Upsilon
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Making my own magnetic stirring hotplate
Hotplates, especially ones with magnetic stirring capability, seem to be really expensive. I'm up for the challenge of building my own for much
cheaper. I'm not looking for anything capable of really high temperatures; just enough to boil water would be sufficient. The magnetic stirring part
seems pretty straightforward to me - just a simple magnet mounted to a motor underneath the heating element. Now, for the heating element, I can
either order some nichrome wire and make my own (the problem would be insulating it) or I can buy a generic stovetop heating element off of eBay.
Since those are meant for high-heat, though, I'm not sure how well they'd work with significantly less current flowing through them. Finally I'd just
need a power adapter, a couple variable resistors, and some electronics knowledge to put it all together. Any suggestions?
And depending on how well this goes, I may up the ante and build an induction heater for very high-heat operations.
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Texium
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I bet that if you look harder on ebay and other sites you'll be able to find a decent stirring hot plate that is affordable. I got mine on ebay for
$45. It is a 1970s model, but it still works like it's supposed to and has a nice ceramic top that is completely spotless and undamaged. It's a bit
slow to heat up sometimes and the stirrer magnets are not quite as powerful as the ones in modern hotplates, but I can't complain. If I really wanted
to fix the magnets I could easily replace them.
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careysub
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I am building one right now. I can tell you my approach, yours would likely differ since mine is based on some things I already have.
I am using a 120mm computer case fan (which I had) with a $2 Chinese pulse width modulating control I ordered yesterday from eBay for the motor, with
neodymium-boron hard drive "supermagnets" (which I had). You can disassemble a dead or no longer needed hard drive to get these (where I got them),
but they are available cheap on (you guessed it).
The fan body makes a good solid mounting structure, so I will put an aluminum pie plate over it with spacers so the magnets are just under the
aluminum. I am pretty sure the plate bottom is thin enough that the magnetic field will penetrate sufficiently (among common metals, aluminum is one
of the poorest magnetic shields).
For the heating part, I plan on using a second hand heating mantle, with I have (I also have a variac).
I plan on mounting it so that there is a little bit of air circulation driven by the fan under the pie plate surface so that heat build-up can't
affect the (not oven ready) case fan.
One heating scheme I was considering was attaching with screws 100 W, 350 C rated, power resistors ($7.13 each from Digikey) to the bottom of an
aluminum plate (1/4" perhaps). Size of plate and number of resistors varies with the desired size and power of the unit. If using a thickish aluminum
plate I would drill a hole in the center the size of the magnet assembly and put thin aluminum over the top so that the magnet is not shielded. More
powerful resistors are available. With low ohmage these could be driven with a 12 V power supply (like a computer case power supply, free from a
junked computer, but otherwise only about $15) and a dimmer switch. The Curie point on NeB magnets is at least 300 C by the way, so it you keep the
temperature of the heating surface below this magnet should not lose its magnetization.
[Edited on 19-9-2015 by careysub]
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alive&kickin
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zts16, where's the fun in that? careysub, I guess my electronics knowledge ain't what it should be (way short of competent). I have a question about
the heating part. If you have a variac for temperature control, wouldn't it stay on constantly at any temperature or am I just being a little on the
stupid side (probably proves for sure that I don't know what I'm talking about)? If it's not too much trouble, is it possible to see your schematic
of one used in this situation (even a U2U would be fine). Like I said, I'm way short of knowledge on this. Early thanks for any help.
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Texium
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Well, I'm no electrician, just a chemist. So to me there's not really any fun in making something as
complex as a stirring hotplate when I know it would probably turn out to be a piece of crap!
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alive&kickin
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zts16, never underestimate what you're capable of, you might surprise yourself! You never know until you try and you might find another enjoyable
hobby.
