Electronics inside the microwave.

[Jonathan Kirwan]

Some amusing things to try in a microwave.

Small flourescent tube, they light very brightly.

Incandescent bulb, light very brightly, very briefly.

Xmas tree lights.

LEDs

Blue bottles buzz like a demented thing.

And finally, worms can travel much faster than I thought.

I'm just glad she can't read this stuff!

Jon

 

[ChrisGibboGibson]

I'm just glad she can't read this stuff!

Jon

But think of the fun she'd have.

Gibbo

 

[Rich Grise]

Well, you are expecting an interaction with the microwaves used to heat the food
to now generate your externally sensed signal.

I agree. If all you're doing is monitoring the microwaves, you might
as well be monitoring "is the magnetron on". It will tell you nothing
about the temperature of the food, which is why people have been talking
about probes. I have used a pyrometer to measure the temp. of an object
in a chamber - this was bread in a vacuum cooler.

<story>
When you get the bread out of the oven, you can't slice it until it cools
some. For a bakery, this is very expensive, having bread just sit there on
a rack cooling, and not going out the door. I've seen a loaf of bread go
from about 250F to about 80F in about 30 seconds. You pump it down, the
water that was going to evaporate anyway evaporates, and sucks the heat
out of the bread as it does. The vacuum lowers the boiling point, you see.

</story>

But anyway, a pyrometer is another way to measure the food temp,
but that's only on the surface, which would be misleading anyway.

I'd go with the thermistor probe.

And about controlling - there's a microwave at my office that has a
"defrost" setting, and what it does is run for about 15 seconds,
and idle for about a minute, and run for about 15 seconds, and so
on, so you don't need any kind of fast switch, if you were thinking
of PWM control. I guess it's still PWM, just with a period of about
a minute or so.

Cheers!
Rich


Cheers!
Rich

 

[Rich Grise]

....
Where
I've seen serious damage occur is in the mica covering the magnetron waveguide
into the chamber -- which often develops burn holes and is eventually demolished
(perhaps, it's good design here creating a weak point intentionally.) But also,
the wave guide itself is melted at the corners and rim at the exit -- this is
permanent. None of this particular damage takes place when there is no metal in
what she inserts (which may still, of course, catch fire, though.)

Except for the fire, this is the kind of damage I'd expect from not having
enough moist stuff in the chamber to absorb all the RF energy, so it
reflects back into the waveguide, and into the maggie, and makes standing
waves, and you've seen the rest. I worked with maggie transmitters in the
USAF, and they usually had a load isolator to protect the maggie from
reflected power. Apparently, your oven(s) didn't. Sorry.

Whether or not some bit of bimetal can cause all this havoc is another thing, of
course. But intentionally putting reflectors/edges in the chamber volume isn't
'good'.

True. It can be done under the right circumstances, and with care, but in
general, don't just throw metal in there and expect it to cook. ;-)

CHeers!
Rich

 

[Mac]

My Amana RadarRange has a metal sheathed thermistor for use in the
food. It has a standard microphone plug that plugs into a bulkhead
connector on the microwave chamber. Not sure how they avoid the
problems you mention but they do somehow. The sheath is rarely
totally immersed in the food when in use so there is some method
besides being shielded by the food.

Bill

Very interesting. I'm glad you posted that. I wonder how they do that.
They might just use high dielectric strength coax or something.

I certainly don't have any kind of explanation.

--Mac

 

[Mac]

The horrors of "metal in the microwave" have been vastly overrated.
I inherited one of the first microwave oven cookbooks, when nobody
knew about these mysterious new cooking rays. In one picture, there's
an ice cube wrapped in aluminum foil sitting next to a cup of water. The
water boils, and the ice stays frozen. It didn't burn down the house. If
you're defrosting a pound of burger, and the corners get soft before the
inside, they recommend covering the "done" parts with foil to shield them
from the invisible rays. ;-)

Interesting!

My point is, just putting metal in the oven doesn't automagically cause
mayhem and destruction.

No. Of course not. I'm not stupid. You have to turn the oven on for the

The walls of the thing are metal, for goodness
sakes, how do you think they keep them from causing plasma storms?

It can make nice coronas and arcs if there are little pointy things
and stuff, but even if you happen to put in a piece that's a good antenna
at 2.4 GHZ, if you haven't got a detector of some kind, it will just
reradiate.

The best thing to put in a microwave oven is a CD (AOL CD's are handy for
this). Just go for one or two seconds. This may not be good for the oven,
but it is well worth it as a visual display.

And a foil-wrapped potato won't even get warm.

Wow. I honestly didn't know you could put anything foil wrapped in the
oven.

They used to say "don't run it empty", but I'd think somebody's put
in some kind of load isolation by now, so even that's kind of a wash
these days.

Cheers!
Rich

--Mac

 

[Mac]

Some amusing things to try in a microwave.

Small flourescent tube, they light very brightly.

Incandescent bulb, light very brightly, very briefly.

Xmas tree lights.

LEDs

Blue bottles buzz like a demented thing.

And finally, worms can travel much faster than I thought.

