Alternate Stir Plate Design
Posted: Wed Oct 20, 2010 8:52 am
A few people have ping'd me with questions about how I built my stir plate, so I took a couple of pictures and posted them to Flickr:
http://www.flickr.com/photos/33275630@N ... 097900761/
As you can see, it's much cruder than the fancy ones built in the last tech meeting (what with their "housings" and "solder"). My design was largely driven by whatever I had lying around. The only piece I had to purchase was the magnets (Magcraft NSN0575/N40 - about $10 at Amazon for more than I needed). I tried using hard drive magnets but could never get them to work with a 50 mm stir bar. They'll probably work with a smaller stir bar, though, if you wanted to try.
One of the differences I've noticed between this design and the one built at the last tech meeting is the magnet attachment mechanism. In mine, a steel backer plate is fixed to the fan (via two-sided 3M foam tape) and the magnets are held to the plate by magnetic force. This provides the opportunity to easily relocate the magnets to ensure they are both displaced equally about the axis of rotation and separated by approximately the length of the stir bar (i.e. so it doesn't wobble and throw the bar). The steel also doubles the strength of the field seen by the stir bar by shorting out the magnetic circuit on one side. The magnets will move a little bit if I turn the fan all the way up without a load on top, but this never seems to happen when I'm actually stirring a flask.
As far as the fan speed control is concerned, I used a universal DC supply. I typically get the stir bar to start moving at around 6V and then run it at 9V.
I'm pretty sure you could do the same thing with an AC fan if you rigged it up with a dimmer switch. I have all the parts to try that but have just never gotten around to it. If anyone wants to give it a try, let me know and I'll get the parts to you. I dug the fan out of the trash and I think I payed $0.10 for the dimmer switch at ReStore (which is a great resource for building brewing control circuits on the cheap).
Lee
http://www.flickr.com/photos/33275630@N ... 097900761/
As you can see, it's much cruder than the fancy ones built in the last tech meeting (what with their "housings" and "solder"). My design was largely driven by whatever I had lying around. The only piece I had to purchase was the magnets (Magcraft NSN0575/N40 - about $10 at Amazon for more than I needed). I tried using hard drive magnets but could never get them to work with a 50 mm stir bar. They'll probably work with a smaller stir bar, though, if you wanted to try.
One of the differences I've noticed between this design and the one built at the last tech meeting is the magnet attachment mechanism. In mine, a steel backer plate is fixed to the fan (via two-sided 3M foam tape) and the magnets are held to the plate by magnetic force. This provides the opportunity to easily relocate the magnets to ensure they are both displaced equally about the axis of rotation and separated by approximately the length of the stir bar (i.e. so it doesn't wobble and throw the bar). The steel also doubles the strength of the field seen by the stir bar by shorting out the magnetic circuit on one side. The magnets will move a little bit if I turn the fan all the way up without a load on top, but this never seems to happen when I'm actually stirring a flask.
As far as the fan speed control is concerned, I used a universal DC supply. I typically get the stir bar to start moving at around 6V and then run it at 9V.
I'm pretty sure you could do the same thing with an AC fan if you rigged it up with a dimmer switch. I have all the parts to try that but have just never gotten around to it. If anyone wants to give it a try, let me know and I'll get the parts to you. I dug the fan out of the trash and I think I payed $0.10 for the dimmer switch at ReStore (which is a great resource for building brewing control circuits on the cheap).
Lee