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Temperature controlled fan circuit

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Newbie level 3
May 3, 2011
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Hello, i am made a temperature controlled fan circuit, but since im new to this im not sure if i made it as optimal as i could.
I prety much used the standard recommended applications from the datasheets, and mixed it up into a circuit.

The temp. censor i use is a LM35, and i used it like this:

I just added a 3 pin connector for the lm35, as the actual sensor will be located a foot away from the PCB.

The circuit will run of a battery, so i put a 12 stabilized supply on it as shown here:

I added a jumper so i could manually disconnect, else it should be prety much as shown in the datasheet.
now as for the "fan ON/OFF" bit, i used a comperator, to set the ON/OFF reference, as shown here:

I added an LED to show when the fan should be on, just to have something to look at.
And here is the full schimatic:

lemme know what you think :)

Well, i guess i will have to just build it and seem what happends, had hoped for a little feedback.

I have since found a possible error with the 100K POT, i think that needs to be made part of a voltage devider, instead of just a plain variable resistance.
So i will change that and make a muck up.

Maybe i will even post my results here for other inexperienced people to learn from.

/electronewb - out

Yea, i never got round to actually building it, im not motivated not knowing if it would work or not.. i don't have the equipment nor the skills to bug find it once build.

/electronewb - out

Hi. You're well underway to developing circuit designing skills. And troubleshooting skills once you build it.


Please explain the use of two transistors. Is that meant to reverse the output polarity from the comparator?

Comparators are not the same as op amps. The comparators I've seen do not provide current at the output. It will pull current from a positive source. This means it will operate the bias of a PNP transistor directly.


Regulators of the 7800 series need an input V which is 2V greater than the output V. Since you're running off a 12V battery, you might consider using a regulator of 8 or 9 or 10 V.


You may be able to get away with a simpler arrangement between the LM35 and the transistor which drives the fan. I haven't experimented with the LM35. Could it drive the transistor directly? With the right biasing combination you might achieve gradual fan speedup as temperature rises.


You have a 100k resistor across pins 1 & 3 of the comparator. Is this to create a hysteresis feedback effect? On the other hand, if it's meant to amplify, then it should be connected to the inverting input instead. You'll need to rearrange the related components then.
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If you don't need adjustability then a Selco thermostat is
a way simpler solution (Check Digi-Key, thermostats in
TO-220 types leaded packages in 5C setpoint increments).
Use that to run a fan relay coil.

Thank you BradtheRad for your feedback, unfortunately i got away from this project because i (finally) found work :)

1. I used 2 transistors because i did not thing the comperator could turn the larger transistor on by it self, but if i can it would simplify the circuit allot.

2. I did not know that about the 7800 series, but as you say switching to 9V should rectify the problem.

3. Hmm interesting, i shall have to try that, even if this project has gone cold. But could i not encounter some regulatory problems? like uncontrolled oscillation?

4. Yes the 100K resistor is for hysteresis.

Thanks again for your

/EN - out

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