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Old 02-28-2009, 07:59 PM   #1
Stevorino
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Default Stir Plate Help....

Hey All, So I have put together a stir plate and this thing is barely not working.

Here's what's going on:

With water in the Erlenmeyer, if I have the fan running on low, it'll catch the magnet, spin it about half a second, then instead of spinning, it'll start swirling for another half second or so, then it throws it....catches it again and the cycle repeats.

If I turn up the speed of the fan, it just launches the magnet out of reach. Never once does a whirlpool begin to form.

Thoughts on what's going on?

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Old 02-28-2009, 08:22 PM   #2
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I had some trouble with this at first as well. Are you running a 12v DC power supply? How are you limiting to voltage to the fan? Some guys run potentiometers but I just used a 12V DC power supply that came from Radio Shack. Like $10 and it allows me to choose from 3V, 6, 9, and 12V. With a 2" stirbar, it usually throws if I get above 5-6V. Just make sure you have the fan turned down as much as possible. You don't need much to get the yeast in suspension.

Also, make sure that your magnet is as close as possible to the top of your enclosure. This will ensure that the stirbar has the most amount of attraction to the magnet as possible. Good luck!

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Old 02-28-2009, 08:35 PM   #3
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+1 on fan speed too fast.

I started with a 12v PS and potentiometer on slowest speed stir bar was thrown right away. With a 9v PS and minimum POT setting the stir bar spun but the vortex would go all the way down to the stir bar and rattle it. I found a 6.5v PS and added some more weight to the fan to spin (some washers glued in balance) and now have it working well am able to use the POT to adjust the speed as needed.

One thing, I still can't get a 2" stir bar to work as it is thrown right away. The 1" stir bar works great. Maybe I need to raise the fan a bit to get the magnet closer the flask.

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Old 02-28-2009, 08:36 PM   #4
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Slow down the stirplate by using a smaller power supply or add more resistance... or, get a smaller stirbar.

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Old 02-28-2009, 09:43 PM   #5
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right now i have to sorta flick the fan just to get it going on the slowest speed and every once and a while it'll stop completely at the lowest speed...

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Old 03-01-2009, 01:09 AM   #6
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Quote:
Originally Posted by Stevorino View Post
right now i have to sorta flick the fan just to get it going on the slowest speed and every once and a while it'll stop completely at the lowest speed...
If that's the case, I would be looking the strength of the magnet on the fan or proximiety to the flask
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Old 03-01-2009, 03:07 AM   #7
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Sounds like you have a couple issues... It sounds like you don't have the magnetic center of your drive magnet concentric with your fan axis. This causes the stir bar to get dragged around rather than spun and thrown easily, especially during speed changes. Try shifting the drive magnet around until the stir bar is really pivoting on it's center.

Two, did you just use a potentiometer or rheostat for speed control or build it around a voltage regulator? It sounds like a pot/rheostat. That combination is very susceptible to voltage fluctuations. That coupled with the interference of the motor's magnetic field from the magnets causes the fan to be unreliable at low speeds. (when it stops do you see it flip backward a little?) Google "LM317" and /or "LM317 stir plate" and you'll find info on the LM317 voltage regulator. It makes a much more stable circuit.

The ideal solution is to use a PWM (pulse wave modulation) speed control. They work really well for this type of application because they don't vary the voltage but they vary the amount of time the voltage is applied. Simply put, if you want the fan to run at half speed, you supply 100% voltage 50% of the time. The only downside to that is the only circuit I was smart enough to make work was at 400Hz, well within our ability to hear it and it made a humming noise I didn't like. I don't know how to make a simple enough circuit that'll operate at a higher frequency.

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Old 03-01-2009, 03:39 AM   #8
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I worked about 10 hours on different stir plates, bought a 80 dollar professional one from the brew shop, then looked on ebay and found really nice home made ones for 40 bucks. I would hit ebay up personally

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Old 03-01-2009, 04:00 AM   #9
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I worked about 10 hours on different stir plates, bought a 80 dollar professional one from the brew shop, then looked on ebay and found really nice home made ones for 40 bucks. I would hit ebay up personally
Yep.... Or you could also get a really nice home made one at Welcome to Stir Plates.com ;-)
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Old 03-01-2009, 02:27 PM   #10
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Quote:
Originally Posted by Stevorino View Post
Hey All, So I have put together a stir plate and this thing is barely not working.
If it's barely not working you're golden. It's when it's completely not working that you have a problem.

General stirplate troubleshooting:
* start the spin after the flask is in place and the stirbar is "caught"
* slow the spin down
* try different size bars (usually smaller)
* if the plate is homemade, play with the position/number of magnets
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