camiller

Recipes 

Look at HenryVance3's first pic in post 981 of this thread, that is how your magnets should look. He, like a lot of us, glued a largish fender washer to the hub to make it easy to mess around with magnet placement before gluing them. You don't have to do that but it is easier than messing with tape.

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petey_c

Recipes 

I recently built a stirrer using a 120V muffin fan. I tried to cobble one together using materials I had readily available. The fan motor's normal operating speed was about 2700 rpm. I decided to use a three speed fan controller from HD. Even at the slowest speed it would throw the stir bar. I then decided to use an incandescent dimmer, even though I know they're for different types of loads. There was just about a 5° range where the motor would turn at an acceptable speed using the dimmer. It also couldn't be depended on to stay at that speed once I turned my back. So I decided to do a complete rebuild of my stirplate. I used the materials list on page one here (I used a 12 VDC wall wart and a 12 V motor) and sorta bread-boarded a test jig. (Proof of theory). The 25Ω pot didn't offer as much control of the speed as I'd wanted. I found a pot that fit the bill. I didn't check the value of the pot and can get that info a little later. I'm using a 2 quart pitcher with a relatively flat bottom. I had it stirring at a good speed for a few hours. Next time I'm brewing (this week?) I'll test drive it with actual wort.

twd000

Recipes 

OK, got some time to work on mine again today. I measured 8V at the lowest setting, and 12.8V at the highest setting. I am running at low speed while troubleshooting magnet configurations.

I have two neodymium disc magnets. They are super-glued to a fender washer that is temporarily taped on the center hub of the fan. The discs are oriented with one N-pole up and one S-pole up. I have a 2" stir bar with center-ring, and the outside edges of the two discs are 2" apart.

I have the stir bar in a Pyrex measuring cup full of water. When I turn on the fan and place the cup right over the magnets, the stir bar rattles around the bottom and turns slowly - probably 5-10 RPM. This is while the fan is going 100's of RPM. It's like the magnets "grab" the stir bar, then drop it, grab and release, etc.

Any help?

mightynintendo

Recipes 

the problem sounds like there isn't a strong enough magnetic force acting on the stir bar. maybe your magnets are too far away from the bar, maybe they're too weak, maybe they're not centered well.

twd000

Recipes 

I don't think I can get the magnets any closer to the stir bar. I don't even have the fan in the housing - just hovering the vessel directly above the fan and magnets. The magnets are centered well enough to prevent them from being thrown off the fan when it spins up.

The magnets I'm using are 18mm diameter by 3 mm height. What size magnets is everyone else using?

mightynintendo

Recipes 

well the build calls for hard drive magnets. most people are using those or rare earth magnets.

twd000

Recipes 

neodymium IS a rare-earth magnet, right? which is the same material as a hard-drive magnet. So the only difference would be the size

mightynintendo

Recipes 

Oh. I guess I don't know then. You are using disc-shaped and everything i've seen used is rectangular-ish.

Jukas

Recipes 

I'm running into a problem with the stir plate I built. I'm using a 12V DC wall wort and a 120mm Antec fan. The magnet I'm using is a circular magnet from another 120MM fan.

When I put the stir bar on the plate it spins perfectly, however when I add a 1L flask the stir bar gently walks to the side and then does nothing, even with the rheostat turned to maximum.

I've added pictures as I can get them of the setup. Should I try and get the magnet closer? Try a different wall wort, or a different magnet?

Attached Images

   

Accidic

Recipes 

Second the magnets aren't strong enough for the build throwing stir bars. Mine wlll do 5700rpm constantly without throwing the bar on a flat surface container.