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picking components for a stir plate

Corbmonster

Apr 3, 2015
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I'm wanting to build a stir plate to grow yeast for home brewing. If you want to know what it is or how it works, you can check this out. But it is not necessary.

In a nutshell, I need to get some DC power (12V, 1 - 2 amps), and send it to a switch, then a potentiometer, then to a regular 120mm computer case fan. That's it. When I went to Radio Shack I saw that their porentiometers said they were 1 watt, or half watt. And when I googled amp, volt, watt calculator and put in what I'm working with, it gave me 12 watts for 1 amp, and 24 watts for 2 amp (depending on which DC power supply I get). That having been said, I can't use those potentiometers can I? The guy there didn't have a clue. And I don't want to burn my apartment down.
 

Corbmonster

Apr 3, 2015
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Or should I be using a Rheostat? The purpose is to control the RPMs of the fan.
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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This sounds like a natural application for an Arduino Uno with a PWM output driving a MOSFET which drives the fan motor.

Or you could go on Ebay and find a 20 watt wirewound potentiometer with about 100 ohms or so resistance. Wire it up as a rheostat (two terminals) in series with the fan motor and your power supply brick. You may have to experiment a little to find the right value for the potentiometer. Too large and there will be little range of control from dead stop to full bore. Too small and there will be very little effect on speed from minimum to maximum. That's why I suggest PWM instead of rheostat control.

No need for anything fancy in the Arduino Uno... just drive the gate of the MOSFET with one of the PWM outputs, ground the MOSFET source, place the motor in series with the drain and the +12 VDC supply. It might be a good idea to place a diode across the motor to protect the MOSFET during PWM switching. You will need a separate power supply for the Arduino, but this could be a 9 VDC "transistor radio" battery or the USB port of a cell-phone charger. Someone here can write a Arduino sketch for you that uses two push-button switches to increase or decrease the speed of your fan motor.
 

duke37

Jan 9, 2011
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Go down to your local scrap yard and get a switch and resistors which control a vehicle heater fan. With luck, you will get a choice of speeds which will suit.
 

Corbmonster

Apr 3, 2015
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I was trying to stay really simple and analog here. I'm really just wanting to know what type of rheostat I should be using. Can I use the 1 or 0.5 watt potentiometers I see in Radio shack? Or Something like this?
 

duke37

Jan 9, 2011
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An ordinary potentiometer such as you show is just not man enough for the job. That is why car heaters use a switch and bigger resistors.

If you wish to stick with a potentiometer, then you will have to follow it with an amplifier. This can be as simple as a power transistor mounted on a heat sink to dissipate the heat. To do calculations of what is needed, then the characteristics of the fan would be required.

A PWM (pulse width modulation) circuit could be made to control the output with little power dissipation.
 

Corbmonster

Apr 3, 2015
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It's just a regular computer fan. I've tried looking on new egg, and amazon, they aren't giving the specs on them.
 

TenderTendon

Dec 20, 2014
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A typical 12 volt CPU cooler fan will draw 1-2 watts at full speed. A 1 watt potentiometer should be fine. If in doubt, you can cut the blades off with heavy scissors or side cutters. That will unload the motor and it will draw much less current.
 

Corbmonster

Apr 3, 2015
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I would suggest that you get a bunch of 10Ω 1W resistors and see how many you can put in series and still have the max speed you want, then add a 100Ω pot.

Or, you can get a PWM motor controller off EBay for just a few bucks that will be far superior.

Edit: something like this:

http://www.ebay.com/itm/12V-40V-10A...552?pt=LH_DefaultDomain_0&hash=item5d44d25ce0

Bob

I see what was meant by using a PWM now. Thank you for the example. Ya it does seem like those would be more ideal. I just with I could find one where the knob had a longer shank and securing nut that could go through the wooden box I'm working with.
 
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BobK

Jan 5, 2010
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You could always replace the pot with a panel mounted one.

Bob
 

hevans1944

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Jun 21, 2012
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The PWM controller listed on EBay does not work with brushless DC motors. Scroll to bottom of listing page to read vendor disclaimer. I think most small computer fans are brushless, incorporating some sort of internal electronics that apparently does not play well with either a PWM or a variable DC or a fixed DC with rheostat current-control power supply. You need to see how your particular fan responds to variations in series resistance using a fixed-voltage DC power supply before jumping in here. My suggestion to use a PWM controller may not fly, whether Arduino-based or cheap Chinee import. Try @BobK 's suggestion to experiment with cheap resistors first. Or find a small DC motor that has brushes and a commutator instead of re-purposing a computer cooling fan. Many, if not most, small PM motors will have brushes.

It's just a regular computer fan. I've tried looking on new egg, and amazon, they aren't giving the specs on them.
The one in the picture on the beer-making website is rated 12 VDC at 0.15 A, which is less than two watts. A 100 ohm, 3 watt, potentiometer may work for you.
 

Corbmonster

Apr 3, 2015
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oh, since soldering really isn't my thing, could get this and cut the wires and re-attach a pot I preferred right? Would I still be looking at the same requirements of a 1-3 watt pot?
 

Corbmonster

Apr 3, 2015
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for what its worth, I was finally able to dig up some posts about what I'm wanting to do that were specific about the components used. It seems 9V DC is preferred as the 12 volt power supplys seem to be just a touch too much. The pots used are 25 Ohms 3 watt
 

BobK

Jan 5, 2010
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That sounds more like it.

I believe computer cooling fans will operate correctly on PWM since the ones in computers are always controlled that way. I think the disclaimer was for brushless motors that need a separate controller.

Bob
 

hevans1944

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Jun 21, 2012
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for what its worth, I was finally able to dig up some posts about what I'm wanting to do that were specific about the components used. It seems 9V DC is preferred as the 12 volt power supplys seem to be just a touch too much. The pots used are 25 Ohms 3 watt
Sounds like your problem is solved!
 
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