Troubleshooting DIY stir plate

[gedennis]

So I just finished wiring my stir plate together. I bought a new 3" 12VDC cooling fan from Radioshack. I used a 25 Ohm Rheostat along with a 30A - 12VDC rocker switch and an old 5V phone charger as my power source. Once I connected everything together and turned the rocker switch on my fan won't begin to spin until I turn the rheostat all the way up. Thus, I can only achieve one speed, which defeats the purpose of having a rheostat. I am certain I have wired it correctly and have no idea why I am not able to adjust the fan speed. Anyone know what could be causing this problem/how can I fix it?

 

[Poobah58]

I'm thinking the 5V power supply won't work on the 12V fan...

 

[grathan]

Well technically it does work wide open at 5v...

 

[k1ngl1ves]

Yep. I use a 6v charger on mine. The fan doesn't have enough power for variable speed when trying to spin a stir bar in a solution.

That's ok. 6v seems to be perfect for my 1.8 to 2L starters.

If you want variable speeds, get a 12v adapter and toss the 5v in the trash.

 

[gedennis]

Yep. I use a 6v charger on mine. The fan doesn't have enough power for variable speed when trying to spin a stir bar in a solution.

That's ok. 6v seems to be perfect for my 1.8 to 2L starters.

If you want variable speeds, get a 12v adapter and toss the 5v in the trash.

I ended up getting a 12V DC adapter and wired it in just the same. Now my problem is that it is causing it to spin too fast. I turn my power switch on and it goes very fast and I am able to make it go even faster with the potentiometer. Even at the lowest setting the stir bar gets thrown off when in solution. Anyway to fix this problem, i.e. make the fan not spin when the power switch is on and the potentiometer is at the lowest setting?

 

[GilaMinumBeer]

I ended up getting a 12V DC adapter and wired it in just the same. Now my problem is that it is causing it to spin too fast. I turn my power switch on and it goes very fast and I am able to make it go even faster with the potentiometer. Even at the lowest setting the stir bar gets thrown off when in solution. Anyway to fix this problem, i.e. make the fan not spin when the power switch is on and the potentiometer is at the lowest setting?

Do you keg?

Try creating a pivot around the bar using a poppet o-ring. I had the same problem. Bar would start up spinning so fast it would get thrown. Pivot point solved it.

Beyond that, use a 12 guage and you won't have trouble shooting your DIY stir plate.

 

[HumulusHead]

What if you got another reostat and wired it in sequence with the other?


Sent from myPhone

 

[petrolSpice]

Similar issue with mine... I use a 120mm 12V fan and with the 12V power supply and rheostat on lowest it will throw the bar. With a 6V power supply it will just barely spin on the lowest setting and will spin up to about 75%, beyond that it will throw the bar.

I need to rethink my magnet placement on the fan.

The surface the bar is spinning on plays a HUGE role. I can use the 12V supply up to max when I use a stainless pot. But when I use my flask I have to use the 6V power supply on a low setting.

 

[mainiac]

I recommend wiring up a small voltage regulator circuit using an LM317 . . . it will give you much better control over the voltage you are feeding to the fan. That's what I did with mine and it works great.

 

[GilaMinumBeer]

Similar issue with mine... I use a 120mm 12V fan and with the 12V power supply and rheostat on lowest it will throw the bar. With a 6V power supply it will just barely spin on the lowest setting and will spin up to about 75%, beyond that it will throw the bar.

I need to rethink my magnet placement on the fan.

If you don't have a pivot bar, try it first.

 

[petrolSpice]

If you don't have a pivot bar, try it first.

Mine have a pivot bar, I've also tried without. And I've tried multiple sizes of each. I've had the best result with a 50mm bar with a pivot.

Next I want to step up and get a rare earth magnet bar, but only after adjusting my fan magnets.

 

[raouliii]

I recommend wiring up a small voltage regulator circuit using an LM317 . . . it will give you much better control over the voltage you are feeding to the fan....

Have to agree there. The usable fan speed range for a stirplate may only be the result of 3 or 4 volts of difference in the supply voltage. The trick to stirplate design is to figure out what those min and max speeds are and create a circuit that only provides the min and max voltages necessary to the fan. Given all of the other variables in the design, from container shape, stirbar size, distance between stirbar and magnets, type and configuration of magnets, etc., the design of the voltage controller must have flexibility.

Using a single variable resistor/potentiometer/rheostat to control the speed of a DC fan is just about the least flexible option there is. Essentially, one is creating a simple voltage divider between the potentiometer and the resistance of the fan. Full range of the potentiometer will result in either full voltage at the fan for a 0 resistance or a small fraction of full voltage for a max resistance. The resistance of the potentiometer must be chosen very carefully based on power supply voltage and fan resistance. Unfortunately, potentiometers in the tens of ohms are very limited in selection. The addition of an appropriately sized fixed resistor in series with the potentiometer may help in establishing a usable voltage range. The end result oftentimes is a design with limited use of the potentiometer's range and essentially one speed.

A LM317 based voltage regulator can be designed fairly easily and with a little experimentation with resistor values, fixed and variable, can provide use of the entire range of the variable resistor. This solution is essentially unrelated to fan resistance and uses far more readily available resistor and potentiometer values. A power supply matched to the fan's spec'd requirement is usually all that is needed. This solution provides a wide measure of flexibility that can be used to make it compatible with the other variables mentioned earlier, as well.

A PWM design would also have very effective results.

 

[FuzzeWuzze]

I ended up getting a 12V DC adapter and wired it in just the same. Now my problem is that it is causing it to spin too fast. I turn my power switch on and it goes very fast and I am able to make it go even faster with the potentiometer. Even at the lowest setting the stir bar gets thrown off when in solution. Anyway to fix this problem, i.e. make the fan not spin when the power switch is on and the potentiometer is at the lowest setting?

The problem is 12V is just too much power...if you cant find a 9V power supply you could always just do a simple voltage divider with your 12V circuit with a 470 and 1k resistor, which should get you to ~8V, which should keep the fan's high end from being so ridiculous.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/voldiv.html

 

[jwa120]

I had a similar problem with the bar spinning too quickly, I replaced the potentiometer with a cheap PWM. I now have greater control on speed but the fan generates a high pitched hum.

http://www.amazon.com/gp/product/B003L4KKF2/ref=oh_details_o05_s00_i00?ie=UTF8&psc=1

 

[Riot]

Have you considered a variable voltage power supply? A lot of them have a 8-9 volt setting, should fix you right up.