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Old 04-01-2011, 12:16 PM   #1
Hops
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Dec 2008
Orlando, Fl
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I am trying to make a DIY stir plate but all the "pea" does is jump around. It won't spin. I am using a 12v dc 4 inch computer fan powered by either a 5.9 v or a 9 v wall wort. I have tried 1 magnet, 2 magnets one on top of the other, and 2 magnets on opposite sides of the top of the fan. I have adjusted the distance from the spinning magnets to the "pea" from about 1/4 " to 1 1/2 ". I have read every post about the DIY stir plates and looked at all the pictures. But this *@%#$^ "pea" won't spin. Someone out there must have had the same problem... HELP.
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Old 04-01-2011, 01:50 PM   #2
wyzazz
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How large is your stirbar? My 2" barbell is way harder to get spinning than my 1" one. What kind of magnets are you using? If you use a Hard Drive magnet 1 should do the trick, the round ones will take 2. And use a spacer (PVC/SS Washer/Wood) of some sort between the fan and magnet.
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Old 04-01-2011, 01:53 PM   #3
carrotmalt
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I don't know what "pea" stands for, but what kind of stir bar do you have? Mine's about 1.5 inches long, and I have two magnets about a cm in diameter each that are placed just under the outer edges of the stir bar. Mine took a while to get right, but it's pretty much as close to the plexiglass it'll go before the fan tries lift off the table (mines not attached yet). You might let it sit on the magnets for a minute or so and make sure the poles aren't confused. I think this helped mine finally get locked in. Now it works every time w/o fail.
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Old 04-01-2011, 02:12 PM   #4
marnel
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Have you checked the poles of the magnets?

 
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Old 04-01-2011, 02:19 PM   #5
Hops
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Dec 2008
Orlando, Fl
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Sorry about the technical terminology... "pea" equals stir bar. I am using a 1 inch stir bar. The magnets are from a hard drive. I will try the spacer between fan and magnet. The stir bar is currently sitting in the magnet hopefully that will allow it to orient itself correctly.
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Old 04-01-2011, 02:27 PM   #6
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Quote:
Originally Posted by Hops View Post
I am trying to make a DIY stir plate but all the "pea" does is jump around.
If it doesn't sound like your fan is struggling to spin, it might be a polarity issue... I had a similar problem in that the bar didn't want to center itself on the magnet. I went to Ace (they have very cheap Neodymium magnets BTW) and bought circular magnets.

At first I though I was having the same issue...

But by flipping one of the magnets, the poles lined up and the stir bar found its happy place...

On the actual fan, I epoxied a thin steel washer so I could adjust the magnets if necessary...
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Old 04-01-2011, 02:34 PM   #7
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Link to those magnets I found at ACE. $4 is a good investment to avoid having to jack around with a hard drive, IMO...

http://www.acehardware.com/product/i...Id=51388772714
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Old 04-01-2011, 02:44 PM   #8
Catt22
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Old 04-01-2011, 02:56 PM   #9
a10t2
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Is the bar getting thrown to the side immediately? If so you might just be starting it off too fast. I've found it's best to start the motor off barely spinning, let the bar get started, then increase the speed.

If the bar stays centered but doesn't spin, you just need more powerful magnets. HDD magnets didn't work for me either, so I got some rare-earth magnets, and now I can spin the bar in a 3 gal carboy. If you have a Harbor Freight nearby, they'll probably have some in stock.
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Old 04-01-2011, 03:27 PM   #10
camiller
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Other thing to consider ... is the center of the magnetic field (not the center of mass) centered on the fan? With a hard drive magnet I don't think the two are the same due to the odd shape. Let the stir bar find it's location on the magnet, the point in the middle of the stir bar needs to be centered on the fan.

 
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