DIY Stir Plate question. PWM? LM317? Washer?

[Fastforwardx2]

Hello,
I am currently in the process of making a stir plate and after going through a lot of forums I am clueless with these three questions: 1) Does a PWM really provide that great of a benefit that it warrants using one? 2) LM317 circuit. What is the benefit of using this circuit over a simple power-fan-variable resistor circuit? 3) Why do people use washers to mount their magnets to the fan? Do the magnets interfere with the fan's motor?

Thanks you for any help you can provide.
-John-

 

[Fastforwardx2]

ok so I think I see why you want a LM317 circuit, because it is less wear on the hardware. At least that is what I was getting out of what I read.

 

[raouliii]

LM317 or PWM circuits provide stable voltage regulation. The usable range of fan speed / operating voltage can be determined and the circuits designed to provide the necessary voltage swing to the fan. This results in finer voltage adjustment and more usable potentiometer swing.

A potentiometer in series with the fan, producing a simple voltage division, can't come close to that flexibility.

A washer helps to magnetically decouple the fan from the magnets. I epoxied a can lid to my fan and marked concentric circles on it while spinning. This allows for easy magnet alignment when experimenting with magnet placement.

 

[Fastforwardx2]

Thank you raouliii. I think for now I will use a potentiometers in series for now and then when I can figure out the circuit for stable voltage regulation I will be able to compare the two and this allows me to build this stir plate now instead of later.

For the washer could I use a PVC pipe for stand off and to glue the magnets on to it? If I understand what you are saying this should be fine.

Well I will get this stir plate built and let you know how it goes. Thanks for the help.

 

[yukonhijack]

Like some others, I built a stir plate using the fan, pot, and hard drive magnet. It never really worked the way I wanted it to (magnet was not very strong causing the bar to get thrown). So I ended up going with a Thermaltake II usb powered fan that comes with its own pot/speed controller, and some 1 inch diameter rare earth magnets from Amazon. I just finished refitting mine and it works MUCH better.

Good luck, it is fun to build these things isn't it?

 

[yukonhijack]

Like some others, I built a stir plate using the fan, pot, and hard drive magnet. It never really worked the way I wanted it to (magnet was not very strong causing the bar to get thrown). So I ended up going with a Thermaltake II usb powered fan that comes with its own pot/speed controller, and some 1 inch diameter rare earth magnets from Amazon. I just finished refitting mine and it works MUCH better.

Good luck, it is fun to build these things isn't it?

 

[Fastforwardx2]

Thanks for the reply yukonhijack. I was going to use hard drive magnets but quickly realized they wouldn't cut it so I decided to use 2 button magnets that I bought that were stir bar keepers. My stir bar keeps getting thrown so I will have to see if I need better magnets. Where did you get your magnets at? How many at you using and are you stacking them. I have my spaced about 1 inch apart ( the length on my stir bar) but I think they are off and that is why my bar keeps getting thrown. thanks

 

[yukonhijack]

I used four button rare earth magnets from Amazon. I stacked them 2x2 next to eachother with one stack North up and the other North down. Works really well and doesn't throw the stir bar at all.

 

[Westermans]

6 dollars for a pack of six at Home Depot

 

[wbarber69]

It's usually not the magnet that throws the stirbar, more often it's that the magnets are not perfectly aligned with the fan. Just ripped mine apart last week to solve this issue. I'm using the off the shelf thermaltake as well. But decided that the harddrive magnets I have are much more dependable than the ones I got at lowes. If the magnet is too strong it will interfere with the coils in the fan. Or it will get pulled on too hard by the stir bar and stop the fan from moving. How I finally remedied this was by getting out my hot glue gun. Positioned the magnet where it didn't make the fan vibrate. Then just used a dab of glue to hold it place. Put the project box back together and placed the stirbar on top of the box without a flask. Turned on the fan and noticed that the stirbar was not making a perfect circle with its rotation. Pulled the magnet back off and placed it back on the fan without any glue. Then put the box back together and put the stirbar back on top. This time the 2 magnets balanced each other out and after carefully removing the stirbar and project box so as not to disrupt the hd magnet. Then I reglued the magnet down. It's worked flawlessly ever since. Now the only time u can make the magnet throw is by turning it up so high that the water spot goes down and pushes the stir bar off center.

 

[Fastforwardx2]

Wbarber69,
Thanks for the reply. After inspecting my magnets they are a bit off so I will redo them. I will be away the rest of the week and will let you know how it goes this next weekend. I also like your idea of running the stir bar not in a container for testing.
Thanks again,
John

 

[wbarber69]

Mine was off by less than a couple mm.