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Old 09-02-2012, 04:06 PM   #1
Nil
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Dec 2011
Canton, Ohio
Posts: 141
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I constructed a stirrer box and I have some problems.

Here are the parts

-Project Box
-Car switch
-25 amps rheostat
-12V, .012 amps computer fan
-3 nuts + bolts
-One 1-inch stirrer rod (flat)
-12V PSU
-Two earth magnets

I wired all the parts together, and the RPMs of the fan were too high even in the zero position. The stirring rod was not spinning, just trembling all over the place.

I made some calculations and found that, for a 12V PSU, 12V/0.12 amps fan, I need a 140 ohms rheostat.

I found another 5V PSU and the rheostat resistance requirement turned out to be zero.

I re-wired the stirrer with the new PSU and still and I have the same problem even though the RPMs were decreased significantly.

Any help will be appreciated.

Thanks, Nil

 
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Old 09-02-2012, 04:23 PM   #2
gromitdj
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Feb 2008
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There are a few possible solutions, however my first inclinations are:

1. Check that you have the opposite poles of the magnets facing up. Since you are using two magnets I am assuming that they are the disc type that are polarized along their length.

2. Make sure that the magnets are as close to the bottom of the flask as possible. Depending on what magnets you used, they might not be strong enough to work through the enclosure.
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Old 09-02-2012, 04:38 PM   #3
weirdboy
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May 2009
Los Angeles
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Does the fan draw .12 amps, or .012 amps?

 
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Old 09-02-2012, 05:59 PM   #4
Nil
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Dec 2011
Canton, Ohio
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0.12 amps

 
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Old 09-03-2012, 04:26 AM   #5
Brewshock
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Apr 2012
Santa Rosa, California
Posts: 21


You just have to play with it. I started with a 12v psu and dropped to a 9v with no success. Yet when I installed a second rheostat, left one down all the way and fined tuned the second one. I was finally able to see some results. Don't give up. You'll get it.

 
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Old 09-03-2012, 12:47 PM   #6
krazydave
 
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Aug 2010
Santa Clarita, California
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I threw a couple resistors on mine to drop the voltage. Worked a charm.
Then I got this brilliant idea to use a on-off-on switch I had lying around to put different resistance on each side. I now have high and low speeds as well.

 
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Old 09-04-2012, 01:04 PM   #7
Nil
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Dec 2011
Canton, Ohio
Posts: 141
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Thanks, Folks. I found that I got ceramic magnets and it seems not appropriate. I will dig some old hard drive to get the magnets out. I will also try to use some resistors as well as a second rheostat.

The stirring rod also seems wired. It is totally flat and square, as opposed to others that are round. I don't know. You may advise.

Thanks!, Nil

 
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Old 09-04-2012, 04:53 PM   #8
Nil
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Dec 2011
Canton, Ohio
Posts: 141
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Well folks, I got it thanks to your advice.

I purchased an internal 10K rheostat and new magnets (I don't have any old hard drive). The magnets are bigger, meaning a higher surface area for the magnetic field.

I placed the new rheostat is series with the external rheostat, and tuned the current using a small nut. Just perfect.

Thanks a lot, Folks! Nil

 
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