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Upsilon
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That's a pretty resourceful approach, careysub. I have a bunch of raw materials and basic electronic parts around my house, so I'll probably be making
it from scratch. I've got some motors, aluminum sheets, and some old light dimmer switches to use as potentiometers. Only things I'd need to order are
the magnet(s) and heating element. I've discovered this Kanthal wire stuff that's a lot cheaper and can tolerate higher temperature than nichrome (not
that it'd be getting that hot anyway). For insulation I've decided just to wrap it up in Teflon tape (they use it in cookware so it won't be an
issue). I must ask though, what are you going to use for a power source? An old power adapter you have laying around?
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aga
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If you're heating with Mains electricity then see if you can find an old broken electric heater in the bins - the type that has 2 or more 'bars' wound
with wire.
The wire is the correct type, and the correct length for your country's voltage.
Electronics is a bit like everything else - once you learnt he basic building blocks, it's all a whole lot easier.
For Heat control of a Mains powered heater you'd use a Triac with zero-crossing detection in the triggering circuit.
If that sounds too complicated just use a Light Dimmer rated at least as much as your heater - they use exactly the same technique to dim the lights.
Just remember that messing with Mains electricity is always dangerous, and can easily kill you.
The stirrer is easier, and a PC fan could work.
Personally i'd use a DC motor and add a longer shaft so the motor was right down at the bottom of the rig, away from the heat.
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careysub
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Quote: Originally posted by alive&kickin  | zts16, where's the fun in that? careysub, I guess my electronics knowledge ain't what it should be (way short of competent). I have a question about
the heating part. If you have a variac for temperature control, wouldn't it stay on constantly at any temperature or am I just being a little on the
stupid side (probably proves for sure that I don't know what I'm talking about)?
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No you are perfectly correct, this does not include thermostatic control feedback.
Heating mantle set-ups do not normally include those, you adjust the power by adjusting the voltage going in, and tweaking it to settle at the right
heat output. When thermostatic control is used you normally have a "bang-bang" system where it switches between full on and off as the heat dictates
so a thermostatically controlled relay is used instead of a variable voltage source.
I have a separate project in the works for a thermostatically controlled heat bath (using aluminum shot) that does not stir. I am using one of these
for the control:
Lightobject ETC-JLD612-A Dual Display PID Temperature Controller
Currently $28 on Amazon, which $8 cheaper than the price I paid two days ago. For that build I plan to use a 12V computer power supply for the
constant voltage.
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careysub
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| Quote: Originally posted by zts16 | | Well, I'm no electrician, just a chemist. So to me there's not really any fun in making something as
complex as a stirring hotplate when I know it would probably turn out to be a piece of crap! |
You have to like making things to make it a good choice.
I am a software architect by profession, and an "architect" by nature - I like designing things. I design and build telescopes as a hobby currently,
and am building my own portable UV light for fluorescent mineral collecting.
I usually don't do "cookbook" projects (not even when cooking) - I look at project plans and think "now how would I like to make it?"
and use other plans as a jump-off point.
Typically a DIY project is not perfect on the first go, and requires a little tweaking after you make it. This is part of the fun and with I have
confidence from experience that what I choose to build will work, once the bugs are worked out.
Some people insist on building their own electronics, using Arduino or what-have-you, so far I have resisted this and look for components like the
ready-made controller I mentioned here. Minimizing parts costs is usually a guiding principle. I can get ready made controllers for cheaper than an
Arduino build.
A key reason for building your own equipment is to get something exactly the way you want it - an equivalent as a commercial product is either
unavailable or far more expensive.
My stirring hot plate build is to provide one suitable for micro-scale work. I have a used Thermolyne stirring hotplate for large vessels, but it is
quite unsuitable for a little 10 mL RBF. For this I have a little heating mantle, but no stirring, thus my stirrer build.
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careysub
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| Quote: Originally posted by aga |
Personally i'd use a DC motor and add a longer shaft so the motor was right down at the bottom of the rig, away from the heat. |
Gluing a spacer to the hub of a case fan accomplishes the same thing.
If you need to buy the part, then I would also look at a separate DC motor, unless you want the case fan to use the body as the stirrer structure.
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aga
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Main difficulty with gluing on an Axle is getting it centred properly.
