DIY stir plate help

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foxtrot

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Need some help from those that are electronically inclined...

I am in the midst of building a stir plate and the problem I'm having is too fast of a fan. Here's my components:

12V / 2.8W 92mm fan
Magnet from hard drive, centered on fan hub
9V, 300mA AC to DC wall adapter
25 ohm, 3W pot
(got the plans from onebeer.net)

I'm trying to spin a 2" stir bar in a 2L flask. The bar starts spinning, then spins out as the fan revs up. The pot does not slow it down enough. I'm not into soldering and don't have much experience w/ electronics, so I'd like a "simple" solution.

Here are two ideas I have:

1) reduce the input voltage by getting a universal wall adapter that adjusts from 1.5 to 12V, in 1.5V increments (btw, these come in various mA ratings- as long as it exceeds the fan, I'm good, right?)
-or-
2) get a fan speed controller

Which method would work better or should I be trying something else???

Many thanks in advance!
 
Between the two I would buy a fan controller, it will cost about the same as that adjustable wall wart.
 
I built my 2 Liter stirplate from the same onebeer.net. I tried using 2in, 1.5in and 1in stirbar. The 1.5 in. bar was the only one that wouldn't throw off in regular tap water. Also, keep in mind that water + dme mixture will cause more resistance on stir bar so it might slow the 2 in stir bar enough so it won't throw it. I don't know cuz the 1.5 in. worked fine and I was just tired of dicking around.
Make sure to start it off slow and gradually ramp up speed, too.

Works wonders when it works. Good luck!
 
Yeah, I think I'm going to try a fan speed controller; I believe a slow ramp up in speed is one of the keys to getting this to work. I wonder if I should run the speed controller down stream from the pot? for additional control?
 
foxtrot said:
Yeah, I think I'm going to try a fan speed controller; I believe a slow ramp up in speed is one of the keys to getting this to work. I wonder if I should run the speed controller down stream from the pot? for additional control?

Remove the pot from the circuit. The fan controller will give you all the control you need.
 
foxtrot said:
Need some help from those that are electronically inclined...

I am in the midst of building a stir plate and the problem I'm having is too fast of a fan. Here's my components:

12V / 2.8W 92mm fan
Magnet from hard drive, centered on fan hub
9V, 300mA AC to DC wall adapter
25 ohm, 3W pot
(got the plans from onebeer.net)

I'm trying to spin a 2" stir bar in a 2L flask. The bar starts spinning, then spins out as the fan revs up. The pot does not slow it down enough. I'm not into soldering and don't have much experience w/ electronics, so I'd like a "simple" solution.

Here are two ideas I have:

1) reduce the input voltage by getting a universal wall adapter that adjusts from 1.5 to 12V, in 1.5V increments (btw, these come in various mA ratings- as long as it exceeds the fan, I'm good, right?)
-or-
2) get a fan speed controller

Which method would work better or should I be trying something else???

Many thanks in advance!

I think you mean 25K ohm pot...

Another angle to this is to mess with the ratio of impedence between the potentiometer and the resistor that connects the pot to the output from the voltage regulator to the fan. The amount of power transferred to the fan has something to do with the ratio of the impedence rating of those two resistors. So for instance, if you get a 10K pot and the resistor is 1K, your ratio would be 10/1. That should have a shallower range of adjustment and therefore finer tuning of adjustment than a 25/1 setup. Maybe even try a 5K pot if it's still not good enough. Either that, or increase the resistor to 2K or 5K if you want to keep the pot.

I'm no electrical engineer, but when I put my stir plate together I had my buddy from high school who got his degree in electronics help me out, and this is what I remember from our messing around with it.
 
Since his problem is he cant slow down the fan enough I am inclined to think the Pot is very small. Unless he has it wired wrong. If you put another resistor in series with the pot it will do two things. A. Give a finer resolution of adjustment and B. Limit the maximum speed of the fan.

This is how it should be wired. +12 side of the power supply goes to one end of the pot. If you have an extra resistor it would goto the resistor first and then from the resistor to one side of the pot. Now wire the + side of the fan to the wiper arm of the pot. That would be the pin with the arrow pointing to the squiggly line. Then wire the negative side of the fan to the negative terminal of the power supply.


If you want to add a soft start put a 500k resistor and a big 20uf cap in parallel with the fan. This will give you a 10 second ramp up to speed.

I'll get some schematics up tonight, ASCII art isnt working out well

*editied to give specific values
 
Virtuous said:
If you want to add a soft start put a 500k resistor and a big 20uf cap in parallel with the fan. This will give you a 10 second ramp up to speed.
You could achieve a soft-start behavior with a resistor and capacitor, but not in the way you have described, and not with such a small capacitor... perhaps you can clarify what you are suggesting.

As for the original poster, I would suggest that if you don't want to build a voltage regulator circuit like the one Yuri's linked in his signature, the best bet is a PC fan controller. You can find them for like 5 bucks online. Some of them are active, and some of them are nothing but a potentiometer, but at least they're of the right value to properly control a PC fan - if you're looking to avoid messing with electronics it's a much easier plug-and-play solution, and still affordable.
 
T=RC. 500,000 ohms * 0.000020 farad = 10 seconds.

Yeah the description sucks, a schematics is worth a million words. I'll get one tonight
 
Virtuous said:
T=RC. 500,000 ohms * 0.000020 farad = 10 seconds.

