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Author: Subject: DIY magnetic stirrer
Quince
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[*] posted on 15-4-2005 at 00:48


V1 is a voltage source. That's a very common symbol so you should remember it.

Check the NE555 datasheet for acceptable voltages. Better run the simulator to make sure everything works right with the parts values and voltages you are using. The one pictured is good and completely free.

Yes, you connect it instead of the NAND. That gate seems to be shown there to indicate that you are using a logic signal. Again, make sure you are using compatible voltages so you don't fry something, and reformat the signal if needed. Rin needs to be set to create the proper current through the emitter; currents are given in the datasheet.

DO NOT drive the triac directly! It is not safe; you need the optical isolation; it is almost as bad as cutting off the grounding prong of a three-prong power cord to fit an ungrounded outlet (though the latter is actually illegal). Also, the two main terminals on a triac are not interchangeable, so make sure you check the datasheet of whatever triac you use.

I'll probably build a PID and thermocouple thermometer that can be added for awesome control, and I'll post these when ready.

Anyone use mantles with flat bottomed flasks and beakers? It kind of sucks having to get both a mantle AND a hotplate. Is the air gap under the bottom of a flat bottom vessel and the mantle a problem, other than slowing down the response?

[Edited on 15-4-2005 by Quince]




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Quince
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[*] posted on 18-4-2005 at 04:55


Values changed in schematic. Use a 250K pot instead of 50K and remove R2. An input signal at the pot wiper, through a 250K resistor, will also control the PWM (with pot set to center), but it's not quite a linear correspondance with the control voltage, which might throw off a PID. Maybe I'll post something better later.

12AX7 pointed out (thanks) that a triac doesn't shut off until a zero crossing of the AC going through it. That means that PWM will only work at a good deal lower frequency than the power line's. I'm thinking a MOSFET switch may work, as in the attachment (simulates OK, but I've not built it so don't put much stock in it).

The voltage source in there (on the right) stands for the driver (PWM through optocoupler or small transformer for isolation). On the left is obviously the mains frequency; the amplitude says 169 instead of 120 since LTSpice uses peak rather than RMS (for sine wave it's by a factor of sqrt(2)). R3 is load and R2 is because mains neutral is tied to ground in North America (as in, it's already in your house wiring, I just chose some arbitrary resistance to represent that connection).

This should work fine with a heater as load (such as a heating mantle). As for other loads...should be fine as light bulb dimmer, and for other 'slow', dumb loads (that 'integrate' the supply). No clue if it will work with AC motors. Wouldn't try transformers or electronics as load either, as they depend on a normal-looking AC.

[Edited on 18-4-2005 by Quince]

ac_ss_relay.png - 39kB




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FrankRizzo
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[*] posted on 20-4-2005 at 18:15


Quince,

This cheap PID available from Meci.com might be of interest. Unfortunately, customer service there doesn't have any specs for it, but it may be useful. I ran across it when searching for PID's for my RIMS brewing setup.

http://www.meci.com/Catalog/Item/7c239168-af45-43bc-9795-505...


Back OT, the following sites have some good info on stir plates:

http://brewiki.org/StirPlate
http://www.frontier.iarc.uaf.edu/~cswingle/brewing/equipment...


[Edited on 21-4-2005 by FrankRizzo]
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Quince
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[*] posted on 28-4-2005 at 20:10


Well I had used plaster of paris for the mantle bowl but it broke too easily. I see BrAiNFeVeR (what kind of brain fever did you have, BTW, meningitis or encephalitis? viral or bacterial? :P ) used concrete. Can I use clay? Actually, I don't have clay either, but what if I crush up some earthenware pots, can I make clay from that and bake it in my kitchen stove?



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[*] posted on 28-4-2005 at 23:57


LOL, no that'll be grog. Grog has no binding power. You need real clay, either from the ground or the local pottery store. Then it needs to be "baked" upwards of yellow heat, preferably a good yellow-white. That'll get you in the cone 6 to 12 area which has good strength and density for most clays (assuming it hasn't melted yet).

Tim
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Quince
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[*] posted on 29-4-2005 at 00:45


I don't have a kiln. What's the alternative? Is air-dry clay good enough for a mantle base, or will the heat damage it?

[Edited on 29-4-2005 by Quince]




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Tacho
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[*] posted on 8-5-2005 at 09:09
Magnetic stirrer made from broken diskdrive.


Almost all you need to make a magnetic stirrer is in that old HDD you just didn't have the courage to throw in the garbage.

Here... attached.

