diy stir plate problems

I have seen a lot of post on this topic, but not any answers that will help me. The issue that I'm having is that my 1 inch stir bar won't spin in liquid. It will spin very fast on the stir plate itself and even in a 2000ml empty flask. Once I put liquid in it spins for a second and then stops. One thing I did notice is that it spins for about 10 seconds and then loses control in an empty flask. On the stir plate itself it spins fine without ever losing control albeit very fast. I thought introducing water would slow it down, but thats not the case. I'm using a 12 v ac/dc power supply, rocker switch, rheostat, old computer fan, and a earth magnet from a hd. The only thing I can really think is to lower the power supply. Any suggestions would be helpful because this is starting to get frustrating.

It sounds like too much voltage is being supplied. I assume this still happens with the rheostat turned all the way down?

You said that once you put liquid in "it spins for a second and then stops." You mean that a flask with water and stirbar, on top of your stirplate, will start spinning and then the stirbar just stops spinning but remains centered? What does the computer fan do when this happens? Does it also stop spinning or does it speed up?

Yes the rheostat is all the way down. Yes the stir bar usually stays pretty centered when it stops. Once I turn it off the stir plate the stir bar vibrates and moves. Yes I was talking about when I add liquid to the flask with the stir bar in it. The fan seems to be fine it doesn't seem to stop I can hear it going but I can't see it because its in my project box.

Is is possible that you have your rare earth magnets spread too far apart on your fan? I generally think that having the ends of your stirbar match close to the middle of each rare earth magnet will give the best attraction. Unfortunately, if the magnets are glued, it can be a PITA to fix. Measure the distance between the centers of the rare earth magnets; is it close to your 1" stirbar?

Are you using one hd magnet or two? If two, are they spit in half?

A couple ideas to test that might be easier would be distance between stirbar and rare earth magnets. Using non-metallic medium (thin plastic, cardboard, cardstock, paper, etc), try spacing a little further away (place the paper on your stirplate and put your flask on top of that). See if further distance helps or hurts. If it hurts, then try getting things closer. Move your fan up closer to the project box so the distance to the stirbar is reduced. See if it helps or hurts.

Homemade stirplates are cool but they definitely take a little trial-and-error to get the bugs out. The good news is that once you get your bugs out, it should remain good until the fan dies

I have moved the distance of the fan forward and back. I'm currently using one magnet from a hard drive. I even added two small earth magnets that I bought from radio shack. With the additional magnets on it, it threw the stir bar. I tried as you suggested with a few pieces of paper in between and it was spinning the same, but once again when i added water the same result.

Wild thought here: Could the weight of the flask plus water be pushing down on the fan/magnets causing it to hold still? In other words, could the fan magnets be so close to the project box lid that the weight of the water pushed down on them causing them to stop?

stpug said:

Wild thought here: Could the weight of the flask plus water be pushing down on the fan/magnets causing it to hold still? In other words, could the fan magnets be so close to the project box lid that the weight of the water pushed down on them causing them to stop?

Click to expand...

That was the problem I had with mine. I solved it by placing a dishcloth between the flask and the project box.

stpug said:

Wild thought here: Could the weight of the flask plus water be pushing down on the fan/magnets causing it to hold still? In other words, could the fan magnets be so close to the project box lid that the weight of the water pushed down on them causing them to stop?

Click to expand...

This is likely whats happening, your fan is mounted too close and the weight of the water your adding is drooping your container enough to press against your fan stopping it in place.

This is how I have built all of mine and built for other people and all work very well.

1) Use a wood spacer on the fan so the magnets do not interfere with the magnets in the fan motor. The fan should spin freely if you spin it with your finger.

2) Attach the magnets to your stir bar before gluing to the wood spacer. This gets the polarity and spacing correct so you get the best connection with the stir bar.

3) Use a 6vdc power supply. A 12vdc is too much power and tough to control with a rheostat.

I hope this helps

What are the specs on your AC adapter?

The fan is spinning freely. I did think of trying to put a washer or something on top of the fan and then mounting the magnet to that. I think I'm going to change out the power to 7.5 instead of 10. If that doesn't work I will add something between the magnet and the fan.

I meant 12 not 10. I will check the specs when I get home.