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Electrical Help

I need help with my stir plate. I can’t get the motor to operate slow enough, it keeps kicking the bar. I have a 25 ohm rheostat on it now, if I put a larger one on will that slow down the motor more. I doubt if it’s the location of the magnets because I have them just about scraping the cover.

25 Ohm is too small.
I personally don’t recommend using the rheostat inline to control the motor speed. At lower speeds, you are putting more power into the rheostat itself, and since most of the time is spent at lower speeds, you are wasting electricity, and unnecessarily shortening the life of both the power supply and the fan.

Instead, go to stir starters, and check the docs there. They recommend using the rheostat to tune a LM 317 voltage regulator. It’s just a little more soldering, but a completely different tier of build quality. You can get the part at Radio Shack. And see the wiring diagram here:

I built my own, pretty much to their specs, and it works like a champ. I do get the occasional thrown bar, but it’s pretty rare. I find having a flask just a little of center helps.

One more tip if your still building: Glue an extra magnet inside the box, well away from the fan. I keep yet another magnet stuck to the outside of the stir plate, held in place by that internal magnet. When it’s time to decant, I know right where that spare magnet is stuck. I grab it, and use it to hold the stir bar in the flask while I decant. While not technically a problem, you don’t want to pitch a stir bar with your yeast.

How do I determine what watt of resistor I need for this?

I’m pretty sure 1/4 watt will do. I’m pretty sure I did 1/2 Watt, but only because I had them on hand.

The current output from the regulator is mostly go into go into the fan itself

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