Tight Arse Stir Plate

[therook]

I think I'm in need of a bit of help with this one. I've been collecting all the bits and bobs for building one of these lil beauties, and finally had the time to sit down and try to work out how it all goes together. Well, managed to get it all wired up, fan comes on with the switch, speed modified by the knob (yeah, one turning the other), but when I put a flask with a stir bar on and fire it, the stirrer begins to spin, I get all excited, and then it just starts flicking around the flask bumping all random-like until i turn it off. Tried turning the fan speed down to just before it cuts out, no change.

The magnets are the small rare earth magnets from Jaycar, the stir bar is a 35 x 6mm cylindrical one. I've tried moving the magnets inward on the fan, outward, stacking them 2 high on opposite sides of the fan as well. pretty sure I have one N facing up and the other S facing up.

I guess the questions are: are the magnets too strong/not stong enough? or is the fan still to fast (hard one to answer without seeing it i guess), is the distance between the flask and the magnets too big/small? (currently ~ 20mm gap, but tried smaller, no change), any advise would be welcomed.
Cheers.
Maple

Maple,

I had exactly the same problem?

What size magnets are you using, I was using small ones and changed them to bigger ones ( 12mm wide i think ) and after that it works a treat.....

let me know if you want me to measure mine or give me a call at home tonight

Rook

 

[captaincleanoff]

this only ever happens to me when it is spinning too fast, so maybe thats the problem

 

[Jez]

i had the same problem. stirbar used to jump all over the place no matter what the fan speed.

I sat the flask on a piece of 15mm or so thick pine board on top of the jiffybox & it spins perfect for days.

Jez

 

[Maple]

Maple,

I had exactly the same problem?

What size magnets are you using, I was using small ones and changed them to bigger ones ( 12mm wide i think ) and after that it works a treat.....

let me know if you want me to measure mine or give me a call at home tonight

Rook

Rook, cheers for that, I suspect they may be too small. they are 10mm dia x 3mm, next size up was 25mm, figured that would have been a bit too big, perhaps something in between is what I'm after. Will tinker around with it tonight.

Jez, perhaps a few things to try, cheers Jez.

 

[newguy]

I guess the questions are: are the magnets too strong/not stong enough? or is the fan still to fast (hard one to answer without seeing it i guess), is the distance between the flask and the magnets too big/small? (currently ~ 20mm gap, but tried smaller, no change), any advise would be welcomed.

There are a few things that will influence the coupling (hold) between the spinning magnets and the bar.

1) With magnets physical size is indicative of power (if material is the same). Larger magnet volume = more force. I have 20mm x don't know how thick - probably 3 or 4mm. You definitely want rare earth magnets.

2) Physical gap between bar and spinning magnets must be as small as possible. This means shimming your fan so that the magnets just barely clear the top of your enclosure. It also means thinner is better for the enclosure top, as long as the top is strong enough to support your flask when it's filled. Never use a metal top - only wood or plastic or some other non-magnetic material.

3) Separation between the magnets on the fan should correspond to your stir bar's length so that the ends of the bar are roughly centered over each magnet's center. This is a good starting point but you may find that a bar slightly longer or shorter than your stationary magnet's center to center distance works better. My best "holding" stir bar is about the c-c distance.

4) One stationary N up, the other S up is mandatory.

5) Start on slowest speed, then slowly turn up the speed. You'll find that most bars will eventually get flung off at some speed; either change the bar for a different size or just operate the unit below that speed.

6) It's important to center your flask above the fan. If it's off center, the bar will get flung off at a lower speed than if the flask is centered.

 

[TidalPete]

i had the same problem. stirbar used to jump all over the place no matter what the fan speed.

I sat the flask on a piece of 15mm or so thick pine board on top of the jiffybox & it spins perfect for days.

Jez

Jez has the answer fellas,

There are two solutions to this:
1 -- Raise or lower your fan inside the box you are using it in to increase\lessen the magnetic force on your stir bar & if that doesn't work try "2".
2 -- Place a piece of whatever (Various thicknesses to suit) between your fan & your flask & Bob's Your Uncle.

I have a 240v stirplate with those those 25mm rare earth magnets & had to vary the height between them & the stirbar to avoid the problems above.

TP

 

[Maple]

Cheers guys. Had a bit of a go with adjusting the distance between fan and flask. With about a 1mm gap between the magnets and the bottom of the flask it works beautifully. To me that means these small size magnets are just not enough even when stacked 2 high. Thanks again fellas, will get some bigger Rare earth magnets tomorrow.

 

[MattC]

Hey all, I have gathered some parts to make one of these stir plates. I have a 12 V fan, 500Ohm Pot, switch, jiffy box, wires, panel mount socket, plug pack etc etc.

I was drilling the hole for the Pot to fit in the jiffy box. The Pot only came with a thin fastening nut. Does the hole for the pot only have to be wide enough for the first grooved cyilnder section to fit through, or does part of the threaded section need to fit through the hole and then the nut fixes the pot from the outside? There is a small piece of metal that potrudes and to me would seem to prevent a flush fitting to the jiffy box!

Silly question im sure but for me it aint!!

