Stir Plate Woes

[TriggerHappy]

So I built my stir plate and sometimes it works fine other times not so much. I have an 8"x10" box with a 12v computer fan, potenteometer and on off switch. I have 2 rare earth magnets on the fan and a 1" stir bar. So the issue is when I turn it on I slowly raise the fan speed to where it starts spinning the bar at its lowest speed that will move the bar it will usually kind of work, no pretty whirlpool like my buddies but you can tell it is slowly stirring. If I do this it usually won't throw the stir bar, but sometimes it still does. If I increase the speed at all it will defintely throw the bar. I tried moving the fan closer to the top of the box removing a 1/16th" spacer nut, thinking magnets sloser to bar will hold it better. Now even at full power it will not spin with the flask and stir bar, no flask and bar and it spins fine. Did not know if anyone had any ideas. I am no electrical genius like some of the people who have posted on this subject so please keep it small words and speek slowly, I called it a potenteometer instead of speed knob to sound smart.

 

[PackerfaninSanDiego]

maybe one of your magnets are upside down?

 

[TriggerHappy]

I have one pos and one neg side up so it matches the pos and neg sides of stir bar.

 

[Natdavis777]

I had the same issues with my first build. I started off used 1/4"x1/16" magnets, but I now use 1/2"x1/8" magnets. On my fan, I have 2 or 3 washers glued on first so the strong magnet doesnt impede the motor. I then hot glued the magnets on the washer, place my stir bar on and power on the fan. This way I can see if they are placed irregularly on the washers, and can make adjustments. Once I was satisfied, I secured it to the box, via washer screws, leaving just enough room for the fan to operate.

This setup gives me a really strong hold on the stir bar. I use 12v power sources, so even on the lowest setting I get a solid vortex. Even if I crank up the speed some, I dont have an issue with throwing the bar.

 

[Konadog]

If I increase the speed at all it will defintely throw the bar.

I would check to see if the magnets on the fan are centered around the center of the fan. If they aren't, they will cause the stir bar to wobble and get tossed.

Try this, with the fan running, use a pencil to mark the center, then draw a line thru it. Then make sure your magnets sit on that line the same distance from the center mark.

I tried moving the fan closer to the top of the box removing a 1/16th" spacer nut, thinking magnets sloser to bar will hold it better. Now even at full power it will not spin with the flask and stir bar, no flask and bar and it spins fine.

This sounds like the weight of the flask is pressing down on the box, and resting on the fan keeping it from spinning.

 

[Temp81]

Try different size stir bars. I started out with longer ones thinking it would be better, not the case with my set up. It will be dependant on your magnet spacing on the fan. Also try different shapes of bars. I had to get one with a ridge in the center for the bar to pivot on.

 

[jdanderson1449]

It took me a long time to get things just right. My magnets pull a solid vortex at about 3 volts. Balance. Is. Everything.

 

[rouest]

Does it matter how much liquid is in your flask?

 

[jdanderson1449]

My 2000 mL seems to perform best when there is at least 800 mL in it.

 

[BowAholic]

8x10 seems like a large box...most are way smaller. It could be the weight of the flask pushing down into the fan and the magnets balance. You might add a divider of some kind to support the weight when the lid is shut.

 

[jdanderson1449]

Also, the magnets don't need to spin fast. As long as the surface of your starter has a dimple in it, it's working.

 

[robotinc]

I had the same issue when I built mine. At first I thought the magnets on the fan weren't strong enough, so I ordered more, and threw them in. It didn't help 😔.

Then I swapped the $4 stir at I found on Amazon for the one they sell at morebeer and it works great. I can't throw it now.

The difference between the two was not length but girth. Just needed a stronger magnet

 

[TriggerHappy]

Thanks for all the help. having more liquid definitely helped, I also added more magnets. I will definitely look at a new stir bar. Good to hear it is not just length but girth that is important.

 

[gromitdj]

Before ordering a new stir bar, try using a lower power wall wart. Mine would not work correctly if I used anything above about 5V. Upon further research, I finally built the LM317 circuit.

 

[TriggerHappy]

Wow, that talk is way over my pay grade. But thank you for trying to help me.

 

[gromitdj]

I think it's about 5 dollars in parts and a little time for a much more reliable design. Of course without a soldering iron, it gets more ezpensive.

 

[TriggerHappy]

Thanks for the help, but I was referring to the talk being a little technical for a low level thinker that drools like me. With added magnets, more volume in the flask, and new stir bar things seem to have worked out. You guys are way sharp on this stuff.

 

[RJS]

with my DIY stir plate i was lacking in magnets and bar would get thrown off. i doubled the magnets and it worked fine.

 

[fun4stuff]

Try either placing one o-ring around center of stirbar or 2 o-rings around the ends of the stirbar to break the stirbar into 1/3s.

Your stirbar should be about the length of your magnet(s).