Maker Pro
Maker Pro

Running an empty microwave oven

S

Snap Whipcrack..............

Jan 1, 1970
0
Peter said:
But where do you think the power ends up if it is not absorbed?
It dissipates. Where do you think the microwave transmitters on mountain
tops power ends up? It doesn't go round and round the earth forever.
 
F

Fred McKenzie

Jan 1, 1970
0
"Peter Hucker" <[email protected]> said:
But where do you think the power ends up if it is not absorbed?

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
 
G

g

Jan 1, 1970
0
Fred said:
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.

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
 
P

Peter Hucker

Jan 1, 1970
0
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.

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.
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,

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

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.

--
http://www.petersparrots.com http://www.insanevideoclips.com http://www.petersphotos.com

These questions about South Africa were posted on a South African Tourism
Website and were answered by the website owner.

Q: Does it ever get windy in South Africa? I have never seen it rain on TV, so how do the plants grow? (UK)
A: We import all plants fully grown and then just sit around watching them die.

Q: Will I be able to see elephants in the street? (USA)
A: Depends how much you've been drinking.

Q: I want to walk from Durban to Cape Town - can I follow the railroad tracks? (Sweden)
A: Sure, it's only two thousand kilometres, take lots of water...

Q: Can you give me some information about Koala Bear racing in South Africa? (USA)
A: Aus-tra-lia is that big island in the middle of the pacific. A-fri-ca is the big triangle shaped continent south of Europe which does not... oh forget it. Sure, the Koala Bear racing is every Tuesday night in Hillbrow. Come naked.

Q: Can you send me the Vienna Boys' Choir schedule? (USA)
A: Aus-tri-a is that quaint little country bordering Ger-man-y, which is...oh forget it. Sure, the Vienna Boys Choir plays every Tuesday night in Hillbrow, straight after the Koala Bear races. Come naked.

Q: Do you have perfume in South Africa? (France)
A: No, WE don't stink.

Q: I have developed a new product that is the fountain of youth. Can you tell me where I can sell it in South Africa? (USA)
A: Anywhere significant numbers of Americans gather.

Q: Are there killer bees in South Africa? (Germany)
A: Not yet, but for you, we'll import them.

Q: Are there supermarkets in Cape Town and is milk available all year round? (Germany)
A: No, we are a peaceful civilization of vegan hunter-gatherers. Milk is illegal.

Q: Will I be able to speek English most places I go? (USA)
A: Yes, but you'll have to learn it first.
 
P

Peter Hucker

Jan 1, 1970
0
It dissipates. Where do you think the microwave transmitters on mountain
tops power ends up? It doesn't go round and round the earth forever.

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

A closed microwave has nothing that can absorb it.
 
P

Peter Hucker

Jan 1, 1970
0
Peter Hucker wrote:

(Seriously,


Aha. My Cats like to turn on my Vaccuum cleaner and I get shocked, in
the first moments. :)

(Seriously, I have left the Vacuum cleaner connected to the wall-socket
many times, indeed. But it was never on, when I came back home)

I have had them chew through the flex.
 
C

colin

Jan 1, 1970
0
Peter Hucker said:
Sounds reassuring. But does this also apply to cheap ones?

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.
Most likely they'd turn it slightly, about 5 minutes. When the thing
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.
I can't see them opening the door. The like to twist and chew stuff. The
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 =^.^=
 
C

colin

Jan 1, 1970
0
Peter Hucker said:
Peter said:
I'm 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.

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 =^.^=
 
S

Snap Whipcrack..............

Jan 1, 1970
0
Peter said:
But where do you think the power ends up if it is not absorbed?
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.
 
P

Peter Hucker

Jan 1, 1970
0
Peter Hucker said:
Peter Hucker wrote:

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.

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?
 
P

Peter Hucker

Jan 1, 1970
0
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.

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.

--
http://www.petersparrots.com http://www.insanevideoclips.com http://www.petersphotos.com

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."
 
C

colin

Jan 1, 1970
0
Peter Hucker said:
Peter Hucker said:
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?

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 =^.^=
 
C

colin

Jan 1, 1970
0
Peter Hucker said:
Peter said:
I'm 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.

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 =^.^=
 
G

Geoff

Jan 1, 1970
0
Peter said:
I have had them chew through the flex.

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

geoff
 
B

Bill Janssen

Jan 1, 1970
0
colin said:
Peter Hucker wrote:


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 =^.^=
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
 
J

jasen

Jan 1, 1970
0
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.

that's not happening not unless those sides are superconductive
 
G

g

Jan 1, 1970
0
jasen said:
that's not happening not unless those sides are superconductive

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
 
P

Peter Hucker

Jan 1, 1970
0
Peter Hucker said:
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.

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.

--
http://www.petersparrots.com http://www.insanevideoclips.com http://www.petersphotos.com

Caller: "Can you give me the telephone number for Jack?"
Operator: "I'm sorry, sir, I don't understand who you are talking about".
Caller: "On page 1, section 5, of the user guide it clearly states that I need to unplug the fax machine from the AC wall socket and telephone Jack before cleaning. Now, can you give me the number for Jack?"
 
P

Peter Hucker

Jan 1, 1970
0
colin said:
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.

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

--
http://www.petersparrots.com http://www.insanevideoclips.com http://www.petersphotos.com

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 you would love to have."
The female response was: "Well, spit it out it isn't yours."
 
Top