Yeah the description sucks, a schematics is worth a million words. I'll get one tonight
Yes, I know how to calculate the time constant of an RC circuit, but that's not relevant to what you're proposing. There is no arrangement in which you can connect those components to have it do what you say. If you connect the resistor and capacitor in parallel with the fan, the resistor will do essentially nothing, and the capacitor will take only a fraction of a second to charge up from the relatively low-impedance DC power supply you are running it from, so it will hardly impact the behavior of the fan at all. If you had them in series without a fan, the capacitor voltage would in fact rise with a time constant of 10 seconds as you say, however if you connect the fan in there it all goes out the window - unless you're adding an active buffer in there or something, in which case it's no longer such a simple circuit.

You could achieve it with a resistor and capacitor alone, but the resistor would have to be much smaller, and be in series, and the capacitor would have to be huge. Adding a transistor would make it much more manageable, but then it wouldn't be as simple a circuit.
 
Wow, looks like I stirred up some serious discussion! (no pun intended)

As much as I love to tinker around with things (which is why I brew in the first place), I would rather go the plug and play route in interest of my time and possible frustration. So is this the type of fan contoller you guys a talking about:

http://www.newegg.com/Product/Product.aspx?Item=N82E16811998129

It mentions a DC input of 12V +/- 10%. My wall adapter is 9V. Is this OK? And will the low end of 5V on the controller get me to a slow enough start? Guess I would have to try it out for $7 + shipping.
 
foxtrot said:
Wow, looks like I stirred up some serious discussion! (no pun intended)

As much as I love to tinker around with things (which is why I brew in the first place), I would rather go the plug and play route in interest of my time and possible frustration. So is this the type of fan contoller you guys a talking about:

http://www.newegg.com/Product/Product.aspx?Item=N82E16811998129

It mentions a DC input of 12V +/- 10%. My wall adapter is 9V. Is this OK? And will the low end of 5V on the controller get me to a slow enough start? Guess I would have to try it out for $7 + shipping.

I am feeling the same way you are... let us know how this works, becasue I am thinking about going the same route...
 
That's the general idea, but based on the reviews of the product you linked, I wouldn't go with that one in particular ;)
There are a number of other options at newegg alone, that look like more reputable products: http://www.newegg.com/Product/Product.aspx?Item=N82E16835118217
http://www.newegg.com/Product/ShowI...a=0&Description=Sunbeam+FC+PCI+Fan+Controller

Hard to say if they'd work on 9v - I would guess that they would, but finding a 12v adapter isn't too hard if they don't.

With my stir plate, 5v gives me a pretty decent stir speed, and it's low enough that the fan doesn't have enough torque to spin up that quickly when the stir bar is in place above it.
 
Yes, I know how to calculate the time constant of an RC circuit, but that's not relevant to what you're proposing. There is no arrangement in which you can connect those components to have it do what you say. If you connect the resistor and capacitor in parallel with the fan, the resistor will do essentially nothing, and the capacitor will take only a fraction of a second to charge up from the relatively low-impedance DC power supply you are running it from, so it will hardly impact the behavior of the fan at all. If you had them in series without a fan, the capacitor voltage would in fact rise with a time constant of 10 seconds as you say, however if you connect the fan in there it all goes out the window - unless you're adding an active buffer in there or something, in which case it's no longer such a simple circuit.

You could achieve it with a resistor and capacitor alone, but the resistor would have to be much smaller, and be in series, and the capacitor would have to be huge. Adding a transistor would make it much more manageable, but then it wouldn't be as simple a circuit.


Yeah, I hear yah. I was in a rush at work and really couldnt think it through with a bunch of people bugging me. So anyways. Here is what I was thinking but you're right. Add the load of the fan and it is all shot
fanschem.jpg


I would rather do it with a transistor or fet as well but that gets to be a little hard to explain,
 
ok- silly idea here......

i'm thinking of trying something a little different. i'm going to try to take an old multispeed blender i have, and attatching the rare earth magnets to the spinning blade on the blender, and sitting the flask directly on top of the blender.

i know this would work, but my only question is, if the lowest speed on the blender would be too quick. i suppose if it is, i could always just lower the speed altogether on the blender, but if it works, this would be cheaper to me, and require much less fabrication, and would still accomplish the same results i think.

i'll try it out, and let you all know how it works, unless someone else on here already has tried this, or sees any major problems i'm overlooking.


brian
 
OK, so I got one of these:

http://www.newegg.com/Product/Product.aspx?Item=N82E16835118217

Tried it with my 9V adapter. At full tilt, the fan spins too fast to hold the stir bar (no surprise). As I turn the speed dial down, the fan slows to a crawl fairly quickly and the fan speed begins "stuttering". At this point, I begin to detect a burnt smell from the controller. Thinking the controller requires 12V, I tried a 12V adapter and the exact same thing happens. Argh!!! I even tried it down-stream from a 25K ohm pot, but the speed still ramps up too fast. Not sure what to do at this point. I think I might try some resistors?

On the positive side, I made two, 2L starters (sans-stir plate) for the first time in my brewing life using a SF lager and a Cali Ale yeast. They were acitve for about 3 days. Holy cow, the primaries took off like mad overnight! I'm sure a stir plate will come in handy eventually, though when I start brewing the heavy weights or lagers.

Until the next update...
 
that's funny I bought the same item from Frys Electronics and mine burnt out too from a 12v adapter. (Although I probably hooked it up the wrong way at some point which might have been why it burnt out.)... >.<
 
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