Stir Plate Speed Control

[laho72]

Recently built a stir plate and wired in a 25watt 3ohm rheostat (Radio Shack 271-275). I have seen alot of DIYers online use the same one. Seems as if the lowest speed setting still spins the magnet out to the edge of my 2000ml flask. I have tried both a 1" and 1 3/8" magnet. Anyone have any recommendations/comments?

 

[LuiInIdaho]

I recently put together a stir plate using a fan with the speed control built in. I was also throwing the stir bar, using a 2 inch stir bar. I finally got it to work decently by doing the following. I replaced the fender washer (about 1 inch in diameter) with a 1.5 inch diameter fender washer. I also replaced the two half inch ceramic magnets with two 3/8 inch rare earth magnets. I placed them opposite of each other clear to the edge of the washer. I placed one washer with the north pole up, the other with the north pole down. I got the stir plate to work all day today without throwing the stir bar. I think that the difference is that I used stronger magnets, and I placed them farther apart to put the stir bar more in the center. I should also indicate that the stir bar does not totally center on the stir plate. I hope that this might help. Mark

 

[Hang Glider]

I splurged on this - it took about 2 weeks to come in from Hong Kong - (saw it somewhere on this site!) It works very well. Electricity overcomes inertia about 10 rpm...
Speed Control

View attachment speed control.bmp

 

[captwalt]

I had the same problem when I built mine. I bought a pack of 5 or 6 44ohm resisters and started putting them inline one at a time until I got it right, I think I used 3 of them. Now it works great

 

[Shaft333]

I added some series resistance to get a speed range that was usable.
The stir bar is 1", I have 2l flask, hard drive magnets on a washer, and a 12v supply.
My range of speed is limited but it doesn't matter much to me.

 

[laho72]

Not sure how to wire more resistors in series. Resistor from Hong Kong looks tempting though. May try the rare earth magnets with a different fan too. I appreciate all of the input!

 

[ffaoe]

i power my homemade stirplate using what was a universal AC adapter that has a switch that sets the voltage between 1.5V and 12V. thats how i regulate my speed