Homemade stir plate

 

Apparently I did decide to make that blog (or should I say this blog)! (see video)

There is a lot of info out there on how to make your own stir plate. I am very new to home brewing, but I decided to make one because it seemed like a fun project. Stir plates are used in home brewing to make yeast starters. For beers that start from high gravity worts making a starter can be helpful to ensure the fermentation goes to completion before the yeast succumb to increasing alcohol content and decreasing food levels. As I understand it, a starter serves two functions - it increases the amount of the yeast and activates it before pitching. This allows the yeast to rip through the fermentation with much more ease than if it is simply pitched directly from the vial.

Anyway, here's how I made the stir plate.

You might want to take a look at the parts list first, but I thought it fit better at the bottom of the post.


Mounting the magnet to the fan

The fan that I used must have some kind of metal component at the center because the magnet would stick to it pretty strongly. This made centering the magnet a much easier task. To center the magnet, I stuck it to the fan and put my stir bar on it. I spun the fan by attaching it to the 5 VCD power supply I ended up using (*see note below for explanation of how to do this). By spinning the fan with the magnet attached, I could get a pretty good idea of how well centered it was by the extent of vibration the fan would undergo. Also, with the stir bar stuck to the magnet, I could watch it as it spun and make sure that it didn't wobble at all (I found that this was easiest to notice as the fan slowed to a stop). Once I had the magnet pretty well centered, I marked its location with a Sharpie. Then I put some polymer resin on it and mounted it to the fan for good.

*You can strip the wires of your power supply then just push the bare wires into the contacts for the black and red wires of the fan (don't worry about the yellow wire if there is one) to get it spinning. If it doesn't spin then you probably have the positive wire (red on fan, often striped on power supply) and negative wire (black on fan, often no stripe on power supply) mismatched.


Mounting the switch and rheostat to the box

I put an on/off switch on my stir plate. This is not necessary. You can also set it up so that it turns on once you plug it in, but I had a switch lying so I figured I would put it on.

Don't worry about the wiring quite yet. I will explain that at the end.

As a side note, I think it would be an interesting social experiment to walk around with a briefcase that has a switch on it and see what kind of reactions you get.

I added the rheostat into this circuit to have control over the speed of the fan. A rheostat is a variable resistor. Since it is in series with the fan, it steals away some of the fan's power, making it spin more slowly. The amount of power that the rheostat consumes in this circuit depends on its resistance. The higher the resistance set on the rheostat, the slower the fan spins. Since this resistance is variable, we can adjust the speed of the fan by dialing the rheostat.

Mounting the fan to the box

Mounting the fan to the box was a little bit tricky. That's mainly because the box that I used is thick, and I wanted to mount the fan from the bottom so that I wouldn't have screw heads getting in the way of my flask. I wanted the flask to be able to sit on a flat surface, which meant that I needed screws that were 4 inches long. The only ones I could find were too big to fit through the holes of the fan. Luckily a guy at Home Depot had an idea for me. Instead of using screws I made my own with all thread rod, which I could cut to any length that I wanted. I used this equation to make the screws I needed:

all thread + end caps = screws

I mounted the screws at the correct spacings on the bottom of the box and secured them in place with washers and nuts on the inside. I used lock nuts to set the height of the fan (since the fan vibrates a little bit, I didn't want to use regular nuts since they might slip down over time). I secured the fan in place with some regular nuts on the top. This is all a little difficult for me to explain, but if you look at the photo below I think you will get the idea.

Wiring it all up

This step turned out to be a lot easier than I had expected. All you need to do is connect the positive wire (striped) from the power supply through the switch to the middle prong of the rheostat. Run the positive wire (red) from the fan to one of the rheostat's two side prongs. Finally connect the negaitve wire (black) from the fan directly to the negative wire (solid) from the power supply. That's all it takes. I soldered it all together to make sure I get good connections.

Check out this diagram. I think it might make things a little more clear.

By the way, I know my box has a strange hair issue (black stuff in the diagram). That doesn't bother me.

It seems to work really well! I will try to post another video when I actually use it to make a yeast starter.


Parts List

Computer fan - Free (thanks Randy! can salvage from old computer)
Hard drive magnet - Free (thanks again Randy! can salvage from old computer)
Switch - Free (salvaged)
5 VDC power adapter from cell phone - Free (salvaged)
25 ohm rheostat - $4.50 (Radioshack)
Control knob for rheostat - $3.50 (Radioshack)
Wire - Free (salvaged)
8-32 all thread - $2 (Home Depot)
8-32 end caps - $1 (Home Depot)
8-32 lock nuts - $1 (Home Depot)
8-32 nuts - $1 (Home Depot)
Washers - Free (salvaged)
Box - $7 (Goodwill)
Epoxy - Free (salvaged)

Total cost - $20

A lot of this cost was the box. I could have bought a cheaper one, but I liked the one that I found at the Goodwill. It had a lot of character (mostly due to a few bumper stickers that I took off - I will put some more on there when I find the right ones).