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Home Brew Forums > Home Brewing Beer > DIY Projects > Chillers and Stir Plates > Stir Plate too powerful
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Old 01-06-2013, 06:02 PM   #1
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Default Stir Plate too powerful

So I made a Cigar box stir plate using commonly available directions. Components:
Wall transformer outputting 12V @ 1000mA
25 Ohm Rheostat (speed control)
Case fan - 1.8 Watts rated power @ 0.15A

I am using a 1.5 inch stir bar - it was octagonal with a ring but I did my best to smooth out the rough spots and I cut off the ring. It isn't perfect but it isn't bad either.

At the lowest speed the stir bar is making quite a tornado:
736758_821295280666_1643864546_o.jpg

And after a few minutes it will get thrown at this speed, even. It won't hold at higher speeds at all, even for a few minutes.

What I would like to do is slow down the base spin rate by 50%, or maybe even more. Then I could actually use the rheostat for speed control and not just leave it pegged at minimum. The only problem is that I don't know anything about circuits or the math thereof. I figured if I could solder in some sort of in line resistor, that would do the trick. I'm hoping one of you can tell me how big that resistor has to be, and where I should put it in the circuit:

- Between the wall and the fan
- Between the wall and the rheostat
- Between the rheostat and the fan

Thanks for the help!

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Old 01-06-2013, 06:16 PM   #2
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What is the size of your case fan?

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Old 01-06-2013, 06:21 PM   #3
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You need a smaller stir bar. It is that simple.

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Old 01-06-2013, 06:50 PM   #4
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80mm with a 1.5" stir bar, neodymium magnets spread roughly 1.5" apart hot glued to a spacer on the fan, as close as possible to the lid of the box.

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Old 01-06-2013, 06:56 PM   #5
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What's the speed range of the fan you used? Sounds like you used the wrong fan to me. If it goes from zero to too fast with nothing between, then you used the wrong fan. You can try a different stir bar if you want. I would NOT try to modify the next one. The ring you removed is there to help it spin.

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Old 01-06-2013, 07:00 PM   #6
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I had a similar issue back when I built mine. I changed the 12 volt ps out for a 6 volt and it worked much better.

Curious as to why you modified the stir bar. That sounds like the same stir bar I use and I've found the ringed ones work better than the 1/2" flat ones I started out with, although that could vary from setup to setup depending on speed, magnet spacing, etc. Mine threw the flat 1/2" bars at least once per use but I can't remember he last time it has thrown my ringed bars.

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Old 01-06-2013, 07:04 PM   #7
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Interesting regarding the info on the stir bars. I read somewhere that the flat ones are better if you have a perfectly flat bottomed vessel, whereas the ringed are better if you have contour or convexity/concavity. I only did that research after the ringed bar was jumping off all the time, even at the lowest speed.

My fan doesn't go 0-too fast, it is too fast right off the bat when I plug it in (I have no switch wired in). There isn't a speed that reliably holds the bar.

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Old 01-06-2013, 07:12 PM   #8
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Quote:
Originally Posted by JNewbs View Post
Interesting regarding the info on the stir bars. I read somewhere that the flat ones are better if you have a perfectly flat bottomed vessel, whereas the ringed are better if you have contour or convexity/concavity. I only did that research after the ringed bar was jumping off all the time, even at the lowest speed.

My fan doesn't go 0-too fast, it is too fast right off the bat when I plug it in (I have no switch wired in). There isn't a speed that reliably holds the bar.
Then you're using the wrong fan, or you need to use a lower voltage power block.

You can get fans with different speed ranges, that can be controlled. Sounds like the one you have is either full speed or nothing. Or it's lowest speed is far too fast. I've bought fans from frozencpu.com when I was building computers. You can get fans that spin slower, and are larger, for not a ton of money. In your case, I'd probably try either a 92mm or 120mm case fan that has a good speed range listed. You'll want something that starts off on the lower end of the scale, so that you can ramp up the speed.

BTW, these kinds of issues is why I just bought a pre-built (Hanna) stirplate when I was looking for one. I could have tried to build one, but I wanted it ready to go on the quick. I've since tried to build one, but haven't gotten the magnets placed correctly, so it doesn't really work.
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Old 01-06-2013, 07:14 PM   #9
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Smaller bar, you only need to keep the yeast in suspension...the local wine lab uses those little kidney bean sized stir bars for all their work. I think mine are 1" and work fine in 1 & 2L starters.

A smaller power supply will also slow it down.

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Old 01-06-2013, 07:47 PM   #10
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seems like you have too much power. find a wall wart with less volt output.

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