Stirplate - 12v too much, 6v won't turn it on???

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Kayos

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I put together a stir plate last night. Have tried 2 different 6v power adapters (cell phone chargers) and they won't turn the 80m comp. fan even when it's straight from the wall. 12v turns it fine. I hooked it all up and it's wayyy to fast. Dialed all the way down and the vortex is much too high.

Why won't the 6v charger do anything? Doesn't even sound like it's getting juice.
 
With the 12v can you just add a resistor? (I'm an accountant, not an electrical engineer so sorry if this is a dumb question.)
 
Pick up a variable wall-wart power supply from wally world. They are cheap, and have a 7.5v position. That should work.
 
you can pick up a rheostat from radio shack. its an adjuster that you can turn the knob and it'll do the job for ya. you'll just have to in-case the whole thing to make everything easier
 
Are your magnets right on the fan?? mine would not turn when the magnets were glued to the fan. I put a 1/2 inch spacer between the magnets and the fan and worked great at 6v.
 
Have you swapped the polarities and tried it? That was a nasty little surprise I had when I soldered mine together drunkenly at 1am. I got mad and went to bed. In the morning I swapped the wires and all was good. Honestly, who figured? I'm just covering the very basics first.
 
Are your magnets right on the fan?? mine would not turn when the magnets were glued to the fan. I put a 1/2 inch spacer between the magnets and the fan and worked great at 6v.

Not sure this is your issue but I too had to put a spacer between the fan and the magnet...damn powerful little thing wouldn't let the fan spin!
 
OK...got it. Thanks for all the replies. I ended up with 9v. Works good, but still too powerful unless it's at least 1.5L. I do have a rheostat, all the way turned down.
 
OK...got it. Thanks for all the replies. I ended up with 9v. Works good, but still too powerful unless it's at least 1.5L. I do have a rheostat, all the way turned down.

Then somethings wrong with your wiring. A rheostat turned all the way down shouldn't spin the fan regardless of the plug. That's why I was asking for your instructions before, or at least a wiring diagram.

Edit: oops, sawdustguy beat me to it.
 
OK...got it. Thanks for all the replies. I ended up with 9v. Works good, but still too powerful unless it's at least 1.5L. I do have a rheostat, all the way turned down.

Also what is the value range of your rheostat? I couldn't find exact one in stirstarters.com's build notes, but was able to drop an additional resistor across two legs to get it close enough.

If the value is too large, you'll just have on/off, but since you only have full on, yes, it's wired wrong.
 
I was having the same problem, putting the spacer in for the magnet did the trick. I used a lid from a film canister.
I have a question too though.
I have mine wired using a 6v power supply and a 25 ohm 3 watt rheostat from RadioShack. http://www.radioshack.com/product/index.jsp?productId=2062299
I tried using a 12v power supply but the rheostat got really hot so I switched to the 6v. If I want to switch out the rheostat so I can use the 12v supply what should I get so it won't over heat?
 
I was having the same problem, putting the spacer in for the magnet did the trick. I used a lid from a film canister.
I have a question too though.
I have mine wired using a 6v power supply and a 25 ohm 3 watt rheostat from RadioShack. http://www.radioshack.com/product/index.jsp?productId=2062299
I tried using a 12v power supply but the rheostat got really hot so I switched to the 6v. If I want to switch out the rheostat so I can use the 12v supply what should I get so it won't over heat?

Rheostats/potentiometers (and resistors more generally) work by converting electricity into heat. Your current rheostat is rated for three watts, which appears to be enough for 6v but not for 12v. In order to give you a specific part recommendation, I'd have to know more about your setup. What are the specifications of your fan, and how do you have it wired?
 
The fan I have is 12v, 0.25A
I have the ground from the power supply and the ground from the fan hooked up to ground on the pot, the power from the fan hooked up to the center post on the pot (variable voltage?)
and the power from the power supply hooked up to the other
Does that make sense?
The power supply I want to use is 12v 500mA.
The reason I want to do this is to get more speed, so if you don't think it would help I won't bother.
 
Forgive the crudeness of my hastily made MS-paint schematic. But this is the method I employed in making mine, the rheostat is wired in a "volume control" configuration. Tying terminals 1 and 2 together insures that as the rheostat wears you will get the maximum fine-control out of it possible. Obviously put your control at either of the spots marked "SW."
164363_10150147485878313_576643312_7508359_7833881_n.jpg
 
here is how I wired mine up

using the 3 watt 25ohm dealybob mine will go from a crawl at lowest setting to warp speed ahead at fastest setting

