|Steve Stephens wrote:
The best way to control speed of a fan is by using a Variac. Good old ones from the 1960s-80s and US made can be had for under $100, often for $25, more or less, at radio swap meets, etc. I run fans from 120 volts down to less than 6 volt battery DC fans using my variac and, when needed for DC, a bridge rectifier, about $5 or less and readily available and tiny.
Yes this is true; however I was thinking he wanted something small which could be installed permanently on a fan. But yes the variac is the way to go, with its undistorted AC waveform and full variability.