Electronics > Beginners
Trying to build a PWM fan controller
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spec:
By the way, you can replace the 5V PSU brick with a 5V Zener diode and resistor running from the 12V supply.
(time for work now)
eamoex:
Hello All,

I've breadboarded Ian.M's circuit and it works way, way better than what I had before. Fan's speed makes a linear, steady progression throughout pot rotation. Thanks everyone for your help!  :-+

I've checked the PWM pin signal without the fan connected and it is a crisp square wave which goes from almost zero to 100% duty cycle. Frequency tends to drift a little along the pot travel, though, and seems just a bit on the low side, ranging from about 19 to 22 kHz.

Hencewhy I wish to trace the output of the tach pin, to check if the fan goes the whole range of RPMs (from 500 to 2000, roughly). I did connect the oscilloscope probe to the tach pin, but I don't understand what I see. It looks like a garbled version of the PWM signal, which makes no sense, because the tach signal should change in frequency but not in duty cycle (actually I have not read this anywhere but it seems obvious enough).

Where do I have to stick the oscilloscope probe and ground clip to properly read the tach pin signal?
Ian.M:
Try a 4K7 pullup to +5V on the tach pin and see if that cleans up the signal.
rstofer:

--- Quote from: eamoex on January 23, 2019, 07:40:00 pm ---Where do I have to stick the oscilloscope probe and ground clip to properly read the tach pin signal?

--- End quote ---

Intel spec says Tach output is open collector and that the motherboard will have a pull-up to 12V.  So, you can probably connect the output to a 10k resistor connected to 5V.  Probe between ground - the common ground for everything - and the pin.

Ii don't know which schematic you are using.  If you don't have a 5V source, you can probably use 12V and a 10k resistor.  Ground is still ground.
rstofer:

--- Quote from: eamoex on January 23, 2019, 07:40:00 pm --- It looks like a garbled version of the PWM signal, which makes no sense, because the tach signal should change in frequency but not in duty cycle (actually I have not read this anywhere but it seems obvious enough).

Where do I have to stick the oscilloscope probe and ground clip to properly read the tach pin signal?

--- End quote ---

The tach signal, according to the Intel Spec gives 2 pulses per revolution.  Whatever mechanism they use internally, perhaps photoelectric or maybe hall effect, will have an on time proportional to speed.

Without the pull-up resistor, all you're going to see is trash.

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