EDIT:
25 ohm 3 watt Rheostat (aka Potentiometer): Part number 271-265 - $3.99

dxQig.jpg


-=Jason=-
 
Wow, how this thread grew. I followed the cheap and easy stir plate in the DIY forum sticky from Anthony Lopez to the letter. 25ohm pot. I figured out it was wire wrong, then wired it correctly and it spun wayyy too fast with 12v. I put a 9v on it and it eemed still too fast. I used it anyway and ended up turning up the pot all the way because of how much more viscous the starter is vs. the water that I tested it with. Now I want to re-wire it with the 12v. - I get plenty of spin with the 9v, but the 12v wouldn't have to be all the way up. I am sure it's wired right now, but it does spin slowly with the pot all the way down.
***Same pot as the guy above****
 
Forgive the crudeness of my hastily made MS-paint schematic. But this is the method I employed in making mine, the rheostat is wired in a "volume control" configuration. Tying terminals 1 and 2 together insures that as the rheostat wears you will get the maximum fine-control out of it possible. Obviously put your control at either of the spots marked "SW."
164363_10150147485878313_576643312_7508359_7833881_n.jpg

Your wiring diagram is wrong. You show the pot shorted out. It should be wired like this:

control.jpg
 
Kayos what I did was wire mine using one of those female adapters so that I can change out my plugs from 6v, 9v, 12v or 18v I currently have a 18v hooked up for testing. but my cord just unplugs very easily.

I too my cord into radio shack and said I need the female dealybob that this plugs into.

http://www.radioshack.com/product/index.jsp?productId=2102489

but make sure you get the right size.. they have different sizes. I had a power supply that was to large to fit into my Size N Panel-Mount Coaxial DC Power Jack
pRS1C-2264647w345.jpg


-=Jason=-
 
Even at 1 ohm it should barely turn.

Knowing that my fan pulls .5a nominal and is made to run at 12v, you can figure out that it offers 24 ohms of resistance while turning in a perfect world. Resistance in a series circuit is the sum of all resistors so that makes total resistance 25ohm on high and 49 ohm on low. With our power supply constant you can now solve for amperage through each circuit. .48a on high and .24a on low. With this you can now calculate the voltage drops. The motor consumes 11.52v on high and only gets 6v on low.

Of course when applied this should mean that it will spin at roughly 95% of it's full speed on high and 50% on low. I assure you mine does exactly this.

You can go on to argue rpm drop due to friction or that fans rated at .3a will be affected differently, but mathematically a 25ohm variable resistor works quite well for accurate speed control of a 12v fan.
 
Wow, how this thread grew. I followed the cheap and easy stir plate in the DIY forum sticky from Anthony Lopez to the letter. 25ohm pot. I figured out it was wire wrong, then wired it correctly and it spun wayyy too fast with 12v. I put a 9v on it and it eemed still too fast. I used it anyway and ended up turning up the pot all the way because of how much more viscous the starter is vs. the water that I tested it with. Now I want to re-wire it with the 12v. - I get plenty of spin with the 9v, but the 12v wouldn't have to be all the way up. I am sure it's wired right now, but it does spin slowly with the pot all the way down.
***Same pot as the guy above****

I just looked at the Anthony Lopez build, and now understand why everybody has 25ohm pots. :D The trouble is, he specifies part numbers for most components, and then just says "computer fan". Different fans have different RPMs, different current draws, etc. Digikey.com stocks 2400 different computer fans, and 1300 of them are 12v. Add in variation in magnets, power supplies, and mounting techniques and it gets complicated. 1-25ohms is a fairly narrow band for a potentiometer, and there are a lot of assumptions that go into getting that band to map properly from the right speed and torque for this application.

I'm certainly no expert with this stuff, but I suspect that a more robust build would use a voltage regulator like an LM317. I'll see if I can dig up schematic tomorrow, but by manipulating voltage directly (rather than using variable resistance to manipulate voltage) is that you don't need to make nearly as many assumptions about the power supply and the fan.
 
Knowing that my fan pulls .5a nominal and is made to run at 12v, you can figure out that it offers 24 ohms of resistance while turning in a perfect world. Resistance in a series circuit is the sum of all resistors so that makes total resistance 25ohm on high and 49 ohm on low. With our power supply constant you can now solve for amperage through each circuit. .48a on high and .24a on low. With this you can now calculate the voltage drops. The motor consumes 11.52v on high and only gets 6v on low.

Of course when applied this should mean that it will spin at roughly 95% of it's full speed on high and 50% on low. I assure you mine does exactly this.

You can go on to argue rpm drop due to friction or that fans rated at .3a will be affected differently, but mathematically a 25ohm variable resistor works quite well for accurate speed control of a 12v fan.


Good post man. You've got a better grip on the fundamentals than I do. Clearly this design works with some fans, but to the extent that I understand the numbers involved here, aren't you showing that this pot would really work only with 500mA fans? Otherwise, your voltage drops wouldn't map to the 1-24 ohm range properly.
 