Cheers

 

[LethalCorpse]

Very silly questions. This should all make immediate sense. But, anyway, The nut fastens from the outside - how the hell could it fasten on the inside? The thread, of course, has to go through the hole - how could you fasten it otherwise? The only remotely difficult part of the whole thing is the little protruding bit. It's a key which prevents the pot from rotating as you turn it. You drill a small hole right at the point where that knob meets the faceplate, so that when you screw the pot to the plate the knob is in that hole and doesn't foul anything else.

It's all moot, as the pot will fail shortly because it's designed to handle a fraction of the power you're forcing it to handle. This has been discussed throughout this thread. If you don't have the nous to build one of the appropriate circuits, buy a fan controller from your local computer store and use that instead.

 

[MattC]

Gee thanks for being so understanding LC. I am not at all mechanically or electrically minded and i do realise that my question was simple for you. As mentioned it was the small key piece icouldnt work out! I have read elsewhere that a 500 ohm pot was suitable, ive already got it so time will tell! Anyway thanks for the advice, albeit with the accompanying lack of empathy for a person less suited to this task than yourself!!

Cheers

 

[LethalCorpse]

Stop whinging - I've flamed harder for less. If you thought about how things fit together for a minute before posting those questions wouldn't have been necessary, although as I said, the locking key might have presented some difficulty.

There's nothing wrong with not being electrically or mechanically minded. In those cases, though, you need to consider if you should really be attempting a particular task. There are plenty of off-the-shelf solutions for what you're trying to do. I'm very handy with electronics and most mechanical stuff, but I don't attempt my own legal arguments, for example. I hire a lawyer.

 

[MattC]

I do not want to waste time entertaining someones ego, but i did look at how they went together before posting. It was the key that presented some difficulty, that part we at least agree on. I searched the net for advice on attatching a pot to a jiffy box but when i couldnt find it, i thought i would post my question here for some friendly advice from someone more electrically minded! As far as suggesting i dont attempt such a task, well i couldnt disagree more. I believe in having a go and if that involves asking what some may call " very silly questions" then i can live with that! As far as the lawyer thing goes, mate not even the same ball park! Get off your high horse and rethink your statement.

To cut a long story short, I appreciate your constructive comments as i got the info i needed, however i did not expect to be ridiculed for asking these questions! May i suggest to you that if people like me ask questions that cause you to be frustrated, then ignore the question and do not post a reply!

Cheers!

 

[gibbocore]

**** you're a ******, LS, get off your high horse for two seconds. The most annoying thing is that you're right.

 

[LethalCorpse]

The question wasn't just about the key, you also asked whether the thread had to go through the hole, and whether the nut locked from the inside or the outside. You're the one who called them silly questions, I just agreed with you. I'm not on my high horse, but if you can't figure out how the pot goes through the box, how are you going to figure out which wire is positive and which is negative? Are you certain of which end of the soldering iron you're supposed to hold, and which end you poke at stuff? All silly questions, but if you don't know the answers to those you're in more trouble than taking a bit of stick from me.

The lawyer example is completely valid. It's something I'm not qualified to do, have no experience in, and, therefore, wouldn't attempt to do it myself. I couldn't use plumber, electrician, mechanic etc, because I know enough in those areas to do what I can and pass on what I can't. I'm not being an arse when I say you don't have the skills to do this properly - that's just how it is. More to the point, you don't need to do it - there are cheap, ready made solutions that will do a much better job than something you can bash together. I'm all for "having a go", but only within reason. You've got to have some idea of what you're attacking first.

 

[MattC]

I did say they were silly questions because I understand they were. I admit I have no idea where to start with this stri plate thingy, but called me determined, I want to do it. So what are these off the shelf thingys you referring to? Do you mean the computer fan controller?

Edit: although humerous LC, I do happen to know the correct end of soldering iron, im not a complete waste of space

 

[brettprevans]

well my newly aquired stir plate has been in action and not living up to scratch. it was struggling with a 2L flask. either the stirbar magnet is too small or the fan is too slow. Im actually thinking its a combo of both. so off to the IT boys to see if they've butchered any PCs lately and I can steal a fan from them. then off to get some bigger stirbars.

 

[LethalCorpse]

So what are these off the shelf thingys you referring to? Do you mean the computer fan controller?

yep, any computer shop should have several different options. Plenty of fans of about the right size are supplied with a little controller that is supposed to go in a PCI slot. Otherwise, they'll have a few aftermarket controllers, though many of those will be multichannel and perhaps more sophisticated than you need.

 

[Fents]

have you finished my stir plate yet LC?

 

[MattC]

yep, any computer shop should have several different options. Plenty of fans of about the right size are supplied with a little controller that is supposed to go in a PCI slot. Otherwise, they'll have a few aftermarket controllers, though many of those will be multichannel and perhaps more sophisticated than you need.

So using your diagram of the circuit, does the fan controller take the place of the LM317? and everything else is the same as in the diagram?

 

[LethalCorpse]

The fan controller would take the place of everything in the diagram - LM317, pot, caps, etc. You'd just connect 12V to one side of the controller, connect the fan to the other side of the controller and you're away. The knob for adjusting the fan speed is part of the controller.