Help with comparator design
Posted: Mon Dec 29, 2008 5:12 pm
My goal for this project is to control a thee wire brushless fan with the PWM signal from a four wire fan header. I have looked at number motherboards and discovered implementation the control signal varies. The INTEL specification is mute on a logic hi value, but requires a maximum logic low of 0.8 volts. The fan header is required to sink up to 5 mA and the signal must be pulled up on the fan side, 3.3 volts is recommended. The control signal must be 25kHz, but it can be 3.3 or 5 volt TTL logic. To overcome this, a comparator with a threshold of 2 volts looks like a viable solution. With a comparator, a fixed amplitude square wave to the low pass filter is possible and allows for an amplifier with fixed gain. At present, I have been unable to find information on op-amp comparator design. The 1 Meg ohm feedback resistor is best guess. How much feedback would give optimum function? The null resistor, R9 is also also best guess at this point. I would appreciate constructive criticism to correct omissions or departure from good design practice. Thank You.