Hot plate for magnetic stirrer

[Fra]

hi all. I have already built a stirrer. works fine, but I have to use a hot water bath to work with high temperatures. so i would like to add a heating plate. I was thinking about using 2 thin aluminum plates and putting a tungsten filament in the middle. They are not ferromagnetic, so they should give no problem with the stirring. I was thinking about using the tungsten from a light bulb an the aluminum from a disposable cocking thing. about the power supply, I'm planning to use a 10v at 700mA one.is it enough? note I'm aiming for temperatures of at least 150°.
The alternative is putting a resistance from an hair dryer in an oil bath. easier, but I don't expect great safety.

[InspiredByMe]

Tungsten wire in light bulbs is to find for an application like this. Try nichrome, especially if you can find it thick, it can be used for a heating mantle.

[asilentbob]

I'd say just find a heating mantle that is the right size and just put it on top of the mag stirrer, with a bit of stand-off if needed. You generally use a variable transformer to control the temperature of the heating mantle.

[Sentinel owl]

Tungsten wire is way too thin and will go POP when exposed to normal air while heated.

What you want to do is cannibalize a hotplate and use the heating element from there. Way, way safer than anything else.

[Zip]

I'd say just find a heating mantle that is the right size and just put it on top of the mag stirrer, with a bit of stand-off if needed. You generally use a variable transformer to control the temperature of the heating mantle.

What he says, also you can make your own with a good FeCrAl resistance wire like Kanthal D (http://www.matweb.com/search/datasheettext.aspx?matguid=35587ef8ab304155a732aff c191329c8) or copper-nickel alloys for lower temperatures.

Probably easier to cannabalize an existing piece of laboratory equipment though.

[eesakiwi]

Find a rice cooker.
They are a aluminium plate with a hole allready inside it.
They are made to stand up off its base a bit too.

[eesakiwi]

I used a rice cooker, cheapo sort.
A large computer fan, its metal & about 6 inch square. Its 240 Volt AC, a set rpm (Damn!)
You need a VERY STRONG fan, a little 12 V fan is far too small,

Several computer hard drive magnets, very strong, easy to get.
This sort, http://www.reuk.co.uk/OtherImages/hard-drive-magnet-polarity.jpg

Wooden baseboard & electrical wire, clips, switches, cords, screws etc.

This shows the hotplate (its not a great pic but if I can find another, I will)
http://www.made-in-china.com/image/2f0j00CRrtnDEoVQmTM/Rice-Cooker-T18822-.jpg
http://www.sinewavepower.com/images/heater12.jpg
http://www.k8gu.com/repair/ricecooker/inside-insulation.jpg
The last pic shows what looks like an exact version of mine, its got the same 360 watts cast in it.
You can see the round bit in the center with the goldish looking lever across the plate.
The center bits got a spring in it, when the rice has absorbed enough water it moves the lever arm & turns off the power switch. All that gets removed.

You can also make out the dish in the center under the hotplate.
It can be turned over & if theres a little metal cut out, the element will fit on the reverse side.
That takes care of the housing.
I cut off all the little bits of protuding aluminium from under the hotplate, except for where the 3 screws screw in.

I used a metal fan that looks like this for the stirrer motor.
http://media.digikey.com/photos/Comair%20Rotron/028422,%20028423,%20028316.jpg

Unfortunatly its only got one speed. I have read that by putting a diode inline with the power you can slow one of these moters down a bit, with a loss of power.
Brush motors can have their speed adjusted, permanent magnet motors can't.

I had to cut off the corners of the top side of the fan (where it mounts up on one side) so it will fit closer into the housing.
Also all the fins were cut off & the little lumps ground off with a grinder (or file).
The fan fits perfectly inside the housing of the rice cooker.

Then I cut a tin baked beans can end off about 1 inch from the end.
That fits over the center of the fan motor & was superglued in place once I got the tin 'on center' with the fan.
Thats to help stop heat getting to the fan & melting the plastic.
It also mounts the magnet(s).

