DIY Stir Plate question/issue

[bigirishape]

Need a little help/feedback on this build...

I've got an old CPU power supply and fan, old HDD magnets on the fan. 1L flask and 1" stir bar.

I lowered the fan about 1/4" below the base of the flask.

My stir bar just chatters around the base of the flask, no vortex action, just barrel-rolling stir bar.

Do I need strong magnets? Do the magnets need to be closer to the stir bar (ie: raise up the fan some more?)?

Any thoughts, or any info I can give to help figure out this puzzle? About the only thing I haven't tried is a stronger magnet (don't have one) or raising the fan (it's a pain in the arse)...

Thanks...I'll be raising the fan while I await feedback...hopefully I'll have a breakthrough, but if not, I'm all ears...

 

[Travestian]

It's possible the fan is spinning too fast to get the vortex started. Do you have some kind of speed control for it?

 

[bigirishape]

I do, but I don't know exactly what the RPMs are that it adjusts to. Maybe I need one of the 3v/6v/9v/12v adapters...

 

[mlinc]

One thing to check is to make sure your magnet is centered on your fan. If your magnet is not centered it could cause your stir bar to be thrown in the flask. A HDD magnet is plenty strong for what you are trying to do.

 

[cooldood]

Another issue is if you stir magnet has a hub in the middle. This will make it chatter and fly off

https://www.homebrewtalk.com/f51/stir-plate-magnet-404859/

 

[bigirishape]

The bar is flat and the bottom of the flask is flat, not sure if the addition of a flat ring would help. May see if I have something to test it with tho...

 

[cjhammel]

I have found there is a sweet spot between the magnet and the stir bar. You can try lifting the flask to get an idea of how far it is. I was surprised at the distance I had to off set the fan. With my magnets I needed about 1/2" to the top of the stir plate.

 

[tcapmidnoces]

How are your magnets oriented on the fan? I found that one magnet works better than two due to the interfering field between the magnets. I'm sure you could still use two if placed with the poles in the right locations.
As far as distance goes, I have my magnets less than 1/8" below the surface of the stir plate.
Also, what method are you using to control the speed? I built a PWM generator on the back of a potentiometer to control my fan.

 

[cjhammel]

I have two neodymium magnets and using a PC fan controller. See my build https://www.homebrewtalk.com/f51/my-new-diy-stir-plate-417849/

 

[bigirishape]

I think I got it licked, maybe...

I had a 12v PSU fan and a 12v power supply from a battery charger, and had them hooked into a scrap potentiometer from an old adjustable case fan. Even with the pot at the lowest setting, it was still going way too fast for the stir bar to do anything but chatter around the base of the flask.

I went to Radio Shack and picked up a 25 ohm (3w) rheostat and wired that into the setup in place of the scrap pot, assuming the pot just couldn't adjust the fan low enough b/c it wasn't designed to let the fan go below a certain RPM. Even with the rheostat at the lowest, fan still going too fast for stir bar.

I pulled the pieces of the stirplate case apart, and turned on the fan while using my finger to adjust the speed of the fan. Magic started to happen.

I scrounged and scrounged around the house for a spare cell charger or something I could use, and I found my Roku power adapter, which is a 5v adapter. Luckily the plug worked with my M-style power adapter plug I installed in the case, and with a little fiddling on the rheo knob, it turned on the fan slow enough to get the stir bar going. With the rheo turned up all the way, the bar works great with vortex going all the way to the bottom of the 1L flask.

Two issues though...one, the HDD magnets were either not strong enough or too far away from the base of the flask, and two, sometimes the 5v adapter is enough to start the fan on the lowest rheo setting, sometimes I have to adjust the rheo up higher to even get the fan to start.

For the magnet issue, instead of disassembling the fan setup I just pulled two small square neodymium magnets off my beer fridge and hot glued them to the fan hub instead of the hdd magnets. Success. Even at the highest rheo setting, the stir bar stays steady.

I suppose I will look around for a slightly stronger ac adapter, 6v or 9v or something, and see if that works...but for now, the 5v works.

Victory tastes pretty sweet, gentlemen. Thanks for all your feedback!

 

[ox45]

It's having trouble starting because most computer 12v fans cannot drop below a certain voltage (usually about 4-5v).

As suggested earlier, you should look into a PWM (pulse width modulated) controller. They are on eBay for about a dollar more than the 25kw rheostat at Radio Shack.

I recently bought one for $8, and it works great. I can get the fan spinning a lot slower than when I tried the rheostat.

http://www.ebay.com/itm/360685305289?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

 

[bigirishape]

Yeah, I figured the psu fan wouldn't go below a certain RPM, but I suppose in this situation the RPM and the voltage are the same beast (not actually, but with regards to fan speed, they have the same effect).

That little PWM is quite schmexy... Maybe v2 of the stir plate may use one of those, or if the wife ever needs her Roku power supply back...

Thanks!

 

[ox45]

You are correct that the fan speed is generally relevant to the voltage as basically it is just a dc motor with blades on it.

The nice part about the PWM is that it is sending the full 12v to the fan, just pulsed in intervals to make the voltage appear to be less. But since the pulses are 12v, the fan can start and keep going at much lower perceived voltages than straight low volt DC.

I guess the important part is that you got it working now!