Tight Arse Stir Plate

[Doogiechap]

thanks L.C.,i was sure that you couldnt change one without the other but thinking too much made me doubt my trade school teachings.

the magnets are very centered,a ive used plastigage extended out from the edge of the fan to see which magnet touches it first,and theyre so close i cant measure much variation between the mounting of the 2.

the lowest hold voltage is 4.8V before it stalls and the start voltage is around the 14V mark,but sometimes i can wind up to max without seeing no movement of the fan.

i still am in the process of finding the sweet spot with mounting height too,all in good time.

if the magnets are further apart are they going to have more "pull" on the stir bar eg larger magnetic field or is closer better? maybe at 15mm they are too close...?

cheers,dan

G'day Dan,
I know I had issues with my rare earth magnets enjoying the attraction to the screws holding my fan together so I replaced them with some stainless jobbies. It's much happier starting now. Just thought that may be a relevant solution for you
Cheers
Doug

 

[newguy]

[snip]at the moment with the magnets on (13mmx5mm) on an 80mm fan i have to crank up to 14V before the voltage can overcome the strength of the magnets,surely not good for longevity <_<

Something that just occurred to me.....Do you have the fan attached to a plastic, wood or metal plate? Plastic and wood are okay, metal would be bad. The spinning magnets would induce currents in the metal.....kind of like an invisible brake. It would mean that the fan wouldn't want to start spinning. But on the plus side you'd have a heated stir plate.

 

[Simon W]

any ideas on wiring diagrams for PWM-based controllers?

Many years ago, before internet and cheap microcontrollers, I 'invented' a simple(well, simpler than other methods at the time) way to do PWM with a 555 timer IC, I was very pleased with myself..... untill I discovered later that the manufacturers datasheet for the 555 had a circuit for PWM . .. could have saved me some time! Since then I keep reminding myself that someone else has always done it before you.

Anyway, here is a simple cheap circuit to do what you want: 555 PWM
If not very readable(I didn't read it), a search with '555 PWM' on google will return many many pages on the subject.

Have just scanned the linked page, a couple of tips, make sure the 555 chip you buy(DSE, Jaycar, Altronics) is a CMOS part, it can then be used at voltages up to 18v.
Shoot for a higher frequency than the 144Hz in the design, maybe 2kHz(2000).

 

[haish]

Just had a thought - it might pay to remove the fan blades to reduce the drag on the fan motor from pushing against a flat surface. You don't need the fan to produce any airflow, so there's no point making it try. Comments?

I'd be wary of cutting off the fan blades because I think they help balance the fan.

Hi guys,

Thanks for the info on this - a good little project.

I've been putting this together and it's all working fine. However I'm now at the point of mounting the magnet on the fan.

I managed to salvage 2 magnets from an old hard drive I had lying around. Unfortunately the magnets each appear to be glued on to a piece of metal and I can't work out a way to get them off! I tried bending the piece of metal to see if the magnet would 'peel' off, but just managed to crack the magnet instead (so I'm now down to 1). I can't seem to get a knife between the magnet and the metal either to prize them apart.

Anyone got any advice on how to get the magnet off the metal?

Screwdriver, hammer and a light tap work well for prising the magnets from their hard drive mount.

 

[benno1973]

Easiest method I found for the glued ones was to put a flat screw driver on the edge of the magnet with the frame on a desk/bench and give the end of the screwdriver a good bang with the heel of your hand.

Actually, I just found this for separating the magnet from the metal.

Thanks PostModern. I actually ended up bending the metal plate back (as per the link) which allowed me to get a screwdriver under the magnet, and after a gentle tap with the heel of my hand it popped off. Thanks for the help...

 

[The King of Spain]

Sorry to keep this one going....

But I'm going to anyway. After stuffing around I found that I got way better control with a 5k Linear pot. I tried 500 Ohm, 1 k OHM and finally 5. I can adjust the 5k Ohm using a magnet that gets thrown with all the others as the sensitivity is just not there. I am using a 12V DC 1.5A transformer on a 80mm 12V fan. I am waiting for a serious rare earth bar magnet from aussie magnets (Link that will allow me to turn bigger vols then its done.

Cheers KOS

 

[LethalCorpse]

That's good, but at some point, you might want to have a read through the rest of this thread at all of the theoretical and anecdotal evidence of pots burning out when used for this purpose. Good luck.

 

[clean brewer]

Found this nice basic one today..

here here..

