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Old 01-07-2009, 01:07 AM   #1
Dec 2008
Kansas City, MO
Posts: 137
Liked 2 Times on 2 Posts

I built a stir plate recently using a cooling fan, cigar box, 25oh 3 watt potentiometer from radio shack, and 9 V DC power supply. Currently, no matter how far or close I place the magnets from the surface, it throws my stir bar (42 mm) at a fairly low to low-medium speed before I can get a good vortex. Right now I have 10x2 mm rare earth magnets. Im waiting on some to come in the mail that are 12x5 mm, so I'm hoping that will solve the problem.

Also, I'm very concerned about the potentiometer. It's getting very, very hot. Is this normal?

Thanks for your help.

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Old 01-07-2009, 02:41 AM   #2
Sep 2007
Bethlehem PA
Posts: 120
Liked 1 Times on 1 Posts

How do you have your magnets mounted? One on each side, both in the center..ect? When i first built mine i had the magnets separated, one on each side of the fan hub and had the same problem. I stacked them in the center and its been fine ever since.

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Old 01-07-2009, 03:52 AM   #3
Brewers Hardware
Dec 2008
Huntington Beach, CA, 92649
Posts: 453
Liked 7 Times on 6 Posts

I've never used individual magnets, but I've built and sold almost 80 of them now with a single magnet from a hard drive and had great success with that. The rheostat will get hot because it's dissipating all the heat from the current the fan wants to draw. That's why that design is just not the best way to do it. Not only that, but it can't compensate for fluctuations in line voltage.

Google the LM317T voltage regulator and built a circuit around that. You can probably use most of what you already have. You don't have to use the capacitors, they just make the fan run a little smoother but at a minimum you just need the LM317, a potentiometer and a resistor.

It works much better!

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