Using a triac to provide phase control of the voltage works poorly for the same reason, with the speed very sensitive ... If you build this circuit in a separate box, with a mains socket to plug in an ...

If you are using this mod on your CPU fan then use the 20 ohm rheostat to prevent fan stall at the slowest speed setting ... to help prevent any short circuits when fitted in the case.

PWM or Pulse Width Modulation has generally been regarded as too complicated for PC fan speed control ... using rheostats or linear voltage regulators such as the LM317. I present here a PWM ...

This results to a voltage increase in R2-P1 and C3 causing the triac to be triggered earlier and the speed increases accordingly. Another version found here. If the main use for this circuit is to ...