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4 pin PWM fan is nuts on anything other than 100%

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Joined: 5 years ago
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Hi all,

I am still pretty new to electronics so I apologize if this post is silly or off topic.

I have a 4 pin PWM fan and when driven at 100%, the tach output is clean. When driven at 1-99 percent, I got 10-15 spikes on every revolution. Since I currently only have ONE fan (picking up another brand in a few days) I was wondering if I should be doing something to clean up the signal, or if my fan is just broken. The pulse width doesn't seem to get longer or shorter as the fan speeds up or slow down as well. I finally noticed the probkem because I have been working with an arduino to set/read fan speed. I can tweak the speed no problem but (I thought) the arduino was going nuts when trying to read RPM's at less than full speed.

When I hooked the fan up to a raspberry pi, the same problem occurred: normal RPM reading at full speed (roughly 2500 rpms) but the indicated speeds would jump to 18k-25k. After seeing the noise, I can understand why my arduino and pi would go nuts!

fan running less than full speed

fan running at full speed

Any thoughts?

This topic was modified 5 years ago by scottplude