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So I made my own stir plate

I followed the typical directions on the internet using an 80 mm computer fan w/ a few magnets in a cigar humidor box, and it at least does the primary function of spinning. However, when I set the flask with stir bar and water on it, no matter what I do it throws the stir bar… So I figured I would buy a few resistors to drop the voltage going into the fan itself. The fan either doesn’t have enough juice to get started or it throws the stir bar after about 1 minute or so… I’m using a 50 mm stir bar with center ring in a 2000 mL erlenmeyer flask, the magnets are rare earth magnets - maybe 3/16 inch if I remember correctly. What can I do to make the stir bar stay centered and spinning? Bigger stir bar? Stronger magnets? It’s a fun little project, but now I’m starting to get frustrated. Any help is appreciated.

Get adjustable resistor. When it throws the stirrer it means the rotation is too fast for that bar. Adjustable resistor would let you control the speed and adjust for different bars in the future.

Get a smaller stir bar. Most people find that a 1" stir bar works far better on home made stirplates then the larger 2" stir bars.

Whoops, forgot to mention I have a 25 ohm rheostat on it as well. I tried fiddling with various speeds and was basically the same outcome. Come to think of it, could I have my magnets too close to the center? I have them glued to polar opposites of the central cylinder of the fan, maybe 1 inch apart. Should I move them closer to the edge of the fan blades?

Heres how mine are spaced and works well with a 1" stirbar. I used a SS fender washer to space my magnets.

Thanks for the replies! It seems I need to get a 1 inch stir bar.

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