controlling PC Fan

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I would like to know if anybody has experience with controlling the speed of a PC power supply fan.

I know that effectively there is a controller in the fan to let the brushless moter rotate.

I can reduce the turning speed of the fan by lowering the power supply, I do this normally with a lab power supply.

I want to use an AVR to control the turning speed depending on a measured temperature.
Now I can do this in 2 ways. either I use a big fat NFET that I set to better or worst conducting, or I use a lighter FEt that I set to switching mode and as such I use a PWM signal to adjust the motor speed.

In both cases I use PWM, but in one condition I use the FET as a resistor and thus lower the voltage over the motor and in the other case I switch ON and OFF the FET and reduce the average voltage over the motor.
The big difference is the amount of power dissipated both in the FET and the system.

does anybody have experience on this?

regards

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Just to say that over the years I've read many threads here about AVRs controlling PC fans so there's probably going to be quite a wealth of prior traffic about this.

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THere are also many IC's designed to control the fan and are programmable with various setpoints and they can also in some cases give RPM's of the fan.

I know of a couple that are made by M*chip :shock:

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Cliff, I have read a number of threads, they only seem to be handling the firmware side of the story. I might have missed them, but I am more interested in the hardware side.
and then to the point: can I just switch on and off the 12V supply to create a slower turning fan or will that in the long/mid/short term kill the fan.

I have seen that there are fans that you can feed a PWm signal into and they self adjust, but I only have antiques around with either no tacho output at all or at most a tacho output and no input control (3 wire max)

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Quote:

but I am more interested in the hardware side.

Surely that depends on the type of fan? And if e.g. PWM at low speed for a vanilla 2-wire fan, the datasheet of the fan?

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I've run into this while designing a fan speed control for an amplifier. I found that 2 and 3 wire fans without a PWM input wire really hate being fed PWM. Most won't run at all if fed a high frequency and make nasty noises at low pwm freq's. Running them linearly solves the sound problem but lowering the voltage also lowers the torque of the motor so unless you integrate a higher 'kickstart' voltage the fan won't start often until you reach 2/3s of the fans supply V anyway. My advice is run the thing at a constant voltage or invest in a 4 wire fan.

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I have scrapped an awful lot of computers in the past, so my intention is to just use 2 of them.
I have not searched, but my first guess is that I will not be able to turn up any datasheet at all on them, So I will have to do with the information on the product sticker.
If the test system does not work out then in the end I might indeed just go and buy me two 4 wire fans and be done with it, but as it is a 1 of project and I have plenty of old fans I wanted to start with those.

mnturner thanks for the reply. I indeed have already read about not starting fans. I was going to let the fan run full speed for the first second always and then throttle back to the wanted value. I will have to investigate what the minimum voltage is that keeps the fans running. But that is the easy part. I will go for the linear option then.
Time is slowly running out, have 8 weeks till holiday and I might need it there so want to have it finished by then. and I also haev a number of other things that ned to be done by then.....