To cool the enclosure of my project I've installed two identical 40mm fans.
I've connected them to an IRF520 which is controlled by a PWM pin like shown here:
http://itp.nyu.edu/physcomp/uploads/arduino_bb_pot_transistor_motor_diode.png
But the weird thing is when I try to use PWM it only works on 1 fan, the other refuses to spin unless it's full on.
How can this be? They are identical and I've tested them both with the same IRF520 and the same pin on the arduino...
Are they BLDC fans? Brushless DC?
If so, you'll find that PWM is not going to work well.
Yes, they are brushless DC...
What type would be better to pwm?
The problem with the brushless DC motor is that it is, internally, driven with a kind of pwm. By interrupting the supply voltage using your own PWM you are breaking the PWM inside the motor. You would need a traditional brushed motor based fan to use PWM.
Alternatively, you can vary the voltage to the fan - that usually works.
You might be able to construct a low-pass filter on the PWM signal to give a varying DC voltage. It might work - worth a try.
Rejected666:
Yes, they are brushless DC...
What type would be better to pwm?
Nice reference here: Driving a three-phase brushless DC motor with Arduino - Part 1. Theory
HI,
Most brushless fans WILL slow down with less voltage applied.
If you put a large capacitor across the fan, say 100uf, it may work to drive that with PWM..
If you put a large capacitor across the fan, say 100uf, it may work to drive that with PWM..
That makes a low-pass filter, doesn't it?
That makes a low-pass filter, doesn't it?
Yes, it "integrates" the PWM signal to result in different output voltages..
See: http://arduino-info.wikispaces.com/Analog-Output
In this case this is all happening at a higher power level..
Well, I put a 100µF cap ont the fans and it works like a charm 
Thank you all for your help 
Yes, it "integrates" the PWM signal to result in different output voltages..
Really useful info Terry, thanks. I'd love to see the output waveform of the PWM RC filter at different PWM levels.
Wish I had an oscilloscope =(.