Need to slow down stir plate - wee bit too fast

[mister704]

So I purchased a stir plate. Actually works fine but I want to slow it down because as of right now I have to start it fast than turn the rheostat down. If I start it slow, it throws the bar.

So this is the items breakdown

Power Supply
Motorola 12v - 2.5a power supply

Fan Info:
Dynatron (Top Motor)
P/N: DF129225SL, DF12922SL-3/4G
12V Brushless DC Cooling Fan
Size: 92x92x25mm Low Speed Sleeve Bearing
Bearing System: Sleeve Bearing
Rated Voltage: 12V
Input Current: 0.24A
Speed: 2200 RPM
Noise(dBA): 29.24 dBA
Connector: 3 & 4 Pin Connector
Safty: UL, CE, TUV, RoHS

Rheostat

25 ohm.

In-Line Resistor

33 ohms
extras = 22ohms and 100 ohms

Question

I went and wired up both of the others and they both had the same result. SO that means I am off to RadioShack tomorrow. I really don't know the math so if someone could point me in the right direction, I would greatly appreciate it. What size should i go buy tomorrow?

 

[the-crowing]

Are you asking what size resistor to purchase? If your stir bar is getting tossed I would guess the magnets are not positioned properly. Have you messed with the position of the magnets on the fan? Also, why are you looking to slow it down?

 

[max384]

I'm confused as to the problem. Why is it bad to start it fast and then slow it down??

 

[mister704]

Sorry let me provide a little more info.

So it is not off balance I believe. It works fine. The thing is I have had to add a small amount of distance between the flask and plate. Lets say something like a piece of cardboard. That was perfect.

But, if I have just the flask on the plate, it will throw it off.

As for the start fast and slow it down, when I slow it down, it is still way too fast. For instance, it is ok if I am making a full 2L starter but for something smaller like 1L...its creating a major vortex. From my readings, it is not necessary to have such a powerful vortex. Basically slowing the rheostat, really does not slow it much. Turning it all the way down, it still is spinning very quickly.

 

[Hello]

So even at the lowest setting, turning the fan down basically all the way, you're still thinking it is too fast?
It shouldn't be though because the power supply is obviously strong at 12v, but the fan should be able to slow down to a near crawl if you turn the rheostat down. If your bar is getting thrown then that is likely a magnet placement issue but if you can fix it by putting more distance between the magnet and the flask then that is good. The vortex definitely can only be fixed by turning it down. I'm not sure why it won't turn down all the way.

 

[max384]

Have you thought about using a lower voltage power supply? Instead of 12V, try a 5V USB, which are plentiful and cheap.

 

[mister704]

Have you thought about using a lower voltage power supply? Instead of 12V, try a 5V USB, which are plentiful and cheap.

I have but its already together and soldered in so I would really like to leave it.

I thought just getting a higher ohm resistor would solve the problem. I was thinking something like 320 ohms or something. The 100 doesn't really make a different so i was debating on going up a bit.

 

[augiedoggy]

you need to either lower the voltage or add a $5 Pwm controller... The more space between the magnet and stir bar the more likely it will throw the bar.

 

[the-crowing]

Adding a 100 ohm resistor in series with the rheostat and fan should drop the fan voltage to around 3.5V which should slow the fan down significantly. Are you positive everything is wired in series? Wiring in parallel would not cause a change in speed

Sent from my SCH-I535 using Home Brew mobile app

 

[mister704]

Adding a 100 ohm resistor in series with the rheostat and fan should drop the fan voltage to around 3.5V which should slow the fan down significantly. Are you positive everything is wired in series? Wiring in parallel would not cause a change in speed

Sent from my SCH-I535 using Home Brew mobile app

So this it how it is wired currently. I am in no way a electric guy so it could be wired funny. Please let me know.

 

[mister704]

So after doing a little reading myself???

Why would you put a resistor between the power to rheostat and not between the rheostat and fan. Isn't in the wrong place?? Like shouldn't it be where that red connector is in the top right of the 3rd photo??

 

[max384]

I'm no expert, but I don't think it should matter. One way may be more correct than the other, but as long as it is wired in series, it shouldn't matter where on the line it is. Yours is wired in series.

 

[mister704]

I'm no expert, but I don't think it should matter. One way may be more correct than the other, but as long as it is wired in series, it shouldn't matter where on the line it is. Yours is wired in series.

Thanks...I am going to go purchase some more 100 ohm resistors. Then I can just twist them together until I get the proper speed. Unless my power supply is lying to me then I would of figured the 100 would of been enough.

But we will see.

 

[BBL_Brewer]

Just be careful about dropping the voltage too much. You really need to be able to vary the voltage. You might need a higher voltage to get the fan started and then a lower voltage to drop the RPM's and get the spin you're looking for. If I try to start my stir plates with the voltage that I spin with, they won't start up. I tried the potentiometer / resistor thing when I built my first stir plate. I got it to work, but I was never really happy with it. I ended up building some new ones with a variable voltage regulator (basically an LM317). It was a little trickier to wire, but man, they're Cadillacs in comparison. Full range adjustable from 0-12 volts and the parts were pretty economical.

 

[mister704]

Screw this...I am pretty sure its the magnet. Its one from a old harddrive. I now have to find a magnet somewhere that has the strength.

 

[mister704]

Confirmed...it was the magnet. It was out some old harddrive. Took the entire setup to my local hobby store and they hooked me up with 2 MagCraft Super Strong batteries and I am up and running perfect.