Stir Plate too powerful

So I made a Cigar box stir plate using commonly available directions. Components:
Wall transformer outputting 12V @ 1000mA
25 Ohm Rheostat (speed control)
Case fan - 1.8 Watts rated power @ 0.15A

I am using a 1.5 inch stir bar - it was octagonal with a ring but I did my best to smooth out the rough spots and I cut off the ring. It isn't perfect but it isn't bad either.

At the lowest speed the stir bar is making quite a tornado:


And after a few minutes it will get thrown at this speed, even. It won't hold at higher speeds at all, even for a few minutes.

What I would like to do is slow down the base spin rate by 50%, or maybe even more. Then I could actually use the rheostat for speed control and not just leave it pegged at minimum. The only problem is that I don't know anything about circuits or the math thereof. I figured if I could solder in some sort of in line resistor, that would do the trick. I'm hoping one of you can tell me how big that resistor has to be, and where I should put it in the circuit:

- Between the wall and the fan
- Between the wall and the rheostat
- Between the rheostat and the fan

Thanks for the help!

What is the size of your case fan?

You need a smaller stir bar. It is that simple.

80mm with a 1.5" stir bar, neodymium magnets spread roughly 1.5" apart hot glued to a spacer on the fan, as close as possible to the lid of the box.

What's the speed range of the fan you used? Sounds like you used the wrong fan to me. If it goes from zero to too fast with nothing between, then you used the wrong fan. You can try a different stir bar if you want. I would NOT try to modify the next one. The ring you removed is there to help it spin.

I had a similar issue back when I built mine. I changed the 12 volt ps out for a 6 volt and it worked much better.

Curious as to why you modified the stir bar. That sounds like the same stir bar I use and I've found the ringed ones work better than the 1/2" flat ones I started out with, although that could vary from setup to setup depending on speed, magnet spacing, etc. Mine threw the flat 1/2" bars at least once per use but I can't remember he last time it has thrown my ringed bars.

Interesting regarding the info on the stir bars. I read somewhere that the flat ones are better if you have a perfectly flat bottomed vessel, whereas the ringed are better if you have contour or convexity/concavity. I only did that research after the ringed bar was jumping off all the time, even at the lowest speed.

My fan doesn't go 0-too fast, it is too fast right off the bat when I plug it in (I have no switch wired in). There isn't a speed that reliably holds the bar.

JNewbs said:

Interesting regarding the info on the stir bars. I read somewhere that the flat ones are better if you have a perfectly flat bottomed vessel, whereas the ringed are better if you have contour or convexity/concavity. I only did that research after the ringed bar was jumping off all the time, even at the lowest speed.

My fan doesn't go 0-too fast, it is too fast right off the bat when I plug it in (I have no switch wired in). There isn't a speed that reliably holds the bar.

Click to expand...

Then you're using the wrong fan, or you need to use a lower voltage power block.

You can get fans with different speed ranges, that can be controlled. Sounds like the one you have is either full speed or nothing. Or it's lowest speed is far too fast. I've bought fans from frozencpu.com when I was building computers. You can get fans that spin slower, and are larger, for not a ton of money. In your case, I'd probably try either a 92mm or 120mm case fan that has a good speed range listed. You'll want something that starts off on the lower end of the scale, so that you can ramp up the speed.

BTW, these kinds of issues is why I just bought a pre-built (Hanna) stirplate when I was looking for one. I could have tried to build one, but I wanted it ready to go on the quick. I've since tried to build one, but haven't gotten the magnets placed correctly, so it doesn't really work.

Smaller bar, you only need to keep the yeast in suspension...the local wine lab uses those little kidney bean sized stir bars for all their work. I think mine are 1" and work fine in 1 & 2L starters.

A smaller power supply will also slow it down.

seems like you have too much power. find a wall wart with less volt output.

Sounds like a lot of solutions, and I thank you guys for that. However I would hate to buy or cannibalize new gear, or order stuff on the internet like new fans or stir bars. I would much rather step down the power from the wall wort with an in line resistor IF that is possible. I'm hoping someone can tell me if that's the case, and if so tell me how big of a resistor I'll need.

And yes the fan is just on or off. The rheostat is how I control the speed - less than ideal but functional and cheap.

The way you’re doing it, you’re going to burn out the control. I suggest doing it right with an LM317 adjustable voltage regulator. Check this circuit at the bottom of the page. http://www.stirstarters.com/instructions.html

If you want use a dropping resistor, I’d guess 100 ohms at 1w. That would give you a little less than half. Your motor has an equivalent resistance of 80 ohms.

The circuit you’re using now varies the motor voltage from 9-12 volts. Adding the 100 ohm resistor would give you approx 5-6 volts.

I did a quick check on radio shack.com and didn’t see any 1 watts. They do sell a 5pak of 100 ohm ½ watts for 1.19. You could wire four of them in series parallel to form a 100 ohm 2w resistor.

It’s not as tough as it sounds. Take two of them and twist them together. Now you have a 50 ohm 1 W resistor. Do it again with two more and put it in series with the other two. You have just created a 100 ohm 2w resistor.

Wynne-R said:

If you want use a dropping resistor, I’d guess 100 ohms at 1w. That would give you a little less than half. Your motor has an equivalent resistance of 80 ohms.

The circuit you’re using now varies the motor voltage from 9-12 volts. Adding the 100 ohm resistor would give you approx 5-6 volts.

I did a quick check on radio shack.com and didn’t see any 1 watts. They do sell a 5pak of 100 ohm ½ watts for 1.19. You could wire four of them in series parallel to form a 100 ohm 2w resistor.

It’s not as tough as it sounds. Take two of them and twist them together. Now you have a 50 ohm 1 W resistor. Do it again with two more and put it in series with the other two. You have just created a 100 ohm 2w resistor.

Click to expand...

After some researching (which I wouldn't have been able to do without your explanation - thanks!) I have verified to myself that connecting the resistors in the fashion in which you describe will create a 100 ohm 2w resistor. Formulas also show that the 100 ohm resistor will drop the total Voltage going across the fan to 5.3ish. Throwing in 25 ohms from the rheostat will drop it to 4.7ish, which seems like a pretty negligible difference, and so maybe I will take the rheostat out all together and avoid the possibility of failure there.

I do have a question about Wattage ratings of resistors. Why do you say I want 100 ohms at 1w? And why would 100 ohms at 1/2 watt not work, and why will 100 ohms at 2w work fine? I imagine it is some sort of a safety rating, but I know how little I know, so out of curiosity I thought I would ask.

Thanks you SO MUCH for answering the question I actually asked, by the way, and thanks for all of the help!

Your numbers are correct, based on the assumption that the power supply is a constant 12 volts. An unregulated supply will vary with the load. Your voltage is probably more like 14-15. That’s why I used rough numbers; if you don’t measure it you’re guessing.

Wattage ratings on resistors are based on how much heat they can take without degrading. The middle turns brown and the resistance gradually increases. If you exceed the rating a lot, they will flat out burn up.

So, basically you have a two watt fan with a resistor that will drop 55.5 % of the voltage, but cut the current in half. Depending on what value you end up with a half watt might work, but it’s pushing it. I’d hate to see you burn up your cigar box.

You can certainly play with series parallel combinations to come up with the sweet spot, but long term, pay attention to the wattage. You want to be at or below the rating.

Awesome, thanks for all the knowledge. I took Phys 1 in college but didn't stick around for Phys 2 (E&M at my school).