USB splicing with 3 pin fan

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honkeypot

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Apr 16, 2015
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Hi there. So I'm building a stir plate for yeast activation by way of attaching a magnet to a Dynex Dx fan101 (http://www.amazon.com/Dynex-80mm-CPU-Cooling-DX-FAN101/dp/B004DWZFTC) that will attract another magnet in a flask that will spin the culture down (this generates constant aeration for microbes that will yield faster growth, see http://www.yeastir.com/#!buynow/cfvg)

I'd like to avoid using a DC power supply so my question is, can I simply cut open the red and black wires on the fan and splice them with a USB cable that I can plug into a phone charger?
 
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Someone Somewhere
I think a fan is going to spin too fast for this, and won't have enough torque.

These are pretty common in chemistry labs - you should be able to get one cheap off ebay that will work far better than any DIY stuff.

But yes, splicing the red and black wires into the +5V and GND wires (colour varies) of a USB cable will work, though at reduced torque and speed because it's running at 5V instead of 12V.
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I think a fan is going to spin too fast for this, and won't have enough torque.

These are pretty common in chemistry labs - you should be able to get one cheap off ebay that will work far better than any DIY stuff.

But yes, splicing the red and black wires into the +5V and GND wires (colour varies) of a USB cable will work, though at reduced torque and speed because it's running at 5V instead of 12V.
 
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Solution
Agree with above.

The fan design is based on only AIR RESISTANCE. It's actually impossible for us to know how much resistance you're adding.

I never, ever recommend someone modify a USB cable and attach it to a computer. It's not hard to damage the motherboard if you short a wire. I'm also not sure if you would draw too much current by overloading the fan, thus damaging the motherboard but unless there's some protection built into the fan somehow that's likely.

*So I suspect the fan might not even spin but could overload the circuit.

Also, the fan you chose has pretty crappy customer feedback.

I'm not sure what to recommend since you seem to be on a pretty tight budget, but I strongly suggest something independent of your PC, with a way to control speed. (On that point, I don't even think that fan was variable).

Summary:
I strongly suggest you ask a science teacher on this subject, but again don't try to whip up something yourself even with suggestions from others who only have peripheral knowledge or you might not be too happy with the results.

A laptop cooler MAY work but I'm just not certain on how much torque is added. Perhaps one with a PWM fan instead of voltage might be safer but we're getting out of my comfort zone.

At the very least use a surge supressor and again don't attach to a PC.
 
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Update:

$60 seems very reasonable for the kit you linked. Even if you bought a cheap fan (better than the one you linked) you still need:

1) proper magnets
2) case
3) power supply
4) Speed control (i.e. potentiometer for voltage control)

(You don't need a power supply with a pre-built USB model but I can't recommend this approach to anyone who isn't quite familiar with electronics. I studied electronics in school and still wouldn't risk my computer to an "ooops" moment.)

If you were pretty good with electronics AND had most of these parts available that's one thing but for what you're doing this seems a small price to pay.

There are several DESIGNS with pictures and complete part lists you can Google but they all require parts and tools to manufacture. Anyway, that's my two cents but I strongly advise against your initial approach.

*Here's a DESIGN you can show to someone who has some experience but I'm not comfortable discussing this further:
http://www.donosborn.com/homebrew/stir_plate.htm
 
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This is not going to be attached to a computer, it will be a stand alone device that will sit plugged into an outlet on a kitchen counter for a day at a time. Your reply clearly indicates you didn't bother reading my original post.
 
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Thanks someone. This system doesn't require a lot of torque, just enough to agitate and aerate the culture so I think this will work fine. Time bust out the soldering iron.
 
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eh, the torque of the fan will be almost irrelevant since there's a gap between the fan motor and the stirring piece inside the flask. if he's stirring rocks or air, the fan will just spin and the magnetic field will "slip".

but yeah, a fan is going to spin too fast, and at 5V some fans might not even start spinning. so, you might have 0rpm, or you might have 800rpm, depending on whether the fan even starts to spin at 5V.

find some other fan or just a plain small DC motor that is made to run at lower voltages and at lower rpms. do you really want the stirring piece inside the jar to be going at 800rpm? or are slower speeds like 100rpm more sensible?
 
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I'm not ask that concerned with the rpm because I'll be putting up to 2 liters of fluid in the flask so having some extra umph in that regard may come in handy.

I actually just stripped the wires and twisted them together. I used an old ipad charger to plug into the outlet and everything is working as expected. Though I haven't been able to solder it yet because the iron is at my girlfriend's place. I'll give it a whirl (lol) with 2 liters after soldering to see how she handles with more than a pint of fluid.
 
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i'm wondering - why not just use a normal hand-blender type of thing, like those little wire ones used to froth milk or eggs? the electric variant.

316YPD35SFL.jpg
 
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Then there's the thing where I'd have to sit there and physically hold it in the jar (or devise some way to suspend it) for 24-36 hours at a time.

Edit: this method would also raise concern for inoculating wild, unwanted microbes.
 
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