Wiring, stirbar, magnet, power questions about stir plate

Ok, I have tried to build a stir plate (3 actually). One of the fans appears to have a speed control on it (Pictured). One has two small magnets and the other two have a semi horse shoe shaped magnet (also pictured).
My stir bars are square shaped that I paid too much for from the LHBS and the power supplies I have so far are 3.7 volts.

My problem is that when I the two motors without the speed control the stir bar just "jitters", it does not stay with the RPMs of the motor/magnet. I thought the small magnets might not be powerful enough but it seems to do this on both. I didn't think it was spinning too fast with just a 3.7 volt power supply. Is it that I want round stir bars instead of the square? What is the ideal power supply? Can anyone give me an idea of how to wire the speed control on the fan shown so that I can use a 9v power supply?

Thanks-

I had the same issue with my first attempt at a stirrer. I had tried it with two angled magnets and a 12V power source. My second attempt, I used a 6V source and a single long magnet right in the middle. This worked like a charm. I used a 25Ω rheostat to dial in the speed and I was able to get it working perfectly.

on my stir plat when its going to fast the bar just jitters
what is your drive motor ?
a computer fan ?

I'm using a fan from a power supply and the magnets are from a harddrive.

I used an old computer fan. Mine looks pretty much like this one. I just finished it last night.

So, that site seemed to say you'd splice a 6V power supply directly into a 12V fan. I don't understand these things, but that's OK? Are all fans typically going to be 12V? I'm thinking about just buying one for $6 on NewEgg, since you can get them cheap and all tricked out with lights and crap (might as well have some fun with this for a couple dollars).

The computer fan spins way too fast for the magnetic bar to keep up. By dropping the voltage, you get a slower spin.

Beauty.

Since I'm lacking in the old-computer department, what size are the rare earth magnets? I'm going to pick up a few from here, I think (I may have a few left over to share).

I would go with these. If you're using the button type, I would put two to create a similar effect of the bar one.

Cool, just ordered those. Thanks!

Suppose I'll have to stop by the (shudder!) RADIO SHACK for a rheostat and a toggle switch... damn, I hate that place.

the_bird said:

Cool, just ordered those. Thanks!

Suppose I'll have to stop by the (shudder!) RADIO SHACK for a rheostat and a toggle switch... damn, I hate that place.

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Me too, but the had the cheapest prices on them. I think all in all, along with a project box I paid about $11.

I've got a couple cigar boxes, including one with a glass top that I'll use if I end up buying a fan with LEDs.

the_bird said:

I've got a couple cigar boxes, including one with a glass top that I'll use if I end up buying a fan with LEDs.

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That sounds good, my fan is very old and still has dust bunnies on it!

FYI, the rare earth magnets showed up today (Satuday) - almost literally 48 hours after I ordered them. That was with the cheapest shipping option, too. Maybe I happen to be lucky and just live near the place, but that's damn good service.

Now, am I really only going to need one of these? They seem so... small. Wicked cool, powerful as hell, but still... just one?

EDIT: Oooh, I just thought of something.... these extras would be PERFECT for holding up a drip tray on the kegerator!

the_bird said:

I've got a couple cigar boxes, including one with a glass top that I'll use if I end up buying a fan with LEDs.

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Wow...disco lives on!

Done. A surprisingly easy project; it's shocking how much NB wants for one. Ended up costing me a bit more than I wanted (had to buy a four-pack of knobs when I only needed one, computer fans are $12 at Radio Shack versus $3 and NewEgg, etc). But, it seems to work great, it's got a toggle switch and a rheostat, plus I found an old adjustable power supply so that I can run it at 6V, 7.5V, or 9.0V (anything less doesn't work).

Not, just need to get a flask and a stir bar. Looks like the_Bean and I will road-trip over to Hennessy after I pick her up on Tuesday, if it works I should be good to go to make a starter with it for next weekend.

Thanks for all the help, gents; Clamper, I basically copied the same design you did.

EDIT: Pictures later.

the_bird said:

Done. A surprisingly easy project; it's shocking how much NB wants for one...

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Nice! Yeah, once I decided to make one, it was done the following day. All it took was a trip to Radio Shack. Have fun with it, I'll be ordering a 2 L flask soon, for now my 1.5 qt. glass juice bottle with do.

What kind of magnet is everyone using? I bought a variable power supply from Big Lots ($5) and a rheostat but the magnet still will not spin it will just jitter or it gets thrown and just sits on the side. I guess next I will try a round stir bar.

jtvinny said:

What kind of magnet is everyone using? I bought a variable power supply from Big Lots ($5) and a rheostat but the magnet still will not spin it will just jitter or it gets thrown and just sits on the side. I guess next I will try a round stir bar.

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I busted open a failed hard drive (I'm a sysadmin so I have these lying around) and used the magnet from inside. There's a good HOWTO here if you have a hard drive to use:

Onebeer.net :: Homebrew :: Harvesting Magnets

Are you using a vessel with a flat bottom like a flask, or a convex bottom like a growler or a jar?

If you turn down the speed almost all the way, can you get it to spin very very slowly? If so, does it look like it's turning around the center of the stirbar?

I would skip the speed controller that's on the fan in the photo, it might not slow down the fan enough to let the magnet center properly. From the photo, your fan is using .23a so you should be good with 6 volts and the 25ohm rheostat.

I do not have a beaker as of yet, so I have tried it with a growler. I have looked for what king of vessels i might have with a flat bottom but when I tried it with a plastic bottle (flat bottom) I got the same results. I have a motor wired up with the rheostat and still have not had any luck that's why I thought the square stirbar was the weak link. I imagine the round stirbars with a plastic ring around the center would spin easier.