# Temperature controlled fan

Discussion in 'The Projects Forum' started by sveik, Oct 6, 2012.

1. ### sveik Thread Starter New Member

Oct 6, 2012
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Hello guy's.
I want to start off by apologizing for my bad English it's not my first language
I'm trying to build an automatic fan controller for a PSU project I'm working on.

The idea was to use some kind of comparator or schmitt trigger to trigger the fan to go ON at certain point(around 50°C). The reason I want to use schmitt trigger is so the fan won't constantly go ON/OFF at the trigger point.

For the PWM i was thinking about using a 555 timer. To control the PWM I thought i might use a NTC resistor.

I'm trying to use only one NTC(they are quite expensive here). The problem is I don't seem to be able to use the NTC both for the schmitt trigger and the PWM.

So to sum up:
Build a fan control with trigger point of 50°C and use schmitt trigger. From 50°c upward slowly rise the speed of the fan until the temp is about 80 and then is on full speed.
I will be running this on 12V dc. And I want to keep the cost down as much as possable

I'm open to all idea's how to do this and if someone can point me in the right direction that would be great. I've already tried google and tho i did find a lot of information on this topic none of them suits me the way i was thinking.

P.S. I would rather not use microcontroller so don't suggest it

2. ### DickCappels Moderator

Aug 21, 2008
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What you are trying to do would be easy with a microcontroller, are you interested in that approach?

As for using only one thermistor, you can do that. Just string the thermistor in series with another resistor to make a voltage divider. Now you have a voltage that is realted to the temperature and you can use it for both the schmitt trigger and the pwm controller. Just do not load down the thermistor circuit.

Could use a circuit like the one below and add the hysteresis with one more resistor. Notice that you can connect many copies of the opamp circuit to the thermistor without having a significant effect on the operation of the thermistor.

http://electronics.stackexchange.com/questions/9611/building-a-temperature-controlled-water-bath

Jun 22, 2012
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4. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
510

Circuit is good except as mentioned you need to add hysteresis (a resistor from op amp output back to POSITIVE input) to keep it from going on and off too much.

5. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
510
Some DC fans will be very unhappy if you try to PWM them as speed control. They contain internal IC's that regulate speed and inrush current when the fan is stalled and other things. They will go nuts if fed square waves.