Knowing that my fan pulls .5a nominal and is made to run at 12v, you can figure out that it offers 24 ohms of resistance while turning in a perfect world. Resistance in a series circuit is the sum of all resistors so that makes total resistance 25ohm on high and 49 ohm on low. With our power supply constant you can now solve for amperage through each circuit. .48a on high and .24a on low. With this you can now calculate the voltage drops. The motor consumes 11.52v on high and only gets 6v on low.

Of course when applied this should mean that it will spin at roughly 95% of it's full speed on high and 50% on low. I assure you mine does exactly this.

You can go on to argue rpm drop due to friction or that fans rated at .3a will be affected differently, but mathematically a 25ohm variable resistor works quite well for accurate speed control of a 12v fan.

Your origonal post was:

Depends on the rating of the pot. I know a lot of us were using 1-25 ohms

My reply was:

Even at 1 ohm it should barely turn.


If you have it wired as shown below:

control.jpg


When the pot is turned to the low end (toward the bottom of the diagram) to 1 ohm the fan will not be turning very quickly no matter what you do. When the pot is set to one ohm the voltage across the motor will only be 0.48 volts. Even flomaster agrees.

using the 3 watt 25ohm dealybob mine will go from a crawl at lowest setting to warp speed ahead at fastest setting

-=Jason=-
 
I just looked at the Anthony Lopez build, and now understand why everybody has 25ohm pots. :D The trouble is, he specifies part numbers for most components, and then just says "computer fan". Different fans have different RPMs, different current draws, etc. Digikey.com stocks 2400 different computer fans, and 1300 of them are 12v. Add in variation in magnets, power supplies, and mounting techniques and it gets complicated. 1-25ohms is a fairly narrow band for a potentiometer, and there are a lot of assumptions that go into getting that band to map properly from the right speed and torque for this application.

I'm certainly no expert with this stuff, but I suspect that a more robust build would use a voltage regulator like an LM317. I'll see if I can dig up schematic tomorrow, but by manipulating voltage directly (rather than using variable resistance to manipulate voltage) is that you don't need to make nearly as many assumptions about the power supply and the fan.

Most of us NEED this site because we aren't in the electronics field and have no idea how this stuff works or is even figured out. That's why the DIY's posts are SOOOOO helpful. I never would have been able to build the stirplate without an exact step by step process. I have no idea how the ohms on a pot are figured. I will help out everywhere I can, but I need someone like you guys to help me with the electronics stuff and just tell me where to solder and to what. I had to do it just as the build suggested because I have no idea how much of a difference a 25 ohm pot is vs a 25K ohm pot or where you would use the two. Fact is, I really don't care. I just want a stirplate that works. It saved me $40. Got a little education along the way as well. Bonus!

P.S. My fan is 12v - not sure if that matters or not ;)

Thanks for all the help guys, and thanks for dealing with the ignorant one (mirror :D)
 
I see that my picture was not pulled though I deleted it from FB account it was linked to. Fantastic. Yes, my diagram was wrong. Yes I will re-do the math for .3a fans in the morning. Was I drunk before? Not nearly as I am now! We-he-hee. Seriously. I apologize, I try to make my info as accurate as possible, but the blondest of us slip up and I will try to repost tomorrow post brew. Perhaps the HR ladies are right; The jack of all trades is a master of none ;-)
(A test light will go a damn long way though!)
 
Most of us NEED this site because we aren't in the electronics field and have no idea how this stuff works or is even figured out. That's why the DIY's posts are SOOOOO helpful. I never would have been able to build the stirplate without an exact step by step process. I have no idea how the ohms on a pot are figured. I will help out everywhere I can, but I need someone like you guys to help me with the electronics stuff and just tell me where to solder and to what. I had to do it just as the build suggested because I have no idea how much of a difference a 25 ohm pot is vs a 25K ohm pot or where you would use the two. Fact is, I really don't care. I just want a stirplate that works. It saved me $40. Got a little education along the way as well. Bonus!

P.S. My fan is 12v - not sure if that matters or not ;)

Thanks for all the help guys, and thanks for dealing with the ignorant one (mirror :D)

Sure thing! :mug: And I got no gripes about any of that. I hope that post didn't come off as impatient; I just kept seeing all these different people with different problems, nobody was posting enough detail to help them, and the only thing in common seemed to be that everyone had a 1-25ohm pot.

Your 6V charger isn't working because it's not giving your fan enough juice. Computer fans are typically built on motors that will spin at a fixed RPM for a fixed voltage. If you bump the voltage up, it will spin faster, and if you bump the voltage down, it will spin slower. It's not so simple, though, that you can say 0V = zero speed, 6V = 50% speed, and 12V = 100% speed. All fans have an operating voltage range, and will only spin at all if given power above a certain threshold. A potentiometer works by introducing variable resistance to the circuit, which causes a voltage drop, which in turn causes the fan to spin slower than its maximum speed.