Attachment: hddstirrer.pdf (192kB)
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[*] posted on 10-5-2005 at 06:34
Triac as rate controller instead of phase controller


Most lamp dimmer type circuits are phase controllers and conduct for a varying percentage of each cycle of AC . The effect
is to not change the frequency of the AC at all , which is desirable for lighting applications , so as not to cause any strobe effect in the light output .

However , the speed of an AC motor is very much related to the frequency of the
AC which establishes the synchronous speed for the motor . The actual speed
slips behind the synchronous speed because of friction and the inertia of the rotor . The lower the applied power , the greater the slippage away from attaining the synchronous speed , until the stall speed is reached and the motor stops .

There can be advantages to using a triac in a different way , so that the triac energizes for a complete cycle of AC ,
and delivers the full voltage and current to the load during each cycle the triac is energized . The triac is used as a percentage controller , and the effect is that as the percentage is lowered , the
actual frequency of the AC power supplied to the load is also lowered .

I have not built this circuit nor compared it side by side with the usual lamp dimmer type phase controller . But it makes sense that it should be more efficient ,
and it would also lower the heatsinking requirement for a triac being used to
control heavy loads such as heating elements .

This type of operation mode for a triac may provide and is claimed by the patent to provide better low speed control and torque output for shaded pole AC motors .
How it may perform with a permanent split capacitor motor is unknown .

In the design of stirplates , it is advantageous for the drive magnet
field to be close to an aluminum or stainless steel plate . The eddy currents
set up in the plate by the rotating field
create a speed regulating and governing
effect which stabilizes the motor speed at any given power setting , particularly the lower rotation speeds . The loading effect of the eddy currents is such that
if the motor speed tries to drift upwards ,
the eddy currents increase and provide
a feedback torque opposing the increase of the speed of rotation of the magnets .
Conversely , if the motor speed is slow and begins to decrease towards the stall speed , the eddy currents decrease and reduce the load on the motor , so that the motor speed comes back up . So ,
an aluminum plate in the field of the magnets functions as an active feedback
speed regulator and stabilizer for a stirplate , and should be of benefit in most designs which do not utilize some sort of active electronic speed sensing and feedback power control for speed regulation . This eddy current speed regulation scheme is used on the Cimarec
stirplates , which control the voltage to the motor via a wirewound power rheostat . Even such a simple design
can provide excellent performance if the separate components are carefully matched to function together well .

The attached patent describes a circuit that has been used by Fisher Scientific in
some of its stirplates . This circuit could perhaps also be useful in custom build
scenarios for stirplates , mantle controllers
and the like where an alternative to the usual phase controller is desired .

Attachment: US44144500 Shaded Pole Motor Variable Speed Contol.pdf (604kB)
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[*] posted on 30-5-2005 at 12:21


I'm building myself a magnetic stirrer from a broken HD as in the pdf you attached Tacho. However: the spindle motor works and I'm hoping that I will be able to use it. The epoxy glue that I fasten my magnets with is now curing and I will see tomorrow if the disk will spind too fast or not. Here's my question:

Do anyone know a moderatly simple way of controlling the speed of the spindle motor?

[Edited on 30-5-2005 by TheBear]
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Tacho
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[*] posted on 30-5-2005 at 13:00


TheBear,

That spindle motor runs at something close to 3.000RPM. I wish you luck, but I see big (imense) problems with centrifugal forces and balance. Besides, will magnets spinning at 3.000RPM drive the stirbar? I don't know.

I also doubt you can slow that spindle motor. I did think of changing a crystal I saw on my circuit to a smaller Mhz value, but I never tried it.

By the way, be carefull with those magnets flying off the disk at 3.000 rpm, they may really hurt you.

Edit: Also, my small motor uses just 150mA. I bet the spindle+driver consumes a few amperes. You will need a big power source.

[Edited on 30-5-2005 by Tacho]
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Quince
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[*] posted on 30-5-2005 at 14:01


Common DC electrical motors are simply too fast for stirrers. Use a reduction gear or belt. I did it as simply as using an elastic band and some plastic wheel from a busted VCR. You get more power as well, so you can use a much smaller motor. If you are using more than a couple of watts, you are wasting energy. My tiny motor spins even a large 5 cm stirrer in thick liquid easily, and still fast enough to make a half full 500 mL beaker spill.

