DIY Stir Plate issues

[TheMattTrain]

Over the weekend I made up a stir plate and it has a few issues.

One issue is that it throws the stir bar. I have a nut on each of the screws spacing the fan away from the lid of the project box. I have had as many as two nuts being used as spacers. When there were two nuts, it didn't throw the bar as easily, so I might move back to that, but was wondering why this is happening. The magnet seems centered, although there is a little wobble in the fan when it's not attached to the box at all.

The second issue is that the vortex fluctuates. What I mean by that is that the dimple goes down very far and then nearly goes away, then comes back and repeats this over and over.

The third issue is that the potentiometer only turns about a quarter turn to control the speed of the fan. I had it wired up in a different box and it didn't even do that. I was almost as if it was either on or not on.

I have my hot wire going from the wall to the potentiometer (which I got for free from a guy from work... no idea about the specs) and then to the fan. Ground goes from the fan to the wall outlet. The two connections that go to the potentiometer are clip connectors that I clipped to the wires I stripped on the fan and the wall adapter then taped.

I have lots of problems... I also have no experience working with electrical... any help would be awesome!

 

[DrummoRC]

I would love to see someone respond to this thread I need assistance also

I would love to know how to size the potentiometer - I turn mine 4/5ths of the way around before the fan starts and when it does it is full speed. I can't really adjust the speed with the current potentiometer I used. I don't remember its resistance level (if that is the right term?) but I have a 12 v power supply and fan.

I am not sure but the way I have it configured it does not spin the stir bar! There are also different size stir bars - does anyone have recommendation on how to size the stir bar itself?

The other thing I have noticed is many of the pics I see have of DIY Stir Plates have holes cut out of the lid - I have not done this. I mean its made out of a cardboard cigar box not lead - so it should not be impacting the magnetic field between the magnet on the fan and the stir bar - OR DOES IT?

Any advise will be welcomed!

 

[captwalt]

Having built a stir plate, I will share with you my experience. I had a similar problem with the bar going flying and the pot not really doing anything. I solved this by buying a pack of resisters and putting them in the circuit one at a time until I was able to slow doen the fan enough to hold a vortex and to make the pot minorly adjustable. I think it has a lot to due with your power supply, the fan and the pot and getting them all to work together the way they are supposed to .

 

[DNKDUKE]

The third issue is that the potentiometer only turns about a quarter turn to control the speed of the fan. I had it wired up in a different box and it didn't even do that. I was almost as if it was either on or not on.

I have (10) full turns of the dial. the first (3) don't spin the fan enough to pull the "pill".
I work at a hospital and our biomedical engineering dept helped me build the electronics.

Please forgive my brag
Oh by the way, captwalt's idea is probably the right solution.

 

[Setesh]

There are a lot of variables here. The most important ones are:

Strength of magnet
Polarity of magnet
Power supply specs
Fan specs which include starting voltage, running voltage, current draw, and RPM range
Distance of magnet to stir bar

In order for your stir plate to work effectively you must have a magnet powerful enough to hold the stir bar in place, a fan that doesn't spin too fast, a power supply powerful enough to drive your fan, and the proper spacing between the magnet and stir bar. When choosing a power supply the voltage should be 12 volts, although it could be less but you can't expect your potentiometer to be of much use if you use a lower voltage. The reason for this is that most computer fans will not start up at less than 7 volts and will run at full speed at 12 volts. If you only started out with 9 volts then your pot will only provide you a 2 volt range (7-9) instead of 5 (7-12). You also don't want to choose a fan that runs at high RPM, you want a lower RPM fan like a standard case fan. If you go too fast you will throw the bar. Also keep in mind that you aren't trying to make a tornado display here, you just want the starter to be in motion to keep everything suspended. You don't need it to look like a blender with a vortex that goes all the way to the bottom. The amp rating of the power supply needs to be the same or more than the amp draw of the fan you use. The fan will only draw as many amps as it needs so using a power supply capable of 2 amps with a .5 amp fan is just fine as the fan will only take what it needs. Voltage does not work this way though. Using a 15 volt power supply on a 12 volt fan is going to overdrive the fan, and at the very least shorten its life and make it run faster. I think most of your problem is the power supply. If you can find the power supply for an external hard drive or a wireless router those are usually 12V 2A and would work perfectly for this project.

 

[TheMattTrain]

I work at an A/V company which is where I got most of my stuff. I'm haven't had the time to go play with my stir plate anymore, but my next step is to try spacing the magnet from the fan slightly. It's currently super glued so I might need to replace the fan and the magnet, no big deal, I can get more for free (Hard drive magnet). After some thinking, I am wondering if the fan wasn't spinning at a consistent speed causing my vortex to fluctuate. I assume the magnet was catching something inside the fan causing it to stutter, if you will. I had it running full speed and holding the stir bar without a problem before I added the potentiometer.

 

[KartRacer54]

I had similar issues, first tried 2 different cheap fans, they did not have the torque that was required. Also bought a 1 1/2" stir bar. Way overkill. I finally pulled an extra case fan from my PC that has a 3 speed switch and replaced with 1" stir bar. Now everything works great without potentiometer. The case fan was not that expensive at Micro center computer store. under $8. Almost gave up on the DYI, glad I Didn't.