I recently built a new (version 2 for me) DIY stir plate. My original version was the typical 80mm PC case fan with a 25 ohm potentiometer. It worked ok, but the Radio Shack project box I had it in was barely big enough for a 2L flask. I am getting into brewing lagers so I wanted a more powerful plate that could handle a full 5L flask.
I wanted to share something I learned that I couldn't find elsewhere on the internet.
Magnet Placement:
I experimented with various magnet shapes and placement.
I used some 1/2" round neodymium magnets as well as various magnets I had pulled out of old hard drives (typically shaped like a very open C).
Note: When using two magnets, you have to be sure that they are flipped the correct way (polarity) so the stir bar sits on them correctly. When using a single magnet, place the stir bar on the magnet and make sure the bar spins at its center.
I tried many different arrangements of two magnets, spacing between the magnets, and various single hard drive magnets. I also tried varying the distance between the magnets and the bottom of the flask.
I am using a relatively powerful 12v 120mm case fan (TA-450DC).
Under most circumstances it was easy to throw the bar by running the fan at or near full speed. I learned that the key to retention of the stir bar is directly related to the strength of the magnet. I also found that stacking the magnets increases the strength and retention.
So, here are my cliff notes on building a robust stir plate that won't throw the stir bar:
1) Don't use two (or more) magnets placed in the same plane, as I have seen people do with two 1/2" neo magnets.
2) Hard drive magnets work great if you stack them on top of each other. Depending on the strength of the HD magnets, you will need two or three of them stacked.
3) The distance between the magnet and the flask bottom didn't matter as much as I would have thought. Closer is better, but moving it a little further away didn't seem to affect stir bar retention much until you get to the point where it doesn't work at all.
4) Balance your fan/magnet assembly with the stir bar thrown on top. I placed the magnets on the fan, dropped the stir bar directly on the magnets and started the fan spinning at low speed. I kept tweaking the magnet placement until I could run the fan at a moderate speed with no vibration. Then I used a small model paint brush to apply epoxy around the perimeter of the magnet and the area where the magnets touched each other. This held them in place well and kept me from disturbing the balance I had achieved.
The trick was discovering that stacking the magnets worked so well.
Here is a short video of what I built. It has two magnets from 15.5K rpm Cheetah hard drives stacked on top of each other (the magnets are thicker than usual HD magnets) and a 120mm 700mA fan. If using thinner hard drive magnets, three stacked should work well.
It will not run at full speed like that forever, as the turbulence will eventually throw the bar. It will, however, run at reasonable speeds, pulling a vortex down to the stir bar forever. That is a full 2L flask. I'm also using a modified version of Kai's PWM circuit to power the fan.
I need to build a kicker circuit to throw full power to the fan for about half a second on startup to get the fan moving. As it is now, if you want to run really low stir speeds you have to turn the potentiometer up to about half way to get it moving and then dial it back down to a slower speed. That isn't a big deal but I want it to be able to recover on its own if there is a power failure.
If this interests anyone, let me know and I'll pull the lid off and take some pics of the magnets and such.
I wanted to share something I learned that I couldn't find elsewhere on the internet.
Magnet Placement:
I experimented with various magnet shapes and placement.
I used some 1/2" round neodymium magnets as well as various magnets I had pulled out of old hard drives (typically shaped like a very open C).
Note: When using two magnets, you have to be sure that they are flipped the correct way (polarity) so the stir bar sits on them correctly. When using a single magnet, place the stir bar on the magnet and make sure the bar spins at its center.
I tried many different arrangements of two magnets, spacing between the magnets, and various single hard drive magnets. I also tried varying the distance between the magnets and the bottom of the flask.
I am using a relatively powerful 12v 120mm case fan (TA-450DC).
Under most circumstances it was easy to throw the bar by running the fan at or near full speed. I learned that the key to retention of the stir bar is directly related to the strength of the magnet. I also found that stacking the magnets increases the strength and retention.
So, here are my cliff notes on building a robust stir plate that won't throw the stir bar:
1) Don't use two (or more) magnets placed in the same plane, as I have seen people do with two 1/2" neo magnets.
2) Hard drive magnets work great if you stack them on top of each other. Depending on the strength of the HD magnets, you will need two or three of them stacked.
3) The distance between the magnet and the flask bottom didn't matter as much as I would have thought. Closer is better, but moving it a little further away didn't seem to affect stir bar retention much until you get to the point where it doesn't work at all.
4) Balance your fan/magnet assembly with the stir bar thrown on top. I placed the magnets on the fan, dropped the stir bar directly on the magnets and started the fan spinning at low speed. I kept tweaking the magnet placement until I could run the fan at a moderate speed with no vibration. Then I used a small model paint brush to apply epoxy around the perimeter of the magnet and the area where the magnets touched each other. This held them in place well and kept me from disturbing the balance I had achieved.
The trick was discovering that stacking the magnets worked so well.
Here is a short video of what I built. It has two magnets from 15.5K rpm Cheetah hard drives stacked on top of each other (the magnets are thicker than usual HD magnets) and a 120mm 700mA fan. If using thinner hard drive magnets, three stacked should work well.
It will not run at full speed like that forever, as the turbulence will eventually throw the bar. It will, however, run at reasonable speeds, pulling a vortex down to the stir bar forever. That is a full 2L flask. I'm also using a modified version of Kai's PWM circuit to power the fan.
I need to build a kicker circuit to throw full power to the fan for about half a second on startup to get the fan moving. As it is now, if you want to run really low stir speeds you have to turn the potentiometer up to about half way to get it moving and then dial it back down to a slower speed. That isn't a big deal but I want it to be able to recover on its own if there is a power failure.
If this interests anyone, let me know and I'll pull the lid off and take some pics of the magnets and such.
Last edited by a moderator:
