Stir plate help

[cdwiggi]

Hello all,

I built my stir plate out of magnets from Home Depot and have a 12v 2 amp DC computer fan turning it. The problem is though every time I place the stir bar or anything mettle on it is throws it off? The stir bar is a square type like these. What I was wondering is why does it seem to be doing this?

Thanks
cd

 

[f0xtr0t]

I used a hard drive magnet. But it was extremely important to line up the axis of the magnet so that the magnet would rotate evenly. I also had to cut a 1.5 inch piece of PVC to raise the magnet off the fan and closer to the base of the flask.

 

[jmf143]

Many possible reasons -

the bottom of your fermenting vessle is convex
the magnets aren't strong enough
the stir bar is too long
the stir bar is too short
the distance between the magnets and the stir bar is too great
12v spins the fan too fast

Change one item at a time and see what works

 

[bcgpete]

I was having problems with mine recently. Realigning the magnets fixed it 100%. If that doesn't help you need stronger magnets most likely.

 

[barrooze]

+1 on an off-center magnet. The way I centered mine was by letting the stirbar spin directly on the plate while the magnet spun. This way you can directly see if it's centered or not. I have a large fender washer centered on my fan so it's very easy for me to adjust the magnets position.

 

[zachattack]

I'm gonna vote for 12V being way too much. Are you controlling the speed at all with a potentiometer? To get the stir bar to start spinning you need to start it slow (off even) and work your way faster.

 

[bcgpete]

I'm gonna vote for 12V being way too much. Are you controlling the speed at all with a potentiometer? To get the stir bar to start spinning you need to start it slow (off even) and work your way faster.

This is a good point too, I never have my pot. turned up more than ~30%.

 

[captwalt]

I bought a pack of resistors from radio shack and started adding them until it stopped throwing the stir bar when I had the pot at the fastest setting. I start the stir plate on the lowest setting and turn it up to about halfway to do its thing. every once in a while i can hear it rattling from the other room but it usually catches the stir bar again pretty quickly.

 

[william_shakes_beer]

I got a variable voltage wall wart for my stir plate. 12V motor, but it stirs correctly at 3 V. Also, gotta be sure the stir bar ends up on the geometric center of the fan to stir properly. That means sticking the stir bar on the fan magnet (to find where it sits magnetically) and then securing the fan magnet so the stir bar is on the geometric center of the fan, then finding a way to clamp it until the glue/epoxy dries. It took me 3 tries to get it right. I used 2 hard drive magnets. One is glued to the fan, the other sits on top of the first magnetically.

Edit to add: the hard drive motor will not work correctly unless you install a 1/2" non magnetic spacer between the motor and the magnet. I used a block of wood cut with a hole saw.

 

[carlisle_bob]

Hi

A silly trick to get things lined up:

First glue on the biggest widest washer you can find. One that's an inch or more is what you really want. Fiddle with it a bit as the glue dries to be sure the fan is balanced. If you happen across a steel disk, about 2/3 the diameter of your fan that would be ideal.

Next just put the magnet on the washer. It's steel, and the magnet will grab on to it something fierce. No need for glue. Push the magnet around until you find where it works right.

I suspect you can just leave the magnet there and it'll never go any place. I always hit it with a little glue just to be sure.

Bob

 

[Chia]

Many possible reasons -

the bottom of your fermenting vessle is convex
the magnets aren't strong enough
the stir bar is too long
the stir bar is too short
the distance between the magnets and the stir bar is too great
12v spins the fan too fast

Change one item at a time and see what works

was going to suggest this. there are several ways to slow the fan down, but I WOULD NOT add weight. it will burn up the fan. go to radio shack and tell them what you are doing, than can get you the parts you need. a potentiometer and resistors are most likely going to be in order.

 

[carlisle_bob]

Hi

One gotcha with slowing down fans:

As the fan gets older and more dirty, it's less and less happy running slow at a low voltage. It decides to stall or stop. If you have a fan that needs 3V to get anywhere near the right speed, that's not a real good sign.

There are indeed fans that start life as low RPM devices. They aren't as cheap as a free fan, but they are less than $10...

Bob