[Edited on 30-5-2005 by Quince]




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[*] posted on 31-5-2005 at 12:36


Safety... umm: plenty of epoxy and a positive (naive) attitude! Great fun seeing those magnets going at ~4500 rpm (nah.. perhaps 3000 when taking the extra weight of the magnets into consideration.. don't know). But I have to say:

When putting the metal top over the spinning magnets you can really hear and feel how those eddy currents do their job. Speed is reduced drasticly! But still not enough.

The speed is of course (as expected) far to great for any magnet to have a chance to keep up with.

And stability: ever seen a harddrive vibrating and moving around a benchtop?... very nice. But this got me thinking: couldn't one make a tiny homemade centrifuge out of an broken harddrive? Should be the most simple way to go I think.

I will probably use another motor.. seems like it would be a hard task manipulating something as complex as the electronics of a harddrive.

And for power source: a PSU


[Edited on 2-6-2005 by TheBear]
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[*] posted on 1-6-2005 at 05:04


I tried to make a driver circuit for the spindle motor, but it didn't work. I used variations on stepper motor circuits, but it seems that this motor works with positioning sensing, probably written on the magnetic disk.

I posted a circuit to drive a stepper motor sometime ago. It's a bit complex because I wanted do use TTL logic and have reverse spinning. One could make a much simpler driver circuit for steppers using a 555, one CMOS chip and a ULN2003 chip.
I may do it one of these days.

I like the centrifugue idea. For the tubes I would use 2 x 5ml syringe barrels. Maybe just two wire loops passed through holes drilled on the disk would work as the swivel (sp?)joints. Humm...

[Edited on 1-6-2005 by Tacho]
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[*] posted on 6-6-2005 at 14:14


Been thinking about a stepper motor myself. Thing is, will they run OK at 1300 rpm? Otherwise, a great way to have perfect speed control.
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[*] posted on 6-6-2005 at 20:54


Its really much simpler to use DC motor with a reduction gear. My local electronics supplier has them for NZ$15. A simple variable voltage supply will give you variable speed.

Tacho, those HD spindle motors are brushless DC motors and use feedback either through Hall sensors, or back EMF from the coils. Chips exist to drive these things.
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Quince
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[*] posted on 6-6-2005 at 21:21


$15, are you nuts? Any little junk motor will do. Why bother with a reduction gear, when you can just use a plastic wheel and an elastic band for mechanical advantage.



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[*] posted on 7-6-2005 at 04:24


Quote:
Originally posted by GreatHampton
Been thinking about a stepper motor myself. Thing is, will they run OK at 1300 rpm? (snip)


To make a stepper motor run fast is possible, but not trivial. You need higher voltages to overcome the impedance of the coils. Then you need power resistors to limit the current. Of course, all electronics must be compatible with such voltages. Maybe 1300 RPM can be reached by commom stepper drive circuits, but I'm not sure,

[Edited on 7-6-2005 by Tacho]
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[*] posted on 7-6-2005 at 11:31
reverse engineer


look at the way a commercial mag stir is constructed.
I don't remember the brand of mine but it is very clever and very simple.

a round disc of ferrous metal is mounted on a shaft of a variable speed (variac) motor.

a round magnet is then balanced on the disc and presto chango = mag stirrer.

dynamic balance can be adjusted very easily.

to get a hot plate mag stirrer add a "thin" nichrome etc surface above the spinner.

just make sure that it is ceramic is non magnetic to pass heat and magnitism.

btw all the components except the wiring and motor are contained in most speakers!!!

just disassemble and there is the magnet and the disc (already center punched).

this info is crudely illustrated on

http://www.geocities.com/jimwig2000/diy-mag-stirrer.html

[Edited on 7-6-2005 by jimwig]

[Edited on 7-6-2005 by jimwig]
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[*] posted on 7-6-2005 at 16:10


Quote:
Originally posted by Quince
$15, are you nuts? Any little junk motor will do. Why bother with a reduction gear, when you can just use a plastic wheel and an elastic band for mechanical advantage.


Because a plastic wheel with a rubber band is not likely to last long inside hotplate! The off-the-shelf geared motor with brass reduction gears for $15 (thats about $10 US) is a hell of a lot simpler and more reliable.
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[*] posted on 7-6-2005 at 21:53


Yes, for a hotplate you are right. I'm using mine with a DIY heating mantle which has insulation on the outside, so it's not an issue.

