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Build a Better Stirplate

It probably isn't necessary to stir Iodophor.

It probably isn't necessary to stir Iodophor.

OK, so the world probably won’t be beating a path to my door. But there’s a right way to do it, and a wrong way – and a lot of home brewers are doing it the wrong way.

The basic idea behind these homebrew stirplates is to control the speed of a motor (computer case fans being a cheap and accessible source) by varying the supply voltage. The best way to do it would actually be using pulse width modulation via a microcontroller, but I didn’t have one on hand, and energy efficiency probably isn’t a major concern for most people interested in powering a motor drawing around a couple watts. The simplest way to provide voltage adjustment (though not regulation, which I’ll get to in a second) is simply to put a resistor in series with the motor, which is what a lot of home brewers do. The problem with that (aside from the engineer in me hating the kludginess) is that electronic components are sold on profit margins more frequently associated with Wal-Mart stores, and are therefore made of plastic. If you have one lying around, or pick one up at Radio Shack, it will typically be rated 0.5 W, sometimes less. We’re going to be sinking around 12 V • 150 mA = 1.8 W into it. It will melt.

So, the bare minimum takeaway here is to use a potentiometer rated for at least 2 W continuous power. A better, or at least more elegant, solution is to incorporate some actual voltage regulation. Which brings us to one of the most useful components ever invented, the LM317 adjustable voltage regulator. The LM317 is capable of providing anything from 1.25 to (Vin – 1.7) V with good regulation (±1%), using only two resistors to set the output voltage:

Vout = 1.25(1 + R2/R1)

The basic schematic for the LM317. Note the potential error in the value of R1.

The basic schematic for the LM317. Note the potential problem with the value of R1.

Replace R2 with a potentiometer and you have a nifty little 1.5 A adjustable power supply using only three components. A couple filter capacitors probably aren’t necessary for our purposes, but they’re cheap insurance. The only downside to the LM317 as a regulator is that it’s inefficient; any excess current is dissipated in the regulator as heat. Again, that could be up to 1.8 W in this application. The most common regulator package is a TO-220, which can only safely dissipate around 1.5 W. So a heat sink is definitely a good idea. The LM317 has a built-in thermal shutdown, though, so you might be able to get away without a heat sink if you’re careful not to run the fan at very low speeds. If you do sink it, be aware that the tab on the TO-220 package is tied to Vout – so be careful to avoid shorts. In the photo of the internals, you can see that I sealed the heat sink bolt with heat shrink tubing just to be safe.

Technically, an LM317 can have a minimum load current as high as 10 mA, although most will be much lower. That puts the maximum value of R1 at about 120 Ω (10.4 mA). Note that the datasheet specifies 240 Ω because it’s lifted from the specs for the LM117, which tops out at 5 mA. It’s also worth noting that I’m using a 220 Ω resistor simply because I had a 2.5 kΩ pot and didn’t feel like buying another. Do as I say, not as I do.

Circuitry aside, the rest of the build is pretty easy. I had a plastic kitchen container that happens to be the perfect size for my 1 gallon starter jugs. I bought a pair of neodymium rare earth magnets, which are simply adhered magnetically to the hub of the case fan. At 4 pounds of lift each, they aren’t going anywhere. If the fan was attached directly to the container, the magnets would make contact with its surface, so you’ll need some sort of spacer. I used roughly 5 mm lengths cut from a plastic drinking straw. The magnets and the power supply both came from American Science and Surplus, and the only other thing I needed to buy was a stir bar: $6 on eBay. If you’re going to use your stirplate with a convex-bottomed vessel, it might be worth noting the type of stir bar that works for me. It’s 1″ x 3/8″ with a ring.

The stirplate internals. Blue LEDs optional but awesome.

The stirplate internals. Blue LEDs optional but awesome.

The complete parts list would be:

  • Project box or other plastic container
  • 12 VDC power supply (at least 300 mA to allow for power spikes on startup)
  • PC case fan
  • Mounting hardware for fan
  • LM317T
  • TO-220 heat sink
  • 120 Ω resistor
  • 1 kΩ potentiometer
  • Knob for potentiometer
  • 0.1 μF ceramic disc capacitor
  • 1 μF electrolytic capacitor
  • 2 rare earth magnets
  • Magnetic stir bar

Plus wire, solder, perf board, etc. If you’re a tinkerer you probably already have some of this stuff on hand. Even if you had to buy everything online and pay shipping, it would only cost about $30. Tayda Electronics is a great source for components. Avoid Radio Shack like the plague.

