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You are here: Home --> Forum Home --> Brewing Forum --> Brewing Discussion --> Stir Plate Build; anybody remember how to work Ohm's law?

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AgedAardvark
SLC, UT
53 Posts


I've got a circuit with a 9v power supply, a fan and a potentiometer.  The pot definitely slows the fan down, so I know it's wired correctly, but it doesn't stop it completely at the zero position.  

If I'm reading Ohm's law correctly, I=V/R.  I have a V=9, max ohms from the potentiomenter = 25, so I = 9/25 (about .36 amperes?).  I know enough about math to know that I approaches 0 only as R approaches infinity.  This being said, my math is mainly in the "perfectly spherical object in a vacuum" domain.  How do I calculate the resistance I need from the potentiometer to stop the fan?  

On a side note:  I had a fan, a ac/9v dc power converter and an old hard drive with a bad-ass magnet in it just hanging around my house.  I figure I'm going to have this thing built for less than about 20 bucks.




Posted 34 days ago.

mchrispen
Bastrop, TX
485 Posts


Your math seems right. If I recall, I had to wire in a small 1 ohm resistor to make mine work...

Also check the resistance of the fan at various speeds and with the magnet in position... that mechanical weight and the influence of the magnet on the motor (if it is too close) may stop the fan at 0.




Posted 34 days ago.

blur_yo_face
Houston, Tx
161 Posts


Ohm's law is pretty straight forward, and it seems like you have it calculated correctly.. If I were you, I wouldn't want to use the potentiometer to stop the current.. I would personally look at either a switch or a pot that can cut the circuit completely, rather than trying to build up enough resistance to stop the fan.. its like using a dimmer switch to "turn off" your light, rather than a regular switch..



Posted 34 days ago.

AgedAardvark
SLC, UT
53 Posts


You know what?  I picked up what I thought was the highest ohm potentiometer at the Radio Shack... 25 ohms.  I didn't notice that all the rest of them were listed in k ohms.  5k, 10k.  I traded the one I had for one at 5k ohms.  Totally good enough to zero the fan out.  



Posted 34 days ago.

homebrewdad
Charter Member
Birmingham, AL
2480 Posts


Hah, glad you got it figured out.



Posted 34 days ago.

brewcrewkevin
Charter Member
Appleton, WI
66 Posts


Glad you got it.

And your math is right on. You'll never get to 0V with just resistors, because resistors are still conductive. But at a certain point, it's not enough to overcome the normal friction in the motor. That's very difficult to calculate. But you got the idea.




Posted 34 days ago.

skunkfunk
OKC, OK
38 Posts


I believe what you want is on off switch. The small trickle you'll still get through that potentiometer could burn stuff out sooner.

I only say that because that's what happened to a buzzer circuit I wired in to my headlights with a 100K resistor on a 12V circuit. Goes bad every few years, should have done it differently.




Posted 34 days ago.

chino_brews
Charter Member
Eden Prairie, MN
301 Posts


This is a bad idea (to stop the fan by dialing up resistance) because resistance leads to heat. Why not wire in a $1 switch? Or unplug the stir plate?




Posted 34 days ago.

ercousin
Charter Member
Toronto, Canada
77 Posts


Just a word of caution for people doing the "cheap stirplate" method. Using a pot to dissipate that much power will cause it to burn out very quickly. Most potentiometers are designed to dissipate less than 1 W. (Watt = Amp x Voltage and also V^2/R)

At low speeds there is going to be a lot of voltage drop over the potentiometer causing it to burn out.

Check out this page for a better stir plate design:

www.stirstarters.com/instructions.html




Posted 34 days ago.

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