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OPZ PWM Armbian Legacy&Mainline unexpected results

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Published in 2017-12-12 02:48:29 | Show all floors |Read mode
Hi,

I wanted to use the Orange Pi Zero PWM to throttle a blower fan in its little case it came with.
The fan at full speed makes too much noise at 10,000rpm though it is effective in cooling it.

So I got a FQP30N06L MOSFET and wired it up to the OPZ pins (Ground to single pin rail and PWM mosfet pin to middle serial header).

The problem is I cannot get a fine range of throttling, the speed changes in steps there might be about 2 speeds I can run the fan at.
For example on the Armbian mainline 5.35 image at 48,000 period setting there is a narrow duty cycle range where it operates 88-89% for medium speed then at 90% it goes full throttle. I wanted the fan to run always at a low speed and increase with temperature so it makes as little noise as possible but does not start and stop the fan, which causes premature wear and tear.

The Mainline and Legacy images have a difference in that the legacy needs a third party PWM driver (hard to use) and the Mailine release the PWM output is set at inverse by default which reverses the min and max throttle settings and here its the same result, I cannot get finer control than when using the "normal" polarity.

At lower period rates, say 10,000 and below I get finer control of the fan and can get a wide range of speeds but it makes a buzzing noise, because the pwm frequency enters the human ear range as its pulses cause resonance in the motor.

Is this just a limitation of the Orange Pi Zero PWM or do I need something more, a resistor or something maybe in the wiring for the MOSFET and fan?

Regards,
Rauli
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