Stir Plate Help

[GrantH]

I have built my stir plate with what I thought was the correct pieces and I can not get things to work correctly.

I am using 2 (or 4) smaller rare earth magnets and can not seem to find my fault. I have the fan "sucking" air down, with the magnets stuck (no glue) to a washer (glued) on the top of the fan. I have 800ml of water and a 1 inch stir stick. I believe the magnets are 1/8 inch in size, the only ones radio shack had. I have 2 other larger magnets I am about to try as well.

The magnets need to be opposing polarity, correct?

 

[Greenbasterd]

the easiest way to tell if it going to work is to not turn the fan on, put your stir bar in and it should stick to where the magnets are. if it jumps away it not right. when it sticks turn the fan by hand and the peice should follow it.. if it follows its your fan speed that is the issue.. start really slow if you have a POT hooked in. as far as the magnets i thinki i heard they must be opposite but im not sure about that, i just used 1 hard drive magnet and had probs untill i realized it was fan speed

 

[GrantH]

I've got it all wired but can't get the stick to run to the middle and stay at all. I think I have tried every combo I can. Can someone please let me know exactly?

 

[GrantH]

My power supply says 12v - 1.5A. I'm thinking it's not strong enough. Anyone? Also, I just got a hard drive magnet that I don't know if it is strong enough or not as well. It is out of a newer slimmer dell that crapped out on us.

 

[fish4fun]

GrantH,

You should really listen to the advice Greenbasterd gave you:

its your fan speed that is the issue.. start really slow

I just built one the other day, and I had a ton of problems until I slowed it down. Most PC fans use BLDC (Brush-Less Direct Current) motors, these motors are typically rated in RPMs/Volt. Most BLDC fan motors for PCs are designed to operate ~12V, but the RPM @ 12V is generally way too high for a stir plate, so you need to lower the voltage, typically to between 3V and 8V. My particular one performs pretty well with a fixed 5V supply, BUT the stir bar HAS TO BE "stuck" to the magnets prior to turning the power on, and even then, it frequently takes "several tries" to get it going. I designed and built a PWM supply for mine, and by starting @ ~1.5V and gradually increasing it, I can get it up to ~8.5V before it "throws" the stir bar, but @ the end of the day, a simple 5V supply works very well on mine. I would be VERY SURPRISED if you ever got yours to work with a 12V supply w/o either a linear or PWM regulator. Try some old cell-phone charger adapters, they typically put out between 4V and 6V.

Fish

 

[GrantH]

I am using a cell phone charger. I will re-look at it and make sure the rating, I may have read it wrong. That being said, I got it working but it is SUPER sensitive. I believe I need a bit stronger magnets, but I also need to figure out how in the world to make it work IN the project box.

 

[thargrav]

Magnets are too weak - you need some that are at least 1/2" across.

Cellphone charger is likely 5 VDC unless it's a really old one then it might be 12 VDC.