BTW, from the discussion of temperature control here I was inspired to PID my espresso machine, and I did that along with some other mods, with awesome results. If anyone here is a coffee snob, check out my post at the coffegeek.com forums:
http://coffeegeek.com/forums/espresso/machines/147405




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[*] posted on 16-2-2006 at 11:21


Here is an AC power handling circuit which I have been
developing and evolving for the dedicated purpose
and niche application that is an open loop speed control
for a laboratory magnetic stirrer . Specifically it was
my purpose to design an AC power handler useful for
speed controlling permanent split capacitor ( PSC ) type
motors . The four pole or six pole motors are most desirable
although others could be used depending on the top
speed desired . PSC motors are more than twice
as power efficient as the shaded pole motors used in
commercially manufactured stirrers , and have more than
twice the starting torque and low end torque , along
with a more linear power slope from locked rotor up to
80% of synchronous speed , so on paper anyway , they
would appear to be the designers choice , even though
the commercial manufacturers are not using the PSC
motors , but using cheap shaded pole motors and often
not even using the best choice of shaded pole motor for
the application :D

I have looked in dismay at the absolute junk which is being commercially manufactured and am unimpressed by most of it , knowing there is something better , and surprised that the " state of the art " is so dismal for laboratory grade instruments that a lab tech would be found at that place where " necessity is the mother of invention " for a thing that the hardware engineers who get paid the big bucks should have perfected years and years , decades ago , but they either dropped the ball or never had a grasp of it in the first place , or the bean counters insisted things be done on the cheap . Whatever the problem , for a lab instrument
I know we can do better , even if we have to build it ourselves in the way it ought to be built :D which is exactly where I am , going to build a good old Labtech Stir - Rite , model 1 , serial number 1 :D

The circuit idea development for the AC power handler for a PSC motor has been chronicled here in this forum since its
beginning on page 4 of the Power Supplies thread in the
" Miscellaneous " category . Thanks for the ideas and suggestions from other members in helping with the evolution of the circuit design . More details concerning
the concept of inherent speed regulation of the motor ,
are described in the thread where the circuit development
can also be followed .

http://www.sciencemadness.org/talk/viewthread.php?tid=387&am...

Even though this circuit idea is general for a sort of
" power supply " , it is more specifically a dedicated
device intended for a niche application , the laboratory
magnetic stirrer . So I am attaching the schematic
for the latest design revision here in this thread where
it is directly pertinent to the topic .

I welcome any comments or suggestions ,
or any simulation data which may contribute to
development of this prototype which is presently
in the parts acquisition stage .

There are a few final changes which I am still making
in the circuit before I consider this a completed design .
So in no way consider the attached schematic as being
a finished work . There are some revisions which
I will be posting with an updated schematic when the calculations are completed and the edits made for
the values of components I will use .

The attached schematic now shows the above mentioned
component revisions / additions . It is now closer to completion . In terms of arrangement of components
I believe the circuit is complete . However the values
for the components are not fully analyzed and checked .
So there may be some changes yet made , in the way of
further tuning and calibrating the system . I am mainly looking at optimizing the RC time constants for the startup pulse controller stage since that is the last section added .

[Edited on 17-2-2006 by Rosco Bodine]

AC Power Handler for Magnetic Stirrer Motor .jpg - 103kB
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[*] posted on 20-2-2006 at 01:03


There is a revision to the circuit design which was
going to substitute for the above file , but the
editing time allowed has expired . So the newest revision
for the experimental circuit is attached here .

At some point in this endeavor it may be fine to
merge the threads . I do not wish to cause any
trouble or waste bandwidth by crossposting attachments .
The attachment above can be removed with no problem
as I only wish to have the current file here with this thread .

AC Power Handler for Magnetic Stirrer Motor  555's.jpg - 120kB
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[*] posted on 20-2-2006 at 10:12


LMAO @ Rube Goldberg designs. Thanks for the joke schems.



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[*] posted on 20-2-2006 at 13:23


No problem on the schematic ,
it was a real labor of love :o

BTW , that's Professor Goldberg to you ;)

learn a little respect for your elders :D

Anyway , if you got something better for the job ,
well I've showed you mine , ........

So now show me yours ,

and we can all see which utensil is a serious tool ,

and which is a joke :P:D

All joking aside .....the theoretical model is sound ,
and was as carefully evolved as my old synapses
can process . If a spice model kicks this thing back
then it would be my assertion that the spice sim is wrong .

[Edited on 20-2-2006 by Rosco Bodine]
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[*] posted on 20-2-2006 at 15:53


Quote:
Originally posted by Rosco Bodine
Anyway , if you got something better for the job ,
well I've showed you mine , ........

So now show me yours ,


In the other thread, someone already posted one as good as I would come up with. Simulation results were even provided.

Tim




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