- #1
Artlav
- 162
- 1
Hello.
I'm trying to control the speed of a fan (12V cpu cooler) with PWM (555 + MOSFET).
Should be quite straightforward, but i noticed that the speed of the fan does not seem to always change in the same way the duty cycle does.
With an optical RPM meter I'm seeing that the lowest RPM is at about 5% of POT's range, and goes slightly higher below that.
In the 5%-60% range the RPM climbs steadily with the duty cycle.
At 60% there is the maximum RPM.
Between 60% and about 80% it decreases.
Then between 80% and 90% it increases again, but at 90% it's less than at 60%.
And 90% to 100% it decreases again.
Even better, that 60% peak is higher that the RPM of the fan just hooked up to the 12V directly, while 100% is less than that.
An LED hooked up to the same controller seem to change brightness linearly.
What is causing such a strange behaviour?
Or rather, how to fix it?
Schematics:
I'm trying to control the speed of a fan (12V cpu cooler) with PWM (555 + MOSFET).
Should be quite straightforward, but i noticed that the speed of the fan does not seem to always change in the same way the duty cycle does.
With an optical RPM meter I'm seeing that the lowest RPM is at about 5% of POT's range, and goes slightly higher below that.
In the 5%-60% range the RPM climbs steadily with the duty cycle.
At 60% there is the maximum RPM.
Between 60% and about 80% it decreases.
Then between 80% and 90% it increases again, but at 90% it's less than at 60%.
And 90% to 100% it decreases again.
Even better, that 60% peak is higher that the RPM of the fan just hooked up to the 12V directly, while 100% is less than that.
An LED hooked up to the same controller seem to change brightness linearly.
What is causing such a strange behaviour?
Or rather, how to fix it?
Schematics: