Easy DIY Stir Plate

[wolfareno]

Aug 29th, 2012 -- EDIT: The double magnet setup did not work at all. I had to use a single hard drive magnet in the center of the fan. This seems to work best with my 2L flask if I use a 1.5" stir bar. The 1" works, but it gets thrown sometimes. Additionally, the wall bug I used, when paired with this pot and fan, produces a little too much speed for the stir bar. I only use it on the lowest setting (which is fine) but less power or a slower turning fan might be better to use.
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Hi Gang,

After perusing the many stir plate builds, I decided I knew enough to make one. Like many others, I used an 8x6x3 project box and several expensive Radio Shack parts. I can post a walkthrough if anyone is interested, but I made a video that explains the build, and you can watch it here: [ame]http://youtu.be/6z3CV5QYDW0[/ame]

(I made the video so my Dad could see how it turned out, so it's not entirely professional.)

Do you guys see any major mistakes I made? This was my first time soldering electronics, and it was very easy (though I didn't do any circuit board work). I'm still waiting on my flask and stir bar, so I won't know how well it works for a few days.

I think the entire project cost me about $25.

Cheers,
Wolf

 

[wsender]

Looks nice. Only thing I might suggest looking at it how many watts the potentiometer is rated for and to make sure you're not exceeding its maximum value.

 

[wolfareno]

It's a 25 ohm 3w pot, and I'm using a 7.5v 700ma (5.25w) wall bug, but I think it will be OK. From what I understand, the 3w rating on the potentiometer isn't necessarily the max load. The fan is 12v (not sure of the wattage).

 

[Boardndave]

Any chance you could put up a parts list? Thanks! Looks great.

 

[wolfareno]

I'll try -- I used some things I had laying around (the fan, the magnets, and the wall bug), but almost everything else came from the Rat Shack:
DC Power Jack type 'M'
Type 'M' power plug
8x6x3 Project Box
Illuminated Round Rocker Switch (note that this was the 125v one that stays lit all the time; you should find one that is 12v and only runs when the switch is "on")
25 Ohm, 3w potentiometer (they sell it as a potentiometer/rheostat -- It's technically a pot, I think)
For the fan I used, I needed 3" 8/32 bolts/nuts/washers to mount the fan.

Please keep in mind that I haven't even tested this design yet. I will get my stir bar and flask on Wednesday, and I will report back shortly after with my findings/adjustments.

Cheers,
Wolf

 

[carlisle_bob]

It's a 25 ohm 3w pot, and I'm using a 7.5v 700ma (5.25w) wall bug, but I think it will be OK. From what I understand, the 3w rating on the potentiometer isn't necessarily the max load. The fan is 12v (not sure of the wattage).

Hi

A pot actually has several ratings on it. The 3 W applies to the gizmo when it's set to 25 ohms and there is free air all around it. If it's set to 12.5 ohms it's likely only rated for 1.5 W. There's also a max current through the wiper rating, but you will not likely run into that in this application.

If the wall wart puts out 7.5 V, and the pot is 25 ohms, max power at full resistance would be 7.5 squared over 25 = 2.25 W. At half resistance you would be at 4.5 W. That all *assumes* there's no voltage on the fan at all. If the fan is turning, there's voltage on it...

Bob

 

[killsurfcity]

This is not really the right way to do this. Motors should not be driven direct from a power source like this if any controllable speed variation is desired. The best thing to do would be to purchase a cheap pulse width modulator circuit to drive the fan. I recently acquired a stir-plate built in the above fashion from a friend, and just couldn't get it to work. It just took off at full speed when turned on, and threw the bar. I added a PWM, and now I can start it slowly and dial in the exact speed I want.

 

[wolfareno]

This is not really the right way to do this. Motors should not be driven direct from a power source like this if any controllable speed variation is desired. The best thing to do would be to purchase a cheap pulse width modulator circuit to drive the fan. I recently acquired a stir-plate built in the above fashion from a friend, and just couldn't get it to work. It just took off at full speed when turned on, and threw the bar. I added a PWM, and now I can start it slowly and dial in the exact speed I want.

Thanks for your comment. I did adjust the magnet placement after I published this video, but the stir plate works great for me with a 1 3/8" bar. I've made 20+ starters with it so far, and it starts up and stays running great every time. I'm sure your method is great, but this one is working for me.