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Old 09-18-2007, 02:07 AM   #1
foxtrot
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Default DIY stir plate help

Need some help from those that are electronically inclined...

I am in the midst of building a stir plate and the problem I'm having is too fast of a fan. Here's my components:

12V / 2.8W 92mm fan
Magnet from hard drive, centered on fan hub
9V, 300mA AC to DC wall adapter
25 ohm, 3W pot
(got the plans from onebeer.net)

I'm trying to spin a 2" stir bar in a 2L flask. The bar starts spinning, then spins out as the fan revs up. The pot does not slow it down enough. I'm not into soldering and don't have much experience w/ electronics, so I'd like a "simple" solution.

Here are two ideas I have:

1) reduce the input voltage by getting a universal wall adapter that adjusts from 1.5 to 12V, in 1.5V increments (btw, these come in various mA ratings- as long as it exceeds the fan, I'm good, right?)
-or-
2) get a fan speed controller

Which method would work better or should I be trying something else???

Many thanks in advance!

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Old 09-18-2007, 02:44 AM   #2
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Between the two I would buy a fan controller, it will cost about the same as that adjustable wall wart.

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Old 09-18-2007, 04:00 AM   #3
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I built my 2 Liter stirplate from the same onebeer.net. I tried using 2in, 1.5in and 1in stirbar. The 1.5 in. bar was the only one that wouldn't throw off in regular tap water. Also, keep in mind that water + dme mixture will cause more resistance on stir bar so it might slow the 2 in stir bar enough so it won't throw it. I don't know cuz the 1.5 in. worked fine and I was just tired of dicking around.
Make sure to start it off slow and gradually ramp up speed, too.

Works wonders when it works. Good luck!

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Old 09-18-2007, 02:30 PM   #4
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Yeah, I think I'm going to try a fan speed controller; I believe a slow ramp up in speed is one of the keys to getting this to work. I wonder if I should run the speed controller down stream from the pot? for additional control?

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Old 09-18-2007, 02:51 PM   #5
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See the link in my signature. There's a huge thread about how to make a speed controller.

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Old 09-18-2007, 03:32 PM   #6
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Quote:
Originally Posted by foxtrot
Yeah, I think I'm going to try a fan speed controller; I believe a slow ramp up in speed is one of the keys to getting this to work. I wonder if I should run the speed controller down stream from the pot? for additional control?
Remove the pot from the circuit. The fan controller will give you all the control you need.
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Old 09-18-2007, 03:41 PM   #7
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Quote:
Originally Posted by foxtrot
Need some help from those that are electronically inclined...

I am in the midst of building a stir plate and the problem I'm having is too fast of a fan. Here's my components:

12V / 2.8W 92mm fan
Magnet from hard drive, centered on fan hub
9V, 300mA AC to DC wall adapter
25 ohm, 3W pot
(got the plans from onebeer.net)

I'm trying to spin a 2" stir bar in a 2L flask. The bar starts spinning, then spins out as the fan revs up. The pot does not slow it down enough. I'm not into soldering and don't have much experience w/ electronics, so I'd like a "simple" solution.

Here are two ideas I have:

1) reduce the input voltage by getting a universal wall adapter that adjusts from 1.5 to 12V, in 1.5V increments (btw, these come in various mA ratings- as long as it exceeds the fan, I'm good, right?)
-or-
2) get a fan speed controller

Which method would work better or should I be trying something else???

Many thanks in advance!
I think you mean 25K ohm pot...

Another angle to this is to mess with the ratio of impedence between the potentiometer and the resistor that connects the pot to the output from the voltage regulator to the fan. The amount of power transferred to the fan has something to do with the ratio of the impedence rating of those two resistors. So for instance, if you get a 10K pot and the resistor is 1K, your ratio would be 10/1. That should have a shallower range of adjustment and therefore finer tuning of adjustment than a 25/1 setup. Maybe even try a 5K pot if it's still not good enough. Either that, or increase the resistor to 2K or 5K if you want to keep the pot.

I'm no electrical engineer, but when I put my stir plate together I had my buddy from high school who got his degree in electronics help me out, and this is what I remember from our messing around with it.
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Old 09-18-2007, 04:39 PM   #8
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Since his problem is he cant slow down the fan enough I am inclined to think the Pot is very small. Unless he has it wired wrong. If you put another resistor in series with the pot it will do two things. A. Give a finer resolution of adjustment and B. Limit the maximum speed of the fan.

This is how it should be wired. +12 side of the power supply goes to one end of the pot. If you have an extra resistor it would goto the resistor first and then from the resistor to one side of the pot. Now wire the + side of the fan to the wiper arm of the pot. That would be the pin with the arrow pointing to the squiggly line. Then wire the negative side of the fan to the negative terminal of the power supply.


If you want to add a soft start put a 500k resistor and a big 20uf cap in parallel with the fan. This will give you a 10 second ramp up to speed.

I'll get some schematics up tonight, ASCII art isnt working out well

*editied to give specific values

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Old 09-18-2007, 06:27 PM   #9
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Quote:
Originally Posted by Virtuous
If you want to add a soft start put a 500k resistor and a big 20uf cap in parallel with the fan. This will give you a 10 second ramp up to speed.
You could achieve a soft-start behavior with a resistor and capacitor, but not in the way you have described, and not with such a small capacitor... perhaps you can clarify what you are suggesting.

As for the original poster, I would suggest that if you don't want to build a voltage regulator circuit like the one Yuri's linked in his signature, the best bet is a PC fan controller. You can find them for like 5 bucks online. Some of them are active, and some of them are nothing but a potentiometer, but at least they're of the right value to properly control a PC fan - if you're looking to avoid messing with electronics it's a much easier plug-and-play solution, and still affordable.
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Old 09-18-2007, 06:48 PM   #10
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T=RC. 500,000 ohms * 0.000020 farad = 10 seconds.

Yeah the description sucks, a schematics is worth a million words. I'll get one tonight

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