Stir Plate Help....

Hey All, So I have put together a stir plate and this thing is barely not working.

Here's what's going on:

With water in the Erlenmeyer, if I have the fan running on low, it'll catch the magnet, spin it about half a second, then instead of spinning, it'll start swirling for another half second or so, then it throws it....catches it again and the cycle repeats.

If I turn up the speed of the fan, it just launches the magnet out of reach. Never once does a whirlpool begin to form.

Thoughts on what's going on?

I had some trouble with this at first as well. Are you running a 12v DC power supply? How are you limiting to voltage to the fan? Some guys run potentiometers but I just used a 12V DC power supply that came from Radio Shack. Like $10 and it allows me to choose from 3V, 6, 9, and 12V. With a 2" stirbar, it usually throws if I get above 5-6V. Just make sure you have the fan turned down as much as possible. You don't need much to get the yeast in suspension.

Also, make sure that your magnet is as close as possible to the top of your enclosure. This will ensure that the stirbar has the most amount of attraction to the magnet as possible. Good luck!

+1 on fan speed too fast.

I started with a 12v PS and potentiometer on slowest speed stir bar was thrown right away. With a 9v PS and minimum POT setting the stir bar spun but the vortex would go all the way down to the stir bar and rattle it. I found a 6.5v PS and added some more weight to the fan to spin (some washers glued in balance) and now have it working well am able to use the POT to adjust the speed as needed.

One thing, I still can't get a 2" stir bar to work as it is thrown right away. The 1" stir bar works great. Maybe I need to raise the fan a bit to get the magnet closer the flask.

Slow down the stirplate by using a smaller power supply or add more resistance... or, get a smaller stirbar.

right now i have to sorta flick the fan just to get it going on the slowest speed and every once and a while it'll stop completely at the lowest speed...

Stevorino said:

right now i have to sorta flick the fan just to get it going on the slowest speed and every once and a while it'll stop completely at the lowest speed...

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If that's the case, I would be looking the strength of the magnet on the fan or proximiety to the flask

Sounds like you have a couple issues... It sounds like you don't have the magnetic center of your drive magnet concentric with your fan axis. This causes the stir bar to get dragged around rather than spun and thrown easily, especially during speed changes. Try shifting the drive magnet around until the stir bar is really pivoting on it's center.

Two, did you just use a potentiometer or rheostat for speed control or build it around a voltage regulator? It sounds like a pot/rheostat. That combination is very susceptible to voltage fluctuations. That coupled with the interference of the motor's magnetic field from the magnets causes the fan to be unreliable at low speeds. (when it stops do you see it flip backward a little?) Google "LM317" and /or "LM317 stir plate" and you'll find info on the LM317 voltage regulator. It makes a much more stable circuit.

The ideal solution is to use a PWM (pulse wave modulation) speed control. They work really well for this type of application because they don't vary the voltage but they vary the amount of time the voltage is applied. Simply put, if you want the fan to run at half speed, you supply 100% voltage 50% of the time. The only downside to that is the only circuit I was smart enough to make work was at 400Hz, well within our ability to hear it and it made a humming noise I didn't like. I don't know how to make a simple enough circuit that'll operate at a higher frequency.

I worked about 10 hours on different stir plates, bought a 80 dollar professional one from the brew shop, then looked on ebay and found really nice home made ones for 40 bucks. I would hit ebay up personally

jamesrm said:

I worked about 10 hours on different stir plates, bought a 80 dollar professional one from the brew shop, then looked on ebay and found really nice home made ones for 40 bucks. I would hit ebay up personally

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Yep.... Or you could also get a really nice home made one at Welcome to Stir Plates.com ;-)

Stevorino said:

Hey All, So I have put together a stir plate and this thing is barely not working.

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If it's barely not working you're golden. It's when it's completely not working that you have a problem. :cross:

General stirplate troubleshooting:
* start the spin after the flask is in place and the stirbar is "caught"
* slow the spin down
* try different size bars (usually smaller)
* if the plate is homemade, play with the position/number of magnets

Derrin said:

Yep.... Or you could also get a really nice home made one at Welcome to Stir Plates.com ;-)

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LOL I bought one of your auctions, I would have bought two of yours but I didnt see a second one up there. "Sings: Its a small world after all!)

jamesrm said:

LOL I bought one of your auctions, I would have bought two of yours but I didnt see a second one up there. "Sings: Its a small world after all!)

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Thanks for the business!

Derrin said:

Yep.... Or you could also get a really nice home made one at Welcome to Stir Plates.com ;-)

Click to expand...

I might get to that soon...my frustration is mounting. Do you mind me asking what size stir bars ya'll use?

I'm using a typical 80mm computer fan, a cell phone charger, one hard drive magnet, and a rheostat.

I'll try recentering the magnet -- I've noticed the fan vibrates a good bit (to the point where it will move on a surface...which makes me believe it's not centered well enough.

The fan shouldn't be vibrating if properly attached. I used 4 long bolts and drilled four holes in the top of the box for those to go through. My fan had holes in each corner which allowed me to bolt it to the top of the box. Actually, the "top" of my box is technically the bottom of my box.

I actually used three nuts on my bolts, one to hold the fan up at the proper height and two to hold the bolt in place at the bottom. Worked out well for me. Just takes some tinkering the first time you build one of these.

By the way, right now I'm using a 2" stirbar with one magnet like your current setup. I want to get a smaller one as well, but then again, there are just so many things to buy!

I've found that a 1 1/8" X 5/16" ringed octahedral bar works the best for my plate.

If the fan is vibrating, you definitely don't have the magnet balanced. Even at that, they have to be balanced with the magnetic center of the drive magnet aligned with the rotational center of the fan. The magnetic center of a hard drive magnet doesn't always match it's center of mass. This can cause a slight imbalance, but it should never be enough to make the whole thing move!

I only briefly skimmed everyone else's replies, so if I'm repeating sorry (too early to read much). I had similar issues with my stir plate when I first tried it. My fan was positioned too low in the enclosure. Moving it up fixed everything.

I'll try rebalancing it this weekend -- anyone ever try putting two hard drive magnets on their fan -- is that even doable?

Check out the link to my stirplate build in my signature... this should get you moving in the right direction without spending anymore money.

Here are a few pointers: If you have your potentiomenter all the way up (lowest resistance) and the fan doesn't spin, you need more voltage (higher output power supply).

Look at the way I mounted my fan in my build. This should help with the vibration a lot. Also, the steel washer centered on the fan helped me get my magnets centered on the fan much better. There is no reason to not use more than one magnet stacked, it will give you more magnetic pull, and keep your bar centered better.