The problem is that you know how much of a voltage drop you want, but you can't know how much of a voltage drop you're going to get from any particular resistor without knowing how much current your fan draws. This should be listed on the label...something in the ballpark of 0.5A (or 500mA). So a 1-24ohm pot will cause one range of voltage drops for a 0.5A fan, and another range for a 1A fan, and yet another range for a 0.25A fan.

There are a couple solutions, but first you want to make sure that your device is wired properly. You say that it's much too high, even at the lowest setting. This probably just means that you've got a relatively powerful fan, and that even 25ohms of resistance isn't enough to get you down to a workable range. Just to make sure you're wired right, though: does the speed of the fan change at all when you adjust your potentiometer?

So solutions:
1) go to radioshack and buy a relatively large range potentiometer (say, 0-200ohms, rated for 10W) and know that this will work for 95% of fans. The down side to this is that you'll have a very narrow "sweet spot" of useful range on your potentiometer, so you'd have to be gentle with it to find a useable setting.

2) build something more like this: https://www.homebrewtalk.com/f11/my-homebrew-stirplate-8850/ It looks more complicated than it is, but this design has the added benefit that it will work with pretty much any 12V fan.

3) post more detailed specs of your fan (particularly current draw in amps or nominal power usage in watts) or a part number and we can speculate about what kind of pot would work better for you. It would be only speculation, though, because the addition of those magnets means that your fan isn't behaving as described in its datasheet anymore.

...and let us know how it goes.

@dzlater - if you are happy with your speed range, you could get the same 1-25ohm pot but just make sure it has a higher rating than 3W. 6W or 10W should be fine.
 
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
@dzlater - if you are happy with your speed range, you could get the same 1-25ohm pot but just make sure it has a higher rating than 3W. 6W or 10W should be fine.

Thanks this stuff gets pretty confusing for those of us who are electronically challenged.
 
...and let us know how it goes.

I think I'm happy with it now. Thanks for the education, if I build another one, I'll definitely do it a but different. I am going to hook up the 12v plug again to so I don't have to have the pot cranked all the way up the whole time. Mybe Add a bit bigger pt, also. What size do you think would work better with the Anthony Lopez setup?
 
I think I'm happy with it now. Thanks for the education, if I build another one, I'll definitely do it a but different. I am going to hook up the 12v plug again to so I don't have to have the pot cranked all the way up the whole time. Mybe Add a bit bigger pt, also. What size do you think would work better with the Anthony Lopez setup?

The whole point of my last post was to explain why I have absolutely no way of answering that question without knowing the answers to the questions I asked you.
 
OK, no prob. I have no intention of taking the whole thing apart to find out the fan info. I did splice both the 12v and 9v wires together and now can plug in either one I choose depending on the size and viscosity of the starter.
 
I built my stir plate with a cigar box, fan, two hard drive mags, and 9V power supply. Don't have a pot on it yet. When i tested it with a 1" stir bar in a 4 cup (1 L) measuring cup, it was perfect. vortex all the way tot he bottom. Now though I'm doing the maiden voyage with a 1.5 L starter (this is the second step) in a 2L flask. The 1" bar spins it but the vortex doesn't go very far down, still getting lots of good stirring though. I tried the 2" bar and couldn't get it to spin. The liquid is not clear so I couldn't see what it was doing, but looked like no spin.

Should I try putting a 12V power adapter back on? When I used the 12V in my testing with water in a measuring cup it was too fast and wouldn't spin.
 
Update, using the 12 volt vs the 9 volt it throws the 1" and 2" ball every time. So I guess I need a 12 volt power with a pot to find that sweet spot between 9 and 12. Before I do that though I will stack a third hard drive mag on top of the other two to see if more mag power and making the mags closer to the lid will let me use the 12v as is. Thinking maybe with more magnetic power the stir bars will stick with 12 volts.
 
Well I added a fourth magnet and now the 9 and 12 volt work great with the 1" bar but the 2" bar still gets thrown every time. So I'll just stick to the 1" bar and use either the 9 or 12 depending on the speed I need. Only thing I had to purchase for this project is the stir bars, had everything else!
 
When I built my stir plate I had similar issues with 12 volts kicking off my stir bar. Fortunately I picked up a universal 300mA power adapter from RadioShack that allows me to adjust the voltage from 1.5V to 12V. I usually set it from 6V - 7.5V, depending on how large my starter is, and by turning the potentiometer on high to get the stir bar started and dialing it back down, I can adjust the vortex however I like. Runs for 2 days at a time with no problems. One of the rare instances where I've had a bit of foresight :eek: . This is the one I use:

http://www.radioshack.com/product/index.jsp?productId=3875402

I tied a knot in the cable just inside the project enclosure to prevent any wires from being pulled loose while moving it around and it seems to work well.
 
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