I need to control the speed of a fan, do it efficiently and not lose the tacho signal from it.
Since the tacho signal is an open collector signal, I can't just PWM a transistor connected to the ground. Since there are some electronics inside the fan and I need feedback from the fan speed, I can't do PWN onto the +12V either. The fan used must be a 3 wire one, so no extra pin for controlling the motor.
Therefore my only option is to change its power supply voltage. I'll be using an LM2674 and mess around with its feedback using a bit of a hack as seen in the picture (similar values will be used, the simulation is for another regulator IC). The Q1 transistor will, when opened, pull the resistive divider down and increase the voltage to the output.
Now, in simulation, when I PWM the transistor, I can modify the output value of the voltage (from 4.8V to 12V). The feedback voltage doesn't look good however, see attachment. I'm not sure the regulator likes it. The output voltage is fine.
So my question is: Is this a good idea? Is there a better way, that lets me keep everything, efficiency included?
I'm using a pretty fast PWM (~40kHz).