Tight Arse Stir Plate

[crozdog]

I recently finished my dual magnetic stir plate after rescuing bits from work which would have been tossed out.

My design uses an incar phone charger to control to fan speed. Details to follow.

i can now make 4l starters for big brews or do 2 different starters simultaneously!

crozdog

 

[crozdog]

The finished insides

 

[crozdog]

details of what I did are in this PDF

View attachment dual_stir_plate_construction.pdf

 

[Pumpy]

Good work Crozdog .an excellent post

Pumpy

 

[Trough Lolly]

Jye / Ross and others...
Thanks for the closeups and info - I'm building my stir plate this arvo - I got the parts from Jaycar on the weekend and have the week off so I should have it done by tea tonight! Lashed out and bought the pair of magnets as you guys did - that's one hell of a stirbar for the 2L flask, eh! I ended up getting a bigger case to sit the big flask on - the original one in the specs was too small...Total cost was just over $100 - bigger case, better magnets, soldering iron and stand etc etc
Will post pics in due course..
Cheers,
TL

 

[Trough Lolly]

:excl: Help Please!! :excl:
I have built the stirrer - exactly the same as Peter and Jye's, but I cannot control the speed of the fan all that well. The power unit is a 12V 300ma unit and I bought the "regulated" version to keep the current under control. I'm finding (as Peter did in his post back in Aug 05 - below) that the 500ohm pot only controls the speed of the fan by a very small degree - and the stir bar seems to behave, but only as the fan is coming to a halt - my assumption is that the 12VDC fan speed is spinning way too high.

...snip...
Whilst I am at it, I do have another slight problem. The pot that I am using is 500 ohms. I find that it does not give very much leeway as far as controlling the speed. If I change the pot to a smaller value thereby less resistance, would I have greater control of speed or less? (I used two rare earth magnet by the way and a 9v power source)
Thanks for your help in anticipation.
PeterS....

Do I need to get a larger pot from Jaycar to slow down the fan?
Should I use a switchable power supply to drop the voltage to the fan?
I have a very basic circuit - fan, bulkhead power connect and 500ohm pot...all in a large box to stand the 2L flask and I used the big magnets that Jye had in his photo which seems to work a treat...
Any ideas would be much appreciated - I need to spin up some starters pronto!!
Cheers folks,
TL

 

[Jye]

The power supply and 500ohm pot should be fine, if you can post some pics or a diagram of your wiring that would help.

 

[Ross]

I find the speed control very sensitive, you only need the slightest turn to go fron stationary to turbo speed - but I don't find it a problem to control - I turn up high for half a second to get the momentum of the big stirbar going & then back off immediately to the required speed.

cheers Ross

 

[goatherder]

I'm in the middle of building the same setup as you TL.

My speed control is way too sensitive also. I think it's because the pot was of a too low power rating (0.2W). At about the point on the dial where the speed is about right the internals of the pot glow red and some of the smoke comes out. Once this happens, the speed of the fan gets a bit harder to control. I reckon i've toasted the pot around the optimal speed setting. I'm going to run some calcs on it and grab a bigger pot on the weekend - **** Smith has 3W wire-wound pots. It won't help you much now, but I'll post the results when I've done it.

 

[Jye]

Instead of using a larger pot, which I think will make the sensitivity problem worse, I would try putting a 500ohm resistor in series with the pot. This will lower the supply voltage to the pot and allow it to supply 0-6V to the fan instead of 0-12V OR try a lower voltage power pack, maybe a 5V phone charger.

 

[Trough Lolly]

G'day all...
Thanks for the replies - here are some pics of the stirrer.
This is a shot of the wiring - power connect on the left and the pot on the right - wired up as per Pete and Jye's original specs...All soldered!!

Here's a shot of the unit with the magnet fixed to the fan - it's not perfect centre and makes a little bit of noise at full tilt but not enough to make the unit jump around...

Last shot is just from the front...note the larger case to sit a 2L flask on.

I'm beginning to think the pot is too small - you have to wind the pot almost completely clockwise to get any action out of the fan and then its only a few millimeters of play between flat out and nothing - and when I wind the pot clockwise, you get a very high pitched squeal which quickly drops in tone and then the fan starts to turn. Surely this isn't right???
Any ideas/suggestions appreciated - I cannot get the fan to turn slowly enough to allow the stir bar to keep pace with the magnet below - so is it a new pot or should I drop the voltage to the fan??? :unsure:
And just to make my day even better - I've cracked my brand new Schott 2L Flask so any info can only make this day better!! :blink: :angry:
Cheers,
TL

 

[Jye]

ah... ignore my last post, I was think you only had to turn a little bit to high speed... when its the other way around.

In that case a larger pot may help.