A standard DC motor comes with the axle/output shaft, so all you have to do is add length.
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careysub
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| Quote: Originally posted by aga | Main difficulty with gluing on an Axle is getting it centred properly.
A standard DC motor comes with the axle/output shaft, so all you have to do is add length. |
Ah, but then you have the problem of mounting the magnets so that they are balanced properly. That is the real difficulty (such as it is).
It is actually not hard at all to mount the spacer (a length of 1" dowel) accurately centered on fan hub (which is 1.5") - you can quite accurately
center it by eye. It is a little trickier to get the magnets placed correctly, but possibly easier than when using a motor shaft.
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zed
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Neodymium Magnets are kinda heat sensitive. I like the powerful magnetic linkage that would ensue, but I worry that the Magnets wouldn't fare well
under heated conditions.
Alnico Magnets on the other hand, are less vulnerable to heat, are fairly strong , are cheap, and you could compensate for their lesser strength,
by using large ones.
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careysub
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| Quote: Originally posted by zed | Neodymium Magnets are kinda heat sensitive. I like the powerful magnetic linkage that would ensue, but I worry that the Magnets wouldn't fare well
under heated conditions.
Alnico Magnets on the other hand, are less vulnerable to heat, are fairly strong , are cheap, and you could compensate for their lesser strength,
by using large ones. |
I did some more research on this and it looks like much above 75 C a hard drive magnet may start to lose some magnetization permanently (though
apparently tall cylinder Ne magnets can go much higher, up to 140 C). Hard drives can encounter temperatures up to 72 C in operation:
https://www.kjmagnetics.com/blog.asp?p=temperature-and-neody...
(This is way below the Curie temperature.)
For my heating mantle stirrer I am not concerned. It should not get hot. I also have ceramic paper insulation from a pottery supply place to use as
insulation if needed. For a hot metal plate heating surface design it is more of a concern.
Alnico magnets seem the best choice for high heat applications (up to 450C);
http://www.duramag.com/techtalk/alnico-magnets/alnico-magnet...
And again, apparently taller (longer) magnets apparently perform better than short or flat ones. The Duramag site recommends a length:diameter ratio
of 4:1.
[Edited on 20-9-2015 by careysub]
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Upsilon
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I've been looking at the resistance per foot of Kanthal wire and it is surprisingly still pretty low. To make a 250W heater, I need something around
500 ohms of resistance when plugged into a 120V receptacle. I guess I am underestimating just how thin the actual wire in a typical heating element
is; it must be hair-thin to be able to provide the resistance necessary to generate the heat it does.
[Edited on 20-9-2015 by Upsilon]
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eesakiwi
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I make a magnetic stirrer/hotplate from a rice cooker element. google image them.
The hotplates got 3 mounting holes underneath and a hole in the centre.
I used a long electric motor which had fan on one ed nd straighteed out the blades and mouted two flat magnets from a harhdrive on it.
The mgnetic field was wrong and \i tried to add two small magnets to the ends of the flat mgnets to make two single magnetic fields that rise from the
magnets.
The fields were not as high as I liked, the round magnets didn't want to stay in place. The ehat messed with the glue and the cetrafugal effect took
over....
So I brought 2nd hand stirrer to see how it works.
The magnets are just rod magnets.
But what they do is machine a solid cylinder of Aluminium about 2 inches dia and 2 1/2 long.
In the base they drill a hole to fit the motor shaft exactly, and a cross hole for' the grubscrew to hold it in place.
That makes it pretty balanced.
Then then drill two 3/8thdia holes (9.5mm id) in from the end of the cylinder, just inside the outside dia.
This fits two 3/8 dia by 1 inch cylinder magnets which are pushed in to the two holes and glued or such in place.
This means that the magnetic field is two mushroom shaped balls above the solid cylider of Aluminium.
The way they varied the speed was with a wire wound resistor. (?!) And they used a common electric motor, without brushes, (induction sort) thats
found in everything. Washing MC pump motors, older record turntables, fans etc etc. The fan motor from a microwave oven.
I hope this helps.
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