PWM to Analog Question

Hey there guys,

So I have a new water cooled computer with three very big 12" fans. I want these to be temperature controlled, so I figgured the best way was off the motherboard fan header (since this can be set of the CPU temp).

The problem is that the output off the motherboard is PWM (pulse width modulated) and these big slow RPM (but very high flow) fans make a huge amount of PWM noise at slow fan speeds.

Does anyone know a really nice trick to take PWM signals and turn them back into analog voltage modulated signals.... Besides just using a active low pass filter, run into a simple foltage follower for current gain?

Thanks, much appreciated,
Jonno

Hi,

You could measure the pulse width of the PWM signal using period
measurement like a counter/timer does and use a DAC to convert this
data into a voltage. Start with a (74HC)4060, use the on board
oscillator as a reference. The counter itself will count the number
of pulses generated by the oscillator, use the PWM signal to control
the reset input of the (74HC)4060. Derive also a command pulse from
the PWM signal to control an eight bit latch which is connected to
the outputs of the counter. The outputs of the latch are fed into an
eight bit DAC. Select a DC coupled audio amplifier to convert the
output current of the DAC into a voltage to drive the fans directly.
You'll have 256 different speed levels for your fans, that should be
enough.

on1aag.