Homemade Stir Plate Problem

[andy6026]

I recently made myself a home made stir plate. I wired a computer fan (12vDC - 0.15A) to a power-supply (12vDC - 200mA). I didn't use any intermediary regulator such as a potentiometer (my local electrical shop didn't have anything in stock), but the speed as is seemed to work fine and I figured I could always wire something in later if needed.

I made a yeast starter tonight and plugged it in. All worked very well for about 4 hours. And then it stopped. The power supply unit was warm to the touch but the fan had more of a hint of warm plastic smell. No visible signs of heat damage (such as melting) anywhere.

I checked my connection between the fan and the power supply - it's fine.

I guess my question is - what is likely the problem, the fan or the power supply, and why did this happen?

One thing I'm guessing with my complete novice electrical understanding is that the rare-earth magnet that I attached to the fan was rather large and seemed quite cumbersome for it -- slowing the fan speed down considerably. Could it have burnt out the fan?

Any suggestions would be helpful. Thanks!

 

[Hang Glider]

The rare earth magnet will most certainly interfere with the motor on the fan.

I took a block of plastic and centered it on the fan, glued it down. Then I glued the magnet on that, raising the magnet an inch above the motor, effectively. Others have hand better luck, but perhaps smaller magnets, or better alignment helps them...

 

[andy6026]

Hmm, interesting. I may try smaller magnet to start. I have a spare fan, but only one, so I want to figure out what fried and why before I put the next one in.

Thanks for the reply, much appreciated!

 

[Roondy888]

The rare earth magnet will most certainly interfere with the motor on the fan.

I took a block of plastic and centered it on the fan, glued it down. Then I glued the magnet on that, raising the magnet an inch above the motor, effectively. Others have hand better luck, but perhaps smaller magnets, or better alignment helps them...

+1 I have also seen people use washers to give space between magnet and fan.

 

[libu]

It could be either. If you have a voltmeter you could check the power coming off the power supply. Otherwise try smelling the power supply and the fan (burnt electronic smell tends to linger).

I would recommend reducing the amount of work the motor has to do the spin the fan. You can cut off blades of the fan and/or move magnets off the center (either up or out, I used a piece of Popsicle stick to move them out).

Also a bigger power supply (I'm using 12V 1A) the fan will only draw what it needs and fans are generally cheaper to replace the power supplies. Most Computer equipment uses 12V power supplies. Look for old routers/switches/modems for power. Call a local computer repair store see if they have anything lying around you can pickup cheap. Also old hard drives have great magnets in them.

 

[Facinerous]

I would be willing to bet its your fan that burnt out.

An electrical motor works through magnets, so attaching another magnet close to the rotor/stator will affect its ability to work(spin). If the fan itself stops it puts itself in a position called locked rotor. This is bad because it will pull an excessive amount of current heating up the windings in the motor and eventually burning it out.

Putting a spacer in will help reduce the influence of the magnet on the motor. Also the size of the fan will imply a strength of the motor. So if you are using a 40mm fan chances are the motor itself is undersized for the addition of a magnet. What is typical is at least an 80mm fan.

It really could just be bad luck on this one though. As many people have done this stir plate build and I have seen very few complaints of burnt out motors with the magnets glued directly on the fan housing. It may be worthwhile to check the actual voltage coming from your power supply. Even though it says 12v it could be much higher. I have a 9v power supply that effectively provides 13.5 volts. Supplying the fan with to high a voltage will burn out the fan prematurely as well.

Good luck dude.

[edit] I just looked back and noticed your power supply is only rated for 200ma. Your fan would have been pulling at least the 150ma that you specified. With the addition of the magnet it could have been pulling more juice. Maybe I wouldn't rule out your power supply going bad. Check the power supply as well.

 

[Pscdouglas]

The key here, IMHO is to use two rare earth magnets (small ones) and mount them as far from the guts of the fan as you can while balancing the rotor. I've seen builds that use the HDD magnet and it might work for them, but Iv'e had the best luck with two 3/8" or smaller magnets mounted opposed and cantilevered off the center area of the fan rotor. I used hot glue to mount them and thus space them 1/16" or so from the fan but I believe that using a conductive material would not aid in the confusing electro/magnetic forces. Also, find a substantially powered AC adapter (think printer or laptop) and get a potentiometer from radioshack for about 3$. THE BEST TIP FOR STIR PLATES: get a stir bar shaped like a X and not a straight bar...much easier to keep going and you wont need a huge one to get a sweet vortex going.

 

[MaxStout]

You mentioned using large magnets. Perhaps they were attached in a way that made the fan go out of balance and burned up the delicate bearings?

On mine I epoxied two rare earth magnets 180 degrees apart and it balances well when rotating. The magnets are small--about the size of aspirin tablets. And I use a power supply that delivers a couple times the current needed to operate the fan. It's good to have some headroom there. If your fan draws 150mA, get a power supply >300mA.

 

[andy6026]

Thanks for the thorough replies everybody! I had almost decided to throw in the towel and buy a stir plate since they're not that expensive. However, given the advice and explanations posted here I'm going to take another crack at it.

Today I'll be going to an electrical shop to see if I can get another power supply that can draw more ampage as well as a potentiometer and spare fan. These are all fairly cheap at the place I go to. I'm also going to try smaller magnets and raise them.

The 'X' shape stir bar sounds like a good idea. I'll google where to do one after I finish typing this.

Thanks again!