Now that I’ve gotten off my lazy ass and put this thing together, I’ll be updating my aeration experiments with one last trial, to test my assertion that frequent shaking and a stirplate are equivalent.

17 comments to Build a Better Stirplate

  • Jez

    Thanks for this! I’ve had a fan and a hard drive laying on my bench since January. I need to get on the stick and will take your advice for components. I will likely get a project box, though, because I’m just that way. That Rubbermaid/Tupperware deal you got there…well, I bet you buy an enclosure like that, they give you a free bowl of soup. Oh, but it looks good on you!

    • Glad you found it helpful. I’d be interested to hear if you have good results with the HDD magnet – I wasn’t able to get one to reliably couple with the stir bar.

      The Tupperware I used was so old and scratched up that it was getting to be unsuitable for kitchen duty anyway. Remember the engineer’s creed: if it’s stupid but it works, it isn’t stupid. Plus using something clear meant I didn’t have to mount an external power LED.

  • [...] ResultsAeration and Yeast StartersLOL @ MazurYeast Ranching and YouYeast Pitching Rate ExperimentBuild a Better StirplateRegulating Fermentation TemperaturesWater, Water EverywhereGood Beer, Easy BeerUnless They’ve [...]

  • Nice design, when I built mine, I ended up buying a case fan that had a 3 speed switch wired into it already (also with blue glowing LED’s which gives off a cool factor and is probably mutating my yeast ). Although that works, its obviously not the most precise so its lacking. I’m learning more about electronics as I build brewing projects, which is fun. I’ll definitely be looking at this again when I build a second. I did use a HD magnet on mine, and it does seem to work well enough if you have one lying around. When I heard the description on Basic Brewing I was actually hoping to see an avocado or harvest gold plastic bowl, which would be pretty sweet.

    • Believe me, my parents still have some harvest gold Tupperware in their kitchen – it’s just in better shape than the newer stuff! [Insert comment about society going to hell in a handbasket here.]

  • [...] stuff online, so it comes down to a question of how much your time is worth. I put together a quick build writeup when I made mine. __________________ [...]

  • BrewerBill

    I cheeped-out and used a cigar box as a case, and a power supply/controller from a model train set as the a rheostat. I had the controller laying around, and since brewing is more fun than watching a model train go around in circles, poof, repurposed.

  • World

    I like your design. Are you using a 3″ or 4″ case fan?

  • [...] components needed to put together a basic LM317 circuit would be a resistor and two capacitors. __________________ [...]

  • Ben

    Could you please post your wiring schematic for this build?

    • Ben,

      That is actually the full schematic, with the exception of the motor, which is wired across VOUT, and the power supply internals. If you wanted to, you could even leave out the capacitors, but they’re cheap insurance. I hope that helps.


  • Eric

    I realize this post is a little old. But I have a couple questions on your circuit. I’m no whiz when it comes to electronics, so bear with me.

    Do you connect the fan to the V(out) and the lower-right node in your schematic (which, I’m guessing is also connected to the power supply, via that ground node)? And, if using the voltage regulator, are we okay with no longer requiring a 2W-rated potentiometer? I only see 0.5W pots on Tayda’s web site.


    • Exactly right. The fan is wired to Vout and the power supply to Vin, and they share a common ground. The potentiometer only needs to be able to sink the forward current of the LM317, which would be about 0.1 W at most.

  • jason

    quick question about the lm317. If I use it with a 12V transformer thats delivering 17v on my meter and between 300 and 500ma would I need to add an additional heat sink to the lm317? I tested it for 30 minutes on a breadboard without and additional heat sink and the top of the lm317 reached 80°F while running the fan with 4.5V. Just wasn’t sure wheather it would get hotter than that over the course of 36-48 hours of continues operation.


  • 80°F is definitely nothing to worry about, and after 30 min you should be at the peak temperature for that scenario. I’d also test it under a worst-case scenario to make sure though. That would probably be either without the motor turning (LM317 sinking all the power) or with the motor running under load at low speed and the case closed.


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