The fan was packed up a bit (10mm? each ones different, you work it out.) on the baseboard so the top of the motor part is as close to the bottom of the element as possible.

The rice cooker housing & element is put over the fan & fixed in place.
You will have to drill a largeish hole/slot into the baseboard to get clearance around the ends of the element where the wiring connects to it.
Theres also another hole needed for where the fans wires come out.

Now look down the center of the element & drop the magnets in there & spin the fan a bit.
For some reason they just wanted to sit in the center of the metal can housing on the fan center.
Now I have used 3 magnets stacked on each other, firstly for strength, it makes a big difference to the stirbar.
Secondly for height, you need at least two to get up thru the Ali element hole.
It also seems to 'pull' the magnetic feild in a bit.

There might be an advantage to placing a smaller magnet on the top magnet at each end of it to bring the feild in better.

Superglue the magnets in place.

Because the element has a convex (rounded over) top plate, I ground it flat in the center, theres a 2mm lip inside the hole, I just ground it down to that height with a grinder.

It may not be nessary to do that, I'll place a sheet of glass fiber over the element when its in use anyway, that'l fill up the gaps & spread out the heat on the flasks bottom, also protect the magnet from the heat.

If I could post pics I would.

[eesakiwi]

I tryed using a diode inline with the power & the stirrer just sat there, wobbling a bit, so that won't work.
I chucked a zener diode in & it was a little better, it won't work, but maybe where I got that idea (to use a diode). Maybe it might work if the correct zener diode is used but I'd rather go to the trouble of getting a electric motor that I can control the speed with.
I think the motor must have carbon brushes & copper windings on both the armiture & the outside part to be able to be controlled by the speed controlers I have.
But with that idea I have to throw away the fan motor & come up with a whole new motor & throw a bunch of time into a housing.

The fan motor will stirr water, but its RPM seems too high & every now & again the stirrer bar jumps off & I have to go thru the 'startup' of the stirrer bar again.
Thats no good, I want something that works 100% of the time.

So I'm gonna go to a few second hand stores & look for something better, damn.

[MunkeyQ]

The motor doesn't have brushes - it's an asynchronous induction motor. It induces eddy currents in the fan hub which causes it to turn.

Putting a diode inline rectifies half the AC waveform, meaning the field coils aren't doing their job.

To alter the speed you can't change the voltage as the motor will spin at the same speed because it's still 50/60hz. It will however get very hot. You need a VFD (variable frequency drive) circuit, which changes the frequency and voltage at the same time. In this case, the voltage is changed proportional to frequency.

For your purposes, a motor from a microwave plate rotator might be a good bet. They're reluctance motors, which means direction depends on whereabouts the AC waveform is when you turn it on...it's essentially random. The motor has a handy reduction gearbox built in, but I don't know how fast stirrers need to be though.

[eesakiwi]

I had a look for motors at home & found one that has a small dia metal fan at one end.
Its off a food mixer I think, the control panel has 3 speed settings & a 'pulse' function.
I plugged it into the motor speed controler & was able to control the speed OK. Happy with that. Its real loud while on 100% but quietens down as soon as you drop the speed down a bit.

There was a mounting plate, pretty heavy, which is good, guess that there had to be a heavy plate as it holds the bearing for the shaft of the actual mixer/chopper part.

I found a tallish round biscuit tin & cut the mounting plate down so it would fit inside the lid.
Found the center of the lid & fitted the hotplate to it, 3 screws, cut hole for the ends of the element to go thru.
Fitted the mounting plate to the underside of the lid, using 3 bolts for alignment.
Ground down the fan dia to fit inside the hotplate hole, flattened out the fan blades a bit too. Have to get some superglue to stick the magnets there.

Thats about it so far.

[313]

damn you guys are samrt

[Kwinnie Bogan]

damn you guys are samrt

samrt? I think the word you were trying to say here was "clever".