Stir plate help

I built a stir plate using a cigar box, computer fan, and rare earth magnet from an old hard drive similar to the one found in this guide: http://www.donosborn.com/homebrew/stir_plate.htm

The problem I'm having is that I can't get the computer fan to start slow enough to not throw the stir bar off. When I slow it down the fan eventually freezes up and I have to turn my potentiometer way up to get the fan to start spinning again. At that point it starts spinning too fast and the stir bar dances/gets thrown off.

If anyone has any advice I'd greatly appreciate it. I tried fixing it by using 10, 15, and 22 ohm resistors (you can see the 22 in use in the picture I attached), but that didn't seem to help. It slowed down the fan, but as soon as the stir bar was introduced, the magnetic pull would be too much for the fan and it would slow to a stop.

I had similar problems and gave up on speed control altogether. Instead I
take a cloth table napkin or dish towel. Fold it in 1/2 and place it on top of the stirplate. Set the flask on top of the towel. If the magnet still gets thrown fold the cloth in 1/2 again. With a little practice & patience you'll find the sweet spot.

Good Luck

Get a piece of PVC pipe about the size of the center of the fan. Put the fan on the bottom of the box and cut the PVC pipe so that the pipe sits on the top of the fan and it comes to about 1/4 inch from the top of the box. Then glue the magnet on the PVC pipe then glue the opposite end of the magnet to the center of the fan. This should help eliminate the magnet field that the magnet puts out interfering with the fan. The magnet is so strong that the "internal magnetic field" caused by the motor cant over-power the earth magnets field till you up the voltage to cause the motor to overpower the magnetic filed. Hopefully this will help you out some.

I recently built my stir plate and had a similiar problem. WhenI started the fan on the lowest setting it would freeze up and wouldn't move. I found that if I turned it on at the slowest speed, then slowly turned it up while listening for the fan to start. As soon as you hear it/see the bar starting to move, turn the speed all the way down and it should hold like that...at least it did for me. Otherwise play with the distance between the magnets or from the magnets to the stir bar like other people mentioned. good luck

I had to end up buying some stronger rare earth magnets. The ones out of my old hard drive weren't strong enough to hold on to my stir bar. I found them on Amazon for a couple bucks. Haven't had a problem since.

How big is your stir bar? I've heard the bigger ones tend not to get thrown as easy. I do my starters in a concaved bottom jug because it was 5 dollars vs what flasks go for, never had a problem and my potentiometer has the same issue u have.

I was able to fix that problem in mine by changing the distance between the two magnets. Wider spacing eliminated the throws for me.

Edit: Oh crap, I just looked more closely at your picture. A single magnet is going to make it tough. It might be worth picking up a couple of small magnets. If you go that route, pick up a few extras; they break easily.

Thanks for the replies. I'm going to try a few things tomorrow and see if I can get it to work finally.

I just finished building mine. I had the problem of throwing the bar... Turns out the magnet was not quite centered on the fan... I pried it off, remounted it without glue, made a few adjustments until it didn't throw the bar and then squirted some epoxy on the edges to hold it.