Dear All, My home-made fan driver circuit seems to be working. Let me know if you are interested and I will describe the circuit; it's just an NMOS FET and a PNP transistor, as suggested in the Fintek datasheet. However, the fan tacho input is not correctly measured at low PWM values / voltages / speeds. I believe that this is because the signal on the wire from the fan switches between ground and the fan supply [*]. The motherboard must have a resistor divider to get the 12V on this wire down to the safe input for the chip (again there is a circuit for this in the datasheet). But if the (average??) fan supply voltage is too low, the input to the chip sees 0. Or, the tacho signal is modulated by the PWM signal. I presume that this is a standard problem. Do other motherboards that have PWM fan control have unreliable tachos at low speed, or do they have extra components between the fan's tacho wire and the chip to deal with this? Maybe it depends on the PWM frequency? Google finds an Analogue Devices chip (http://www.analog.com/library/analogdialogue/archives/38-02/fan_speed.html) where they describe this problem, and suggest that when you want to take a tacho reading you should briefly set the PWM output to 100%. Presumably this would have to be done within the chip for the tacho-tracking automatic mode to work, though it could also be implemented by the O.S. for other control mechanisms. [*] Why they have done it like that I cannot imagine. It would have been much simpler to have the switch in the fan go from ground to open-circuit, and to have whatever pullup resistor to whatever voltage you need on the motherboard..... Regards, Phil.
