources of voltage into the system with an external supply.
 
falseeto said:
I plan to connect it to a seperate power supply (not my computers PSU) to minimize risk. Are either of these circuits what i'm looking for? Which would be more practical?
I have worked with the first one & its worked fine.I used 80mm DC fan.No more humns (noise) & no more heat in the MOSFET.
But I worked with a circuit that produces PWM signals, the noiseless fans became with noisy when reducing the fan speed.:D
 
Circuit 1 isn't very good as the MOSFET will get very hot when it's in its linear region and it could oscillate as there's no hysteresis.

Circuit 2 is probably the best solution, on/off action is all you require and you could get rid of R4, R6, Q1 and the relay and replace them with a MOSFET if you want to cut down on parts but the relay offers the advantage of isolation if you want to power the circuit from two separate power supplies.
 
the fet doesn't get hot, it does get a little warm. speaking from personal experience. I used a crusty old IRF520 fet to build mine.

The reason I stopped using it; it would stall the fan at low speeds. The fan would sit there and growl and whine and stutter until the computer heated up enough to allow more current to flow to the fan.
 
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