chief
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Power-output-controlling of MW-oven, towards Birkeland-Eyde synthesis
First check out how easy a plasma can be generated using a domestic MW-oven (1 minute video)
http://www.google.de/search?hl=de&q=plasma+microwave&...
Of course, most (cheap) MW have no power-output regulation, but get over-used and fail under continuous short-circuiting of the plasma ...
If the MW doesn't fail, the apparatus might ...
So it's time to think about _how_ such a MW may be controlled/limited ...
This is VERY DANGEROUS, because of the high-voltage/high-current electricity in the MW, so don't do it ; but if one cannot resist:
How can the circuitry be modified ?
==> Would it be sufficient to connect the magnetron to just another source of voltage, not the strong one built in ?
==> Maybe then this could be pulsed at higher frequency (not only 100 Hz), thereby having a better burning plasma ?
==> Maybe just an Inverter in the kHz-range (high-power-audio-Amp into some usable transformer ?)
==> Will microwave-radiation be generated also at an occasional DC-discharge from a capacitor ? Then just only the capacitor-Joules would go into
the magnetron, might even be quite precisely be controlled ... ? (maybe trading-in higher peak-power in each impulse, for better ignition,
multi-kW-range ?)
[Edited on 19-11-2008 by chief]
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hissingnoise
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While the idea of making HNO3 from air and water is attractive, IMAO, the NO2 output from B-E generators is so small they're next-to-worthless.
If you had no access to nitrates or H2SO4 you'd have to try it; after an eternity you'd end up with a very small quantity of ~10-20% HNO3, and that's
if you were lucky.
In any case, an NST-driven Jacob's Ladder would seem a better bet than a MWO---'Just my uneducated opinion.
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watson.fawkes
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See The Bell Jar, Volume 10 Number 3/4; it has an article entitled "Microwave Oven Plasma Reactor".
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chief
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Well, I don't mean the Birkeland-Eyde thing; I just mean the MW-regulation for other sorts of plasma-stuff; the B-E HNO3 might get more efficient at
the end, anyhow, since it's a volume-plasma, when compared to the arcing of typical B-E-reactors. Also other gases might be made to react in
interesting ways, laser-tube-experimenting could be done,
but I want a way towards solar-cell-experimenting ! Thats what I want, and some microwave-power might be useful for that.
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chemrox
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BellJar sure looks like a fun pub...
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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12AX7
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Try an inverter oven (you can tell them apart because they don't weigh a fuckton from the transformer).
Tim
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chief
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I now got myself an old 900 W MW; as long as I just want to do a furnace-setup (some absorber, good thermal insulation), might I just monitor the
power-consumtion ? As long as it's not more than 4-5 A (220 V), everything should be safe ? Or can I blow something anyhow ?
How do I know the absorber gets enough wattage out of the field, that nothing internal of the MW get's overheated ? After all these materials have
their phase-change temperatures, and might loose some susceptibility at a point ?
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12AX7
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Meh, once any ceramic gets hot enough it'll start conducting (ionic mobility) and absorb just fine. This is especially true of ceramics containing
fluxes which become molten at lowish temperatures (though the bulk resistivity might still be too high to absorb all that great). Won't necessarily
overlap the magnetic or electromagnetic range though. In general though, it looks like excellent results can be had from magnetite and silicon
carbide.
Power consumption is a good start, but a magnetron should pretty much produce constant power. Actually it might draw more current when lightly
loaded. The best check would be the temperature of the magnetron, if possible...
Tim
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domaani
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chief:
I have acquired a Prolabo microwave digestion device which utilizes a common MW-oven magnetron. It's output power can be adjusted between 30 to 300 W
by some form of PWM control. I haven't got time to test the equipment, so I have no idea of the PWM frequency and pulse forms used. Anyway, it might
be interesting to know that it's possible.
As you can see, in a commercial use the power taken from such device is well below it's operational limits. In that specific application it makes
possible to evaporate samples into dryness without causing damage to the magnetron. It's a simple power RF transmitter, which should transfer the
produced RF power to a properly matched load (here the medium to be heated), or it will have to dissipate it as heat among the loss power it makes
anyway. In most microwave oven designs it's expected that the substance to be heated absorbs almost all the RF power and the magnetron needs to
dissipate only it's usual thermal losses. Thus it's cooling system is not adequate for loads which do not absorb all the power, and using it under
those conditions makes it overheat and finally get damaged.
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chief
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I was thinking about coupling the 0.9 kW of MW-power into some metal-tubing of 10 cm diameter (therefore taking the magnetron out of the MW-oven).
In such a tube, lined with fireproof brick inside, and well insulated outside, the absorber might be placed away from the magnetron (1/2 m), and at
the opposite end there could be some extra absorber and an metal-cap, so the whole thing would be self-contained.
Although the thing might have no MW-leak it still would be operated only from a
distance, eg. 5 m. What about that idea ? 900 W of power may heat an well insula
ted volume of 2 l to more than 1000 [Cels]; also enough power would be consumed
from the magnetron by good additional absorber at the other end of the thing ...?
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triggernum5
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Be really careful if you take out the magnetron, stray microwaves will destroy your eyes before you know what happened, so you'll need to take care
with waveguide coupling..
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chief
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Thats why I was talking about letting the thing run from a minimum distance of 5 m. Or would that be not enough ? As I understand: The MW-power cannot
be focussed much sharper than it's own wavelength, which at 2.5 GHz is more than 10 cm; so at a distance it should be distributed ?
Anyhow I'm just asking precisely of this question: I'm not willing to take any risk on that !
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12AX7
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Easy way to tell, if you feel radiant heat (microwaves heat much the same as IR, which is also hazardous to the eyes if you stare at it for a while),
it's too close or too unshielded.
Tim
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triggernum5
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I'm kinda hazey on details, but when I researched the threat a while back, it seemed that the risk could be alot harder to pinpoint than discomfort,
as effects/pain can lag the event..
One vague number I do remember is that anything above 1mW/cm^2 should be considered chronically dangerous, and 80-100mW/cm^2 or higher should be
considered more immediately dangerous..
I seem to remember finding alot better info when I researched this in the past..
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chief
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If these numbers are true, then I'm just gonna let the magnetron inside the oven.
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Twospoons
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Going through the numbers: if you are radiating 1kW (isotropically) then the 1mW/cm^2 level is achieved at a distance of 2.9m. In practice you wont
be leaking 1kW, unless your waveguide construction is unbelievably bad. The leaks wont necessarily be isotropic, but I still think your planned
distance of 5m should keep you safe, even if you completely screw up your waveguides.
You can also buy microwave oven leak detectors - they're just a little meter with an antenna and rectifier - just for peace of mind.
Helicopter: "helico" -> spiral, "pter" -> with wings
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triggernum5
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I just remember thinking that a worst case scenario sounded like what happened to Homer when he declined the eye-drops after laser surgery..
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Texium
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Thread Moved
19-11-2023 at 12:45 |
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