Running an empty microwave oven

Peter-

If there is nothing in the microwave, there is no load, or at least very little. If there is no load, there is no power dissipated. It is somewhat like having 120 VAC at the wall socket with nothing plugged-in.

The problem isn't heat as much as voltage. With no load, the magnetron voltage will be higher than normal. The question is really whether or not damage will occur from over-voltage.

Someone in another thread a few weeks ago, said that only very early microwave ovens would be damaged by running them empty. I know that one I bought in 1976 came with a warning about running it empty, as well as not putting anything metallic in it. However, two that were bought about ten years ago did NOT come with such warnings, and even came with metal racks!

Fred

While this is true, relative to the normal operation of the oven, I don't think it is quite correct to characterize the unloaded oven this way.

Here's why I say this:

If there is no additional load (an empty oven) there is still a load, but it's mainly reactive. This means that the energy the tube generates reflects, rather than being absorbed. There is a high SWR (Standing Wave Ratio). When the returning comes back to the tube, it sums vectorially with the outgoing wave. This may result in anything from zero to twice the magnitude RF voltage. Put another way, the phase of the reflection may be anything from an open to a short, including everything in-between. As a result, the tube may see *either* excess voltage or excess current.

But, there is in fact always *some* load in the system. This is what causes real energy (heat) to be dissipated within (mainly) the tube. For the unloaded (no user supplied loss) case, these intrinsic losses, R's, whether from dielectric or conductive elements, will tend to see higher voltage or current than they normally would. The power lost in these R's, is generally related to either I^2R or E^2/R. But for the case of doubling(say) voltage or current these loses quadruple.

As a consequence *more* power may be dissipated *within the tube* when there is no user-supplied load than was the case with straight-and-level operation with a normal cooking chamber load.

This increased dissipation along with the potential for increased current and increased voltage within the tube may cause things to be tougher on the tube. Often power devices are specified to operate into some maximum SWR and it may be that modern magnetrons can no take whatever is likely to be thrown their way.

Exactly what happens is probably a pretty complicated function of the whole assembly, dimensions and such. But it isn't unreasonable to think that the tube might get hotter, in addition to being stressed in terms of it's voltage and current limits.

g

I don't think you're right. The magnetron is converting electricity to microwaves and transmitting them, at a rate of several hundred watts. They come out of the magnetron and bounce around the oven until they are absorbed by water. If they are not absorbed, they will eventually bounce back into the magetron and cause damage.

I just bought a very cheap one (£27) which DID come with a warning. It says "damage to the oven may occur".

The metal is only a problem in certain shapes I believe. All it does is provide a short circuit from point A to point B for the microwaves. If the metal is thin (eg. gold plating) it will be damaged by heat, just as a thin wire would be conducting electricity.

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it?

or a build up of microwaves over a certain level?

concerned one of my pet parrots will switch it on! (Seriously, they do stuff like that)

It's absorbed into water, in lakes for example.

A closed microwave has nothing that can absorb it.

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Advice for office managers:
If you keep the sexual harassment complaint forms in the bottom drawer, then
when a woman gets one out you'll get a great view of her arse.

I have had them chew through the flex.

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When eating a tongue sandwich, how do you know when you've finished?

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Some of the ones we had were quite cheap, actually some were pretty awfull, and some of the controllers we had to make had to be so cheap it was a shame. the basic magnetron/oven arangement was hardly any different from the cheap to expensive ones.

keep

turned on they'd probably get a fright and fly off anyway.

I doubt it would go pop after 5 minutes just becuase it was empty, I would be even more surprised if it became dengerous after 5 minutes on empty too, either way the manafacturer would probably have a liability waiting to happen.

If you do decide to try it empty despite what the manafacturer tells you its on your own risk ofc, make sure the glass tray is still in as that does absorb some microwave energy.

a CD makes an interesting display when you put it in the microwave.

door requires a hard pull (no button to push).

One of my cats managed to repeatedly get out of a locked cat flap. I couldnt figure out how untill I watched once. animals can surprise you sometimes.

Colin =^.^=

break it?

overheating, or a build up of microwaves over a certain level?

I'm concerned one of my pet parrots will switch it on! (Seriously, they do stuff like that)

The glass tray will usually absorb some of the microwave, basically when its empty the electric field builds up to such a high value that it eventually gets absorbed by something somewhere, or it ends up disipating in sparking wich can be quite spectacular if have two bits of unconnected metal close to eachother.

Colin =^.^=

it?

or a build up of microwaves over a certain level?

concerned one of my pet parrots will switch it on! (Seriously, they do stuff like that)

Same question, where do the light waves go from the light inside the microwave? Same energy, just higher frequency and lower power. Same place as the microwaves.

Brainiac Science Abuse (TV program) put a set of five swinging balls (one of them office toys) in a microwave on its own. The microwave exploded spectacularly after less than a minute. Can you explain that?

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If there's a fire why not just open the fire exit to let it out?

it?

or a build up of microwaves over a certain level?

concerned one of my pet parrots will switch it on! (Seriously, they do stuff like that)

Light is absorbed into all sorts of things. Microwaves are NOT absorbed into anything inside the oven. They are reflected completely by the metal sides.

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Two cowboys are riding along a trail in the mountains when they suddenly hear
tom toms beating very close to them.
"Oh! That doesn't sound good," one says to the other.
As soon as the words were spoken, an Indian jumps out from behind a tree and
said, "Yeah, our regular drummer is out sick."

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of them office toys) in a microwave on its own. The microwave exploded spectacularly after less than a minute. Can you explain that?

From what ive seen of braniac they probably filled it with something that would explode spectacularly. I wonder what the balls were made of, something explosive perhaps ? ordinary flour or paper dust can make a very good explosive if mixed with air.

Colin =^.^=

break it?

overheating, or a build up of microwaves over a certain level?

I'm concerned one of my pet parrots will switch it on! (Seriously, they do stuff like that)

into anything inside the oven. They are reflected completely by the metal sides.

not completly, even if they were silver coated it would still not be quite

100%, it bounces back and forth so quickly eventually even the smallest loss gets multiplied suficiently to absorb considerable energy.

Colin =^.^=

I suggest that the main problem is your desire to have 14 parrots in your house.

geoff

While the microwave is bouncing around in the oven there will be locations where the signal combines to generate high voltages and other places where high currents are generated. These locations can be in side of the Magnetron.

Bill K7NOM

that's not happening not unless those sides are superconductive

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Bye.
   Jasen

There's also the matter of the efficiency of the magnetron changing when presented with a very reactive (reflective) load. In addition to the tube getting hotter from the larger voltage and current peaks which cause increased microwave losses inside, the tube no longer is as effective at producing microwave energy and a higher percentage of the input energy gets converted directly to heat.

g

You mean they were lying?!?

A colleague suggested that large metal objects might "short circuit" the magnetron and draw more power from it than it expects. I didn't agree or disagree, I know nothing about that sort of thing.

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cleaning. Now, can you give me the number for Jack?"

On Mon, 29 Jan 2007 00:40:56 -0000, Bill Janssen wrote:=

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As long as it doesn't explode. I have a good mind to try it with my old= one. Outside.

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This exchange was overheard between the separated sections of the jail.
A male voice yells over to the female side: "I got 12 inches over here y=
ou would love to have."
The female response was: "Well, spit it out it isn't yours."

13.

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