Stir-plate Troubleshooting

[MaestroMatt]

Hey there,

I have just built myself a stirplate for my yeast starters but am having a little problem with getting the whole thing to operate properly, so I though I would just chuck it out there and see if anyone could help.



My stirplate consists of the following:

- 12V computer case fan (ball bearing I believe)

- 25K variable resistor (Potentiometer)

- plain switch

- 12V power adapter.

The circuit works so I know it's not a wiring thing but when I flick the switch and start turning the variable resistor around it takes almost 3/4 of the revolution before the fan starts spinning and then it spins too fast for the stir bar. It's like it is not actually ramping up the power but suddenly turning on at that power level. Any suggestions with troubleshooting this one?

Also, I have salvaged a couple of rare earth magnets from a HDD and attached one of them to the fan. For some reason, at a good range away from the magnet, the stirbar spins well for a little while then starts to wobble off centre and eventually pop out of rotation.....weird... The flask is dead centre on the magnet and the magnet is as centered on the fan as I can get it....any suggestions of this one as well?

Thanks for all your help.

Once I get it working properly I will put up a guide of how I did it....not that it is any different from a lot of those around at the moment.

Cheers!

MM

 

[roller997]

Hey there,

I have just built myself a stirplate for my yeast starters but am having a little problem with getting the whole thing to operate properly, so I though I would just chuck it out there and see if anyone could help.



My stirplate consists of the following:

- 12V computer case fan (ball bearing I believe)

- 25K variable resistor (Potentiometer)

- plain switch

- 12V power adapter.

The circuit works so I know it's not a wiring thing but when I flick the switch and start turning the variable resistor around it takes almost 3/4 of the revolution before the fan starts spinning and then it spins too fast for the stir bar. It's like it is not actually ramping up the power but suddenly turning on at that power level. Any suggestions with troubleshooting this one?

Also, I have salvaged a couple of rare earth magnets from a HDD and attached one of them to the fan. For some reason, at a good range away from the magnet, the stirbar spins well for a little while then starts to wobble off centre and eventually pop out of rotation.....weird... The flask is dead centre on the magnet and the magnet is as centered on the fan as I can get it....any suggestions of this one as well?

Thanks for all your help.

Once I get it working properly I will put up a guide of how I did it....not that it is any different from a lot of those around at the moment.

Cheers!

MM

Is it possible that since you are using a potentiometer to reduce the voltage the fan doesn't have enough "power" to get going until it gets to a certain voltage level? A slower but more powerful fan might assist.

Cheers

Roller

 

[therook]

Here you go

Tight Arse Stir Plate

Rook

 

[Screwtop]

More R

Screwy

 

[MaestroMatt]

Well I had a good look through the link that Rook gave above....got to page 11 and found some info that might help

Quote from that Page:
"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"

This is what I was thinking but it's good to have some confirmation of a possible solution.

Re: the magnet spinning around, someone suggested on that thread that the magnet might be interfering with the fan motor, and what can help is putting a spacer (e.g. 3/8" washer or something like that) inbetween the fan and magnets.

Will try both above suggestions and post back here with the results.

 

[Wolfy]

I'd assume that you do not have a linear-pot hence why you need to turn it quite a way before the fan starts spinning.
However, the first thing I'd suggest you do to try and fix the problem is to adjust the distance between the magnet and the stir-bar, put some cardboard or plastic between them, and you might find the 'sweet spot' where the magnet and stir-bar work happily together.

 

[Sammus]

yeah speed adjustment isn't that great... it's called tight ass for a reason And the stir bar is probably throwing probably because it's spinning to fast. Some of the smaller bars on mine throw at full biccy. I'm using a 240V fan through and a ceiling fan controller - works like a charm.

What happens if you turn it up full biccy to just get it going, then back off a little?

 

[MaestroMatt]

I'd assume that you do not have a linear-pot hence why you need to turn it quite a way before the fan starts spinning.
However, the first thing I'd suggest you do to try and fix the problem is to adjust the distance between the magnet and the stir-bar, put some cardboard or plastic between them, and you might find the 'sweet spot' where the magnet and stir-bar work happily together.

Nah, it's definitely linear. I've tried a few different ways of getting distance away from the magnets.....still doesn't really solve the problem. But I will definitely keep working with that part of it to get the bar stirring problem under control.

yeah speed adjustment isn't that great... it's called tight ass for a reason And the stir bar is probably throwing probably because it's spinning to fast. Some of the smaller bars on mine throw at full biccy. I'm using a 240V fan through and a ceiling fan controller - works like a charm.