 

[Screwtop]

That's good, but at some point, you might want to have a read through the rest of this thread at all of the theoretical and anecdotal evidence of pots burning out when used for this purpose. Good luck.

But then again you could read about all of the slap dash ones still going strong after 3 years, never having to replace a dreaded sub $2 pot :lol:

Don't ya love forums designed as places for points of view to be aired.

 

[white.grant]

I've finaly assembled all of my parts for my stir plate. I've got a couple of rare earth magnets, a 12v puter cooling fan and a 12v variable speed transformer.



I was thinking that I would use a single magnet glued across the fan hub but have found that the magnet "sticks" to some metal in the fan hub and loads it up. Any ideas how to overcome this problem?

cheers

grant

 

[Doogiechap]

I've finaly assembled all of my parts for my stir plate. I've got a couple of rare earth magnets, a 12v puter cooling fan and a 12v variable speed transformer.

View attachment 24513

I was thinking that I would use a single magnet glued across the fan hub but have found that the magnet "sticks" to some metal in the fan hub and loads it up. Any ideas how to overcome this problem?

cheers

grant

I had this problem too ! I found the main culprits in the fan to be the screws holding the winding assembly together. I carefully pulled the fan off the hub and replaced the screws with SS ones There is still some attraction but not enough to pose any problems.
Cheers
Doug

 

[SJW]

Sorry to keep this one going....

But I'm going to anyway. After stuffing around I found that I got way better control with a 5k Linear pot. I tried 500 Ohm, 1 k OHM and finally 5. I can adjust the 5k Ohm using a magnet that gets thrown with all the others as the sensitivity is just not there. I am using a 12V DC 1.5A transformer on a 80mm 12V fan. I am waiting for a serious rare earth bar magnet from aussie magnets (Link that will allow me to turn bigger vols then its done.

Cheers KOS

I don't doubt what LC is saying BUT, I used a 500 Ohm 3W wire-wound job and its works great. Loads of control and no more burn outs.

Steve

 

[white.grant]

I had this problem too ! I found the main culprits in the fan to be the screws holding the winding assembly together. I carefully pulled the fan off the hub and replaced the screws with SS ones There is still some attraction but not enough to pose any problems.
Cheers
Doug

Thanks Doug,

I just pulled it apart to check and found that there is a circular magnet around the fan hub and a steel spindle. My magnets stick to it like a bad smell. Think I might be scrounging a new fan.


cheers

grant

 

[samhighley]

I was thinking that I would use a single magnet glued across the fan hub

That oddly shaped magnet looks to me like it'll make the whole system horribly unbalanced.

I would think this would fry the fan pretty quickly with that oddly balanced load.

 

[Wolfy]

That oddly shaped magnet looks to me like it'll make the whole system horribly unbalanced.

I would think this would fry the fan pretty quickly with that oddly balanced load.

I think you'll find its a magnet from a computer hard drive, and there are examples all over the 'net of people using these. If centered as well as possible, I'm not sure it would cause any more problems than 2 single magnets.

 

[Sammus]

FWIW, I had a computer fan and pot that was crap, it had no control between full and off, and barely enough torque to stir in a 2L flask, I went and got a 240V fan and ceiling fan switch (stole the idea from someone here.. maybe tony or something) and it's about a billion times better, plus I don't need to lug around my power supply with the stir plate.

 

[LethalCorpse]

Those HDD magnets would probably work better if you used two of them, and put them out over the fins by gluing them to a paddle pop stick or similar glued to the hub. Removes the loading issue, and removing the fins would lighten it further still.

 

[white.grant]

Those HDD magnets would probably work better if you used two of them, and put them out over the fins by gluing them to a paddle pop stick or similar glued to the hub. Removes the loading issue, and removing the fins would lighten it further still.

Thanks LC, I'll give that a go.

cheers

grant

 

[white.grant]

It Lives.




This photo doesn't do it justice but it makes a pretty impressive vortex with the kitchen measuring jug full and half a nail for stir bar. The noise of the thing is fantastic.

I attempted the two magnet and a paddle pop stick approach, but found the wider magnet placement would attract the stir bar to one side and just drag it around instead of making it spin. In the end I went with a modified plan A and glued one magnet to a piece of thin mdf and then glued both to the hub. This has reduced the attraction enough for it to spin happily.

Now just got to get my flask and proper stir bar and I'm away.

thanks for the help guys

cheers

grant

 

[TidalPete]

I went and got a 240V fan and ceiling fan switch

+1

TP