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Old 04-26-2013, 04:06 AM   #1
kontrol
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Default Stir plate - Potentiometer and fan questions...

Planning to build myself a homemade stir plate. I've read couple of tutorials and I'm left with some questions lol.

I'd like to wire the fan to a potentiometer so I can control the speed.

1. I intend to use a pc fan (80mm) or something. I've read against using brushless fan as you can't control the speed. How do you see if it's brushless?

2. What is the potentiometer I need. I see people say 25 ohm (http://www.homebrewtalk.com/f51/diy-stirplate-could-get-any-easier-225340/)
and other people say 1k ohm (http://seanterrill.com/2010/04/26/build-a-better-stirplate/)

Does it make much of a difference? Why does it work with both.
How do I calculate the required ohm? Because I will go the my electronic shop tomorow and see what fans then have in stock. If the ohm is important I should know how to calculate it.

3. I see that I should get a 2W rated potentiometer to avoid it melting because of the dissipated heat. In the second link I provided up there I know the guy says the best method is with the LM317T and yada yada, but fact is I'm a real amateur in this domain so I'd like to keep it simple for now. Maybe one day I'll do something like he said (His instruction are not simple enough for a noob like me).

So if I get a 2w rated one, I should be fine right?



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Old 04-26-2013, 04:19 AM   #2
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More info: Scavenged a fan from an old power supply. This is a DC12V 0.17A fan. Not written Brushless like the other one I had (which made me worry). I know it could be brushless too, let's just hope its not?

So what potentiometer should I get if I have a 12V AD/DC Adapter (I guess it must support at least 500mA right?)



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Old 04-26-2013, 04:46 AM   #3
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http://www.radioshack.com/product/index.jsp?productId=2062299&retainProdsInSession=1 &filterName=Type&filterValue=Potentiometers


If that doesn't work take it back.

I used a rheostat from the 1970's for my stir plate build. 12v DC power supply also.
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Old 04-26-2013, 02:46 PM   #4
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But why 25ohm? And not 2k like the other article I read? Will it change anything?

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Old 04-26-2013, 02:56 PM   #5
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V = IR
12 = 0.17*R
R = 12/0.17 = 71 ohm

The closest common value would be 100 ohm: http://www.parts-express.com/pe/showdetl.cfm?partnumber=023-604

A 25 ohm would only allow you to reduce the speed by 25/71 = 35%.

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Old 04-26-2013, 03:09 PM   #6
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a 100ohm will allow me to reduce by 100%

Why the other guy in his article use a 2k then? Sorry for all these questions, I'm trying to understand.

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Old 04-26-2013, 03:27 PM   #7
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Quote:
Originally Posted by kontrol View Post
Why the other guy in his article use a 2k then?
Because with an LM317 it isn't the value of either resistor that sets the voltage, it's the ratio between them. I was using a 220 ohm fixed resistor, so a 2.5 kohm pot allowed for a range of ~11 V.

With a potentiometer wired in series, all you're doing is sinking some of the current into the pot and dissipating it as heat. Whatever is "left over" powers the motor.


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