What happens if you turn it up full biccy to just get it going, then back off a little?

I have indeed tried that - works about 10% of the time. I turn it up full pelt then ease it off until the bar catches back into rotation. But by the time, the bar is caught and is rotation, the fan is very slow and it just stops. Even in really tiny moves, it doesn't start moving at a slower rate...just doesnt do anything until...BAM!! It's going way too fast for the bar to keep up.


I did head out to Jaycar and pick up a 500ohm pot so I will take that home and see how it goes.

 

[MaestroMatt]

Ok.....

So I fixed the power issue - the 500ohm resistor worked a treat. Now I am only having problems with the magnet and stirbar not spinning properly.

It spins but not on a central axis - in other words, it is like half the magnet is more powerful than the other half. I have swipe another magnet along the stirbar to see if it was a polarity issue but it still happens. Maybe I am not doing it properly.

Any more help would be fantastic.

Cheers

MM

 

[bjay]

Just a thought MM are you running the stirbar with liquid (water) in the flask when spinning
as this would change the whole dynamics of the thing

just a thought

bjay

 

[MaestroMatt]

Just a thought MM are you running the stirbar with liquid (water) in the flask when spinning
as this would change the whole dynamics of the thing

just a thought

bjay

That's a fair thought there.

Unfortunately the problem occurs mainly when there IS water/liquid in the flask. At the moment it's a flat bottom coffee strainer glass but it will eventually be a Erlenmeyer flask.

It's funny though....I took the stirbar out of the flask and put it on a flat surface to see what would happen.....it centered better than when it was in the water......hmmm

Thanks for the continued advice people...

 

[Wolfy]

Unfortunately the problem occurs mainly when there IS water/liquid in the flask. At the moment it's a flat bottom coffee strainer glass but it will eventually be a Erlenmeyer flask.

It's funny though....I took the stirbar out of the flask and put it on a flat surface to see what would happen.....it centered better than when it was in the water......hmmm

Have you tried a different flask/container, to be sure that it is flat-bottomed on the inside?

 

[Fourstar]

do away with the rare earth magnets from the hdd unless you stack them and place them in the middle of the fan. I had issues like this where it would spin off really easily until i got some circular RE magnets from dealextreme. I stacked two either side of the fan with opposite polarities and now she spins up well.

also what i did was stick the RE magnets to either side of the stirbar, popped super glue on the udnerside and stuck them to the fan with the bar magnetically attached. Atleast that way i knew they where evenly spaced enough so they would attach to the stirbar once it was in the falsk.

 

[MaestroMatt]

do away with the rare earth magnets from the hdd unless you stack them and place them in the middle of the fan. I had issues like this where it would spin off really easily until i got some circular RE magnets from dealextreme. I stacked two either side of the fan with opposite polarities and now she spins up well.

also what i did was stick the RE magnets to either side of the stirbar, popped super glue on the udnerside and stuck them to the fan with the bar magnetically attached. Atleast that way i knew they where evenly spaced enough so they would attach to the stirbar once it was in the falsk.

Fantastic, quality advice - Thanks 4*

I did think of the stacking HDD magnets and it does help a little but the bar still comes undone during the spin.....

Even though I didn't want to spend a great deal on this project, in this case, I may have to fork out a few extra bucks for those magnets...will keep trying though.

 

[Fourstar]

Fantastic, quality advice - Thanks 4*

I did think of the stacking HDD magnets and it does help a little but the bar still comes undone during the spin.....

Even though I didn't want to spend a great deal on this project, in this case, I may have to fork out a few extra bucks for those magnets...will keep trying though.

what i think is the issue with them is the polarity on both ends rather than either side. All i know is i had the same problem as you (stirbar flying off and very hard to get going) until i went to the RE magnets. for the sake of 4 bucks! for 10 > 20 magnets you cant really go wrong!

 

[Phoney]

It's this thread that makes me happy and proud that I ordered a stir plate ready made.

(Sorry for being so smug)

 

[Wolfy]

It's this thread that makes me happy and proud that I ordered a stir plate ready made.

(Sorry for being so smug)

You can be as smug as you like, the cost difference between your ready-made stir plate and mine equates to about 3-4 batches of beer, so I'm happy for you.

 

[Phoney]

Hmm lucky you, 42 bucks wont even get me 2 batches of beer

 

[mintsauce]

these guys : http://aussiemagnets.com.au/ have cheap magnets, I ordered $20 worth and got 8 smaller magnets of varied size (the smallest ones will probably work with a stir bar ) and a couple of the friggen strong medium sized ones - this included $10 postage....