You may have a log pot which could explain why the control is sensitive, a linear pot should remove the sensitivity. To check look for an A (log) or B (linear) printed on the back of the pot somewhere.

 

[goatherder]

Instead of using a larger pot, which I think will make the sensitivity problem worse, I would try putting a 500ohm resistor in series with the pot. This will lower the supply voltage to the pot and allow it to supply 0-6V to the fan instead of 0-12V OR try a lower voltage power pack, maybe a 5V phone charger.

[post="129304"][/post]​

I was thinking of that - one of the link from sosman's page has some series resistance in before the pot. I'll do the calcs and see what I can work out. I was still going to stick with a 500 ohm pot - just a higher power rating to help keep the smoke on the inside.

 

[Trough Lolly]

...snip...
You may have a log pot which could explain why the control is sensitive, a linear pot should remove the sensitivity. To check look for an A (log) or B (linear) printed on the back of the pot somewhere.

[post="129313"][/post]​

I think you're onto something here Jye - stamped onto the back of the pot has a lower case "a" in a circle and beneath it:
ALPHA
B25K
followed by 4L3 in black on the bottom edge of the pot. The 1st pic I attached above shows most of those markings when you click on the image...I might take it to Jaycar and get them to confirm - I can always get a linear pot and some other larger 1K or better pots and experiment (it's easy once you know how to solder!!). I also have some lower voltage power units laying around the house - including one that works between 3 to 12 volts. I need to slow down the fan...
Cheers,
TL

 

[Jye]

...snip...
You may have a log pot which could explain why the control is sensitive, a linear pot should remove the sensitivity. To check look for an A (log) or B (linear) printed on the back of the pot somewhere.

[post="129313"][/post]​

I think you're onto something here Jye - stamped onto the back of the pot has a lower case "a" in a circle and beneath it:
ALPHA
B25K

[post="129340"][/post]​

I think the pot you have is linear 25000ohms (B25K), check it with a multimeter or get the guys at jaycar to do it. This is your problem since the resistance is too high until you have turned it all the way to the end. Replace it with a 500ohm and all should be sweet

 

[Trough Lolly]

It's off to Jaycar I go...Thanks Jye.

TL

 

[user 865]

TL,

the "squeal" you talk about from the fan is likley the fan trying to start up on too low a voltage/current. Some fans happily start on a wide range of voltages (even down to 40% of their 'rated' voltage) others struggle even at some 75% of their rated voltage. Usually, once spinning, the fan will happily slow down and keep running. The other catch is by introducing the resistor the current to the fan is limited and thus is more likely to stall. 500 ohms would be a much better size resistor as a first attempt.

I usually use a pulse width modulator (PWM) which pulses 12v to the fan so you get enough voltage to start the fan and the speed is then controlled by the pulse on-pulse off ratio. IF you dont mind the fan making a bit of noise, it is possible to have a virtual 0-100% speed range. I haven't looked at the modified phone charger circuit mentioned in this thread in much detail as I already have a circuit to acheive the same thing, but it might be along the same "concept".

Give me a call if you like and I can etch a board for the circuit for you (uses about 8 components so nothing too complicated).

Darryl

 

[Trough Lolly]

G'day all,
Well, Jye, you were right! It was a 25K pot and the Jaycar staff were a tad embarrased...I ended up getting a 500 ohm pot and they wanted to charge me for it until I reminded them that it was their mistake.
Anyways, it's now soldered in and the fan runs a lot slower and is much more controllable - once you get it spinning.
There is one problem left to sort out now - and that's the orientation / centering of the magnet over the fan. Even at low speed, the stirbar is not working as it should - it spins and then flicks off at a tangent and then spins back to the centre and out to a tangent, etc, etc. I'm also getting cavitation from the fan - which I didn't think was a problem, but it's obviously doing that because the large heavy magnet is not centred on the hub. I can see it's slightly off centre so I've carefully cut the magnet off the fan (I used 2 part epoxy and thought I'd be there all night with the stanley knife but it eventually lifted!!). Anyway, I'll draw some lines on the hub of the fan to help centre the magnet, check with the tape measure and refix the magnet to the fan and test before epoxying again.
And who said Friday nights are boring!!!
Cheers,
TL
P.S. Darryl - thanks very much for the kind offer mate - If I can't get this unit to work after re-centering the fan, I might just take you up on the kind offer to etch a board etc. Thanks...

 

[poppa joe]

TL
Has anyone thought about a Train Controller....Would it work...???? if so
Tell the Kiddies Daddy wants to play with it....
Cheers
PJ

 

[Jye]

TL
Has anyone thought about a Train Controller....Would it work...???? if so
Tell the Kiddies Daddy wants to play with it....
Cheers
PJ

[post="129528"][/post]​

As long as the voltage is about 12V.