I use the same 120v AC muffing fan from RS. I've found that the rotary type light dimmers work well for speed control. There are, however, some variables that should be considered. There will be considerable friction between the stir bar and the bottom of the flask due to the pull of the magnets. This friction must be overcome in order for the stir bar to inititally begin to spin. Once moving, the frictional forces a less and as a result the fan will sometimes spin too fast. The problem is that when you reduce the power with the dimmer, the fan can sometimes stall if there is a voltage fluctuation (not uncommon in household circuits). When the stir bar stalls, sometimes it will not self start when the voltage returns to normal. The trick is to adjust the distance between the fan and the flask so that the frictional force is just right. Not too strong and not so week as to not couple with the stir bar. This requires some trial and error adjustment to the gap. The other thing I have found helpful was to wire a light bulb (25-40watt) in series with the fan motor in addition to the dimmer. The light bulb functions as a big resistor and helps increase the dynamic range of the dimmer. This also requires some trial and error to find the best bulb wattage. So now, you have three main variables to shuffle around. The dimmer, the gap and the light bulb wattage. Much also depends on the strength of the magnets used and the type of stir bar. One last thing, I've learned that a stir plate works best when it is well leveled. I use a small carpenters level to check and adjust the level. That's a small thing, but it can sometimes really help prevent throwing the stir bar at higher speeds. Oh yeah, all of this stuff can vary considerably with the specific fan type that you are using. What works with one, may not work the same way with a different fan, stir bar or magnets.
