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

[Kayos]

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.

 

[MalFet]

What voltage is the fan rated for? And what instructions are you following?

 

[michur]

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.)

 

[Cpt_Kirks]

Pick up a variable wall-wart power supply from wally world. They are cheap, and have a 7.5v position. That should work.

 

[ryan0914]

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

 

[sakeyfour]

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.

 

[tjpfeister]

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.

 

[woodstone]

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!

 

[Kayos]

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.

 

[Sawdustguy]

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.

If when it's turned down all the way the motor still turns you have it wired wrong.

 

[MalFet]

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.

 

[tjpfeister]

If when it's turned down all the way the motor still turns you have it wired wrong.

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

 

[IrregularPulse]

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.

 

[MalFet]

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

True true, but if that's the case, he's not wired wrong but he's got the wrong components.

 

[user 19629]

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?

 

[MalFet]

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?

 

[user 19629]

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.

 

[tjpfeister]

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."

 

[Sawdustguy]

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

Even at 1 ohm it should barely turn.

 

[Flomaster]

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

-=Jason=-

 

[Kayos]

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****

 

[Sawdustguy]

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."

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

 

[Flomaster]

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

-=Jason=-

 

[tjpfeister]

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.

 

[tjpfeister]

Your wiring diagram is wrong.

You are correct. My apologies. Here, look at this instead of my scribblings(I was wrong about the volume thing too. I did wire it as variable resistance):

 

[MalFet]

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. 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.

 

[MalFet]

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.

 

[Sawdustguy]

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:

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=-

 

[Kayos]

I just looked at the Anthony Lopez build, and now understand why everybody has 25ohm pots. 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 )

 

[tjpfeister]

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!)