Tiny Induction melting furnace

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Hi all

I have a need to make a very small laboratory type induction melting furnace for melting about 100 grams of stainless steel pellets - I cannot use any other device that uses oil for burning.

On searching I could find only details of large systems.

Can any of you please help me with a schematic or any other help, maybe a URL for a site?

TIA

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Sandy Archer
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The photos are certainly nice, Tim, but give us some details we can digest and provide feedback upon.

What frequency and duty cycle when under use, how much power drawn from the +/-17V supplies, etc. Some details of the half-bridge, transistor type, drive stuff (25V gate drive?), any snubber, etc. Details about the resonant tank, please.

--
 Thanks,
    - Win

It is possible to make a microwave oven crucible for melting many metals. If this interests you, search using google with the following three words: microwave oven crucible.

Here is the URL for the first hit, which should be enough to whet your appetite:

Apart from that, I have no idea how to make an induction furnace, but I suspect that the schematic is not the most difficult part of the job.

--Mac

Cool. Looks like the floor in my "play" room. ;-)

LECO ( Labratory Equipment COmpany) makes ( made) just what you want......I used to repair them for a local steel mill. Consisted of a 304TL tube as a simple colpitts oscillator with the coil made of 1/4" OD copper tubing. The crucible sat inside the coil. Don'y know if LECO is still around but it's another avenue to try. It was simple and reliable........not cheap though.. Jay

Shouldn't the resonating capacitors be soldered directly to the heavy tubing of the tank inductor?

Yes, I replaced the 3uF saran wrap mega-ESR thing with some polypropylenes soldered on, I don't have a picture of it currently. It handles 100-200A reactive pretty easily.

Tim

-- "California is the breakfast state: fruits, nuts and flakes." Website:

Whatever's needed, it's probably at 30-40%. I haven't scoped the driver in a while, since it's not as finicky as the tank. ;o)

F = tank, right now it's got 4uH + 6uF, you know the frequency.

Mmmm, haven't measured that, can't be more than an amp.

The bridge is powered by a 100V 10A transformer (the clamped MOT) so the circuit gets somewhere around 110-130V at up to 5 or 10A; I haven't measured current consumption there either (and it just so happens I'm out of a DMM, so I'll have to dig out my VTVM I guess).

12V peak gate drive, looks something like this: Except a bit higher duty cycle of course. Transistors = 6 x STW11NB80, sooner or later I'll wind a larger adjustable matching inductor so I can bring up the voltage to suit.

Nope. Can't detect any dissipation at 200-500W output, so it must be switching fast enough (maybe 90% effiency). You could add some C to the bridge output to slow transitions if you want.

30uH (I need variable 0-300uH to step up the voltage and power) series choke to match bridge to tank. Uh 20uF or so coupling cap between the bridge and Lmatch, since I don't have a bipolar supply.

Work coil is 1/4" Cu tubing either 2" dia. 2" tall 5T = 1uH = uh 65kHz IIRC, or 3 1/2" dia 3" tall 7T = 4uH. Cap bank made up mostly of 0.22uF 400V polypropylenes from Digi-Key (uh, PF2224-ND).

Oh, and the hottest thing I've had so far is a titanium strip, with help from a ferrite rod to concentrate flux. Was a good orange glow (using the

1uH coil).

Tim

-- "California is the breakfast state: fruits, nuts and flakes." Website:

Ah, yes. I didn't recognize that wad of foil as a capacitor.

heavy

polypropylenes

100-200A

I didn't recognize this one either. Scroll down 2/3s

Maybe I should make one and put it on the LC meter...

His is much better, a noninductive wind. I just taped aluminum foil on for the terminals, so you can imagine the ESR! ESL was negligible though, being only six feet long and under 5 thou thick.

Tim

-- "California is the breakfast state: fruits, nuts and flakes." Website:

Seems do-able to me,but i'm no expert here.. I'd think the two main problems would be to make a small enough coil,that could handle enough current without melting..But I suppose you won't need a whole lot of power for a small workload. Maybe some solid copper house wire,or large gauge enameled magnet wire.

The other might be the frequency,The frequency will be higher with a smaller coil..But perhaps that isn't much of a problem either.There are plenty of SMPS circuits to get ideas from,etc. And they usually run at a fairly high freq.

Humm..maybe there aren't any real 'problems',just 'differences'. Go for it!

Absolutely...mind that the crucible is conductive. Moderate resistivity is best, I wouldn't go with ferromagnetic materials personally. Something like titanium would be good. Maybe 300 series annealed stainless.

Just hook up a little power LC oscillator. Well, like how I started...look on my website.

Tim (re-redirecting replies to ABSE+SED since I'm not subscribed to the latter)

-- "California is the breakfast state: fruits, nuts and flakes." Website:

["Followup-To:" header set to sci.electronics.design.]

I want to horn in about this: A colleague of mine recently asked me if it was possible to build a supersmall induction heater for heating organic molecules in a UHV chamber. The crucible is about 5mm (dia) x 5mm. Induction heating (to about 300 C) would be elegant because it would permit easy transfer of the crucible into and out of the chamber wihout having to bother about electrical or thermal contacts for resistive heating.

The coil would be about pencil diameter. I don't know it if is possible to build a working furnace with such a tiny inductance and haven't had time to look into the matter yet.

Suggestions? --D.

I read in sci.electronics.design that Daniel Haude wrote (in ) about 'REALLY Tiny Induction furnace (was: Tiny Induction...)', on Fri, 22 Apr 2005:

The heat losses are greater in proportion for small coils, so lag the coil with rockwool or something more attractive. Embedding the coil in compressed powdered magnesia (as used in MICS cable) is a possibility. That would increase the self-capacitance of the coil; MgO has a dielectric constant of around 7.

Overcome the small inductance by using a larger tuning capacitor, so that you are not radiating outside the relevant ISM band. This low-impedance solution minimises the Q (and maybe f) change when you insert the load.

--
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\'What is a Moebius strip?\'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

It is possible, I saw such a beast as a graduate student. It was used for evaporating all sorts of materials for investigation of thin film semiconductors. The big RF cabinet looked as if it were a commercial design but it's been a while so I may be mistaken.

The coil was perhaps twice the diameter of a pencil, maybe less. Ah, the fraility of memory.

Robert

I once made an induction heater out of a 100W audio amp and a 20 turn coil (plus caps etc) about 20mm dia. Worked fine - even heated a small screwdriver to red heat.

--
Dirk

The Consensus:-
The political party for the new millenium
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I wonder how hot the sample would get if you put it in a quartz crucible and hit it with several IR laser diodes? I do know that quartz doesn't absorb IR. Most organic molecules do.

Not my area of expertise for sure, so I may be talking complete nonsense.

--Mac

Tim et al,

Is there any reason to go to the trouble and complexity? Why not just pass 100A at 5V to terminals on the crucible and make it act like an electric stove?

Mike Monett