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Author Topic: 12V PC fan voltage control  (Read 6538 times)
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Hi, I am looking for a simple straight forward way to control the voltage to a PC fan with a 12VDC brush motor. Would like to be able to turn the fan on low speed (5V input) with Arduino pin set to low and high speed (12V) when the pin goes high...

Thanks for the input guys and sorry for being such a damn newb lol
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I've played a bit with a pc-type 12v fan. I changed its speed by changing the duty cycle of the pwm signal coming from Arduino.
To control the fan I used a couple of transistor I had aroud... a BC548 and a BD135.
An Arduino PWM-enabled pin controlled the BC548, which in turn activated the BD135. The latter was acting as an ON-OFF switch on the fan +12v line.
On the software side, analogWrite(fanPin, value) did the trick. Too low values made the fan noisy and still, because you need a minimum torque to have it start rotating.
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I fooled with an array (4) 120MM fans on a full H-Bridge motor controller (Pololu brand) and the PWM wouldn't work... Maybe I needed to set the PWM pulse speed lower (less than the 20kHz)? I don't know, but I'd just like a simple way to switch the supply voltage lines without having to rig up a bunch of relays or anything eccentric like that. Maybe a 5-pin relay or something? Mmm... The fans will always be on, so it's not like I need 12V, 5V and Off... They could always run at 12V and be switched to 5V when needed.
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If I use a 5v relay with both N.O and N.C outputs, I can put a pot on one of the outputs... Any idea's for doing this solid state, such as a transistor or mosfet that has 2 separate outputs depending on control signal?
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http://www.arduino.cc/en/Reference/AnalogWrite

"The frequency of the PWM signal is approximately 490 Hz."

Maybe those 20KHz were actually too high...
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That's my thoughts... Lower the PWM freq. or I might use a different way.
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less than the 20kHz

Yes, like < 100hz.
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If the PC fan really does have has a brushed motor, then you can just control it with PWM. However, all the PC fans I have come across use brushless motors, which don't work very well if you PWM the supply voltage.

You can buy PWM-controllable PC fans. They only cost a little more than regular PC fans. The specification says you should PWM them at 25KHz, which can be done using an Arduino, although most will probably work over a wider PWM frequency range than the spec says.
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Thanks dc42, I know of those pwm-capable fans, but I'm prototyping and I already have a bunch of fans laying around from a previous project and I'm trying to not blow my budget with buying things I already have. Any thoughts on how I can just switch the voltage going to them? I'm using a PC power supply, so I'll have 5V and 12V readily available and it would be great if I could switch between 5v and 12v rather than doing a variable control.

** I thought brushed motors were more difficult to control through PWM than brushless... Maybe they are brushless then, I'm not sure. I just remember that I tried it before and they didn't slow down or respond to the PWM control through the motor controller at all.
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Since the load is small a relay is probably sufficient... Can't understand why you keep running away from pwm though...
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they didn't slow down or respond to the PWM control through the motor controller at all.

Brushless motors are easy to tell: if you rotate the blades effortlessly, they are brushless motors.
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they didn't slow down or respond to the PWM control through the motor controller at all.
Brushless motors are easy to tell: if you rotate the blades effortlessly, they are brushless motors.

I have a pc fan that reads:
"DC brushless, model AD0612MS C70GL 12V 0,13A ADDA CORP."
but if I turn it when it's off I can feel "magnetic force hills" so to speak. Even if I give it a spin, it oscillates a little around a precise position just before totally stopping. Every 360° turn sees 4 distinct poisitions (4 poles ?).
I can control this fan with a transistor and an arduino pwm pin (i.e. standard analogWrite() call).
Is this "real" brushless or what ? I'm confused. Thanks for any hints!
(ps: I've looked for a datasheet but couldn't find nothing but endless part lists...)
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but if I turn it when it's off I can feel "magnetic force hills" so to speak.

That's a brushless motor.
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Then the fact that I could vary its speed with just an analogWrite() and a couple of transistors seems to contradict what dc42 said earlier...  ?-|
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Then the fact that I could vary its speed with just an analogWrite() and a couple of transistors seems to contradict what dc42 said earlier...  ?-|

How wide a speed range did you get? Come to think of it, I've heard of people PWMing brushless PC fans before. I remember they had problems with the tacho output.
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