>
>> Somewhere recently I saw an ad for a fan control chip that
>> monitored the fan commutation spikes instead of requiring a
>> tachometer.
>
>
> http://www.piclist.com/techref/au/net/ezy/www/http/~fastvid/encoder.htm
> http://www.romanblack.com/encoder.htm
>
>> Of course, I now can't find that ad.
>
> I'd be interested to see that as well.
>

The project leader on this wants to use a Cyclone FPGA for this and a
bunch of other stuff, but I'm still leaning towards a PIC (it's what I
know). My current thinking is to use an N channel FET that I PWM with the
PIC. There's a 2 ohm resistor between the source and ground. Measurements
indicate that at full speed, I get 360mv across the resistor, and it
pulses to -200mv at 440 Hz. I could run this into a comparator input on
the PIC, and time how long it is between pulses (when the PWM is on) to
determine the fan speed. Temperature sensing would be on-die diodes with a
pull-up resistor on PIC ADC inputs.

My current thinking, anyway. I can do pretty much everything in one chip.

Harold



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