> 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.

> I'm 
> interested in a single chip (ideally with fan PWM fets on 
> board) that monitor die temperature (external PN junction) 
> and adjust fan speed based on that. It seems that it could be 
> done without a fan speed monitor at all (just adjust pwm 
> based on temperature error), but we do need to know if the 
> fan is stuck.
> 
> So, what have you used? What do you suggest?

You can also read the back emf using an analog switch:
http://www.piclist.com/techref/io/motor/regulate.htm#

---
James.
 

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