Tight Arse Stir Plate

[Screwtop]

Thanks mate. I think I started with 70mm stir bars and keep taking them back and getting smaller ones. I have a 45mm one now and it works good. I might take back the 55mm one and get a 30mm. I bet the 30mm bar looks small in a 2 litre flask? I think the trick is not to have the thing running flat out, just enough to keep it moving. It's a shame I have 4 full kegs at the moment otherwise I would love to give this a crack on some yeast. I am guessing it may perform a little different in yeast slurry rather than water too!

Steve

Why wait Steve, have a play, bet you want to. Make a big ass starter, chill, drop out, pour off the beer and store under sterile water till you need to pitch. If you stir until the yeast starts to drop out of suspension the bar has it's work cut out, whirlpool reduces in size until just about invisible.

Screwy

 

[LethalCorpse]

updated diagram to make it a bit neater and include the new values.
Jaycar parts:
LM317T - ZV1615
5k linear pot - RP3508 or RP7508
knob to suit - pick one. Not the ones which say "spline" or suit 9mm pot
680ohm resistor - RR0568
switch - pick one. Any will do, so long as it's a latching type, not momentary. Pushbuttons, toggle switches, rockers, etc.
10u cap - RE6070
100n cap - RC5360

 

[SJW]

Why wait Steve, have a play, bet you want to. Make a big ass starter, chill, drop out, pour off the beer and store under sterile water till you need to pitch. If you stir until the yeast starts to drop out of suspension the bar has it's work cut out, whirlpool reduces in size until just about invisible.

Screwy

Good point. I wondered if the unit would strugle to keep a load of yeast movin once it had fermented out. I guess by that time its done its job and reproduced to sufficient levels.

 

[SJW]

I now have a 25mm, 35mm and a 45mm Teflon stir bars. The 25mm one looks way too small for the 2 litre conical flask but it still gets a good vertex going. My point is that the smaller the stir bar the easier it is to control the speed and keep it in the centre of the flask. I think I will settle on the 35mm for the large flask and the smaller one for the 1 litre & 500ml flasks.

steve

 

[goatherder]

Can I ask where you got the teflon stir bars Steve?

I'm thinking of converting my rig from the oblique spheroid magnet pairs to a real stirbar unit. I've cracked a few flasks using the big magnets.

 

[SJW]

At Australian Scientific at Kotara. If you know where Red Funnel is, go down that street and turn left and they are down there, you cant miss em. 35mm teflon stir bar with the ring in the middle was $4 the 45mm one was about $6. Too easy. They have very cheap Schott Duran conical flasks too!

Steve

 

[goatherder]

Thanks mate - I've been meaning to go there soon to replace my broken flasks.

 

[Screwtop]

I now have a 25mm, 35mm and a 45mm Teflon stir bars. The 25mm one looks way too small for the 2 litre conical flask but it still gets a good vertex going. My point is that the smaller the stir bar the easier it is to control the speed and keep it in the centre of the flask. I think I will settle on the 35mm for the large flask and the smaller one for the 1 litre & 500ml flasks.

steve

Them's the ones

 

[WitWonder]

Thanks to LC for the original circuit and others on this board I managed to build my own TASP. Parts from Jaycar - about $6, container from Bunnings about $5, pwr supply - free, rare earth magnets also free (one slightly broken from childrens demonstration...), flasks and stirbars from Perth Scientific around $40.

 

[LethalCorpse]

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?

 

[benno1973]

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?

 

[LethalCorpse]

Maybe acetone? You need to two magnets anyway, so perhaps you're better off just buying some cheapies off fleabay.

 

[WitWonder]

Boiling water?

 

[benno1973]

Yeh, both good ideas. Does boiling water do anything to magnets (i.e. demagnetise them or reduce their magnetic strength or something)?

 

[PostModern]

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

I play around with these in quiet times at work. We've pulled apart maybe 7-8 old drives now. 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.

 

[benno1973]

Actually, I just found this for separating the magnet from the metal. Wish I'd found that before breaking the other magnet! <_< Will try it tonight when I get home...

 

[danman]

for the more electrically savvy out there,i built myself a stirplate yesterday and used a LM317 and made the circuit below which i got from an electrical website,very similar to everyone elses but different resistors and pots.

the question i have is that, with my limited understanding, the motor should be recieving 12V at all times and the current should be adjusted :huh:

have i got this right or are we just adjusting voltage or have i fudged it up somehow.

with my multimeter across the motor i get awesome linear control of the motor speed (voltage) from 1.2V thru to 15.98V (output from laptop power supply) but i would like to have 12V always at the motor so the motor starts spinning easy.
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 <_<

any help welcomed,these things will turn out great i hope :lol:

cheers,dan

 

[LethalCorpse]

Nope, with the LM317 circuit you're adjusting the voltage, not the current. They're inextricably linked - you can't change one without changing the other. But the 317 is a voltage regulator - it delivers a fixed voltage no matter how much current is drawn from it. It is unusual to have to turn it up so far to get it spinning. Once it's spinning, what's the minimum voltage you can turn it down to before it stalls? This, BTW, is the reason I've been advocating PWM-based controllers, because with a pulsed 12V supply the fan should be able to start no matter how low you've got it set, whereas with a fixed voltage reg you've got a minimum start voltage and a minimum hold voltage.
Perhaps your magnets are too large or (more likely) not balanced on the fan. If you can, reposition them so the weight is evenly distributed. If the minimum you can tun it down to without stalling is low enough for your purposes, though, I wouldn't worry too much about it. Cranking it up to 14 for a few seconds won't hurt it appreciably, or you can always just start it by hand.

 

[danman]

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

 

[danman]

any ideas on wiring diagrams for PWM-based controllers?

im from the old school of "poor man pays twice" and "only build it once-to last" so i would rather play around now than have to rebuild it in 12mths because of a shortcut i took

cheers for your advice L.C.,dan