Gibbo

Also try compact disks. I recommend that you convince your friends to do
it rather than risk doing any slight damage to your own microwave.

One or two seconds is plenty of time to see the effect.

--Mac

 

[Mac]

I'm talking about something well, well under a wavelength.
Maybe 2cm dia externally.

Right. But how to get any signal out? Maybe a heavily shielded cable would
actually do it. If all else fails, apparently, you can just wrap the
cabling in aluminum foil. ;-)

--Mac

 

[Mac]

This just ain't so. I work with a group researching nanomaterials and
their properties. We use a donated microwave oven as a reactor to create
silver nanowires. It's a cheap home-type oven. The rotary timer was
replaced with a solid state relay. The relay is controlled via an Omega
temperature controller. The controller uses a simple (J-type?)
stainless-clad thermocouple. The thermocouple is grounded to the oven
chamber. The turntable mechanism was removed from the bottom of the
oven, and the only problem we have had is some slight sparking where the
mesh shielding covering the hole where the turntable was removed was not
in suffecient contact with the oven chamber. Scraping paint off and
tightening the screws solved that problem. The hole in the top of the
chamber where the thermocouple is inserted is covered by a Farday cage.
As long as you insure that whatever you stick into the oven chamber is
grounded to the chamber, you will minimize problems. Otherwise, you will
need some sort of ferrite (beads, rods, etc) to reduce RF feedthrough
into your measurement circuit.

Wow. Thanks. That's very interesting.

For the OP, a thermocouple is just what he wants (but not what he
proposed), as far as I can tell.

--Mac

 

[Rich Grise]

Very interesting. I'm glad you posted that. I wonder how they do that.
They might just use high dielectric strength coax or something.

I certainly don't have any kind of explanation.

The probe and wires are shielded. This not only make a lousy antenna
at 2.4 GHz, but it's grounded at one end and embedded in a resistance
at the other. And, since it's a conductor, anything it picks up that
doesn't get shorted out will just reradiate, ultimately to be absorbed
by the chicken. I don't think it's about to cause any problems.

Cheers!
Rich

 

[Rich Grise]

Wow. I honestly didn't know you could put anything foil wrapped in the
oven.

Well, people have been baking potatoes wrapped in foil in an ordinary
oven since there's been foil. When uW ovens first came out, consumers
had no clue about the magic invisible rays, so had to be edjamacaited.

Cheers!
Rich

 

[Ian Stirling]

Right. But how to get any signal out? Maybe a heavily shielded cable would
actually do it. If all else fails, apparently, you can just wrap the
cabling in aluminum foil. ;-)

A diode causes generation of double the frequency of EM radiation
it's exposed to.
You then pick up the 5.8Ghz from that.

 

[Mac]

A diode causes generation of double the frequency of EM radiation it's
exposed to.
You then pick up the 5.8Ghz from that.

So are you saying that you'll attach an antenna to the diode and pick up
the 5.8 outside the oven? I'm pretty sure ovens are designed to keep the
radiation IN. ;-)

Or will you carry the 5.8 out in coax?

Anyway, I still see a couple of problems with this, both of which may have
already been mentioned in this thread.

1) All this will tell you is whether the magnetron is on, whereas what it
seems like you would want is to know the temperature of the food.

2) Don't you think the magnetron puts out a fair amount of 2nd harmonic? I
mean, if it is putting out, say, 500 Watts of primary, and it has a 2nd
harmonic which is 60 dB down (I'm making this number up), that is still
half a Watt. Will your little diode pump out enough to even be detected on
top of the 2nd harmonic?

It seems to me that the way to go is to rig up something like what Dan
Major described elsewhere in this thread.

--Mac

 

[Ian Stirling]

So are you saying that you'll attach an antenna to the diode and pick up
the 5.8 outside the oven? I'm pretty sure ovens are designed to keep the
radiation IN. ;-)

Or will you carry the 5.8 out in coax?

Small antenna to pickup the 5.8 inside the cavity, with inherent blocking
of 2.4Ghz.

Anyway, I still see a couple of problems with this, both of which may have
already been mentioned in this thread.

1) All this will tell you is whether the magnetron is on, whereas what it
seems like you would want is to know the temperature of the food.

Short the diode by a bimetallic thermometer.

2) Don't you think the magnetron puts out a fair amount of 2nd harmonic? I
mean, if it is putting out, say, 500 Watts of primary, and it has a 2nd
harmonic which is 60 dB down (I'm making this number up), that is still
half a Watt. Will your little diode pump out enough to even be detected on
top of the 2nd harmonic?

Isn't 60dB down 500uW, not 500mW?

 

[Mac]

Small antenna to pickup the 5.8 inside the cavity, with inherent blocking
of 2.4Ghz.

This is pretty far from anything I have any real practical experience
with, so I can't argue with you about it. ;-)

Short the diode by a bimetallic thermometer.

Again, I don't know what the properties are of a bimetallic thermometer,
so I'll just have to let this go.

Isn't 60dB down 500uW, not 500mW?

500 Watts / 500 mW = 1000.

I am pretty sure that 20 *log (1000) = 60, so I will have to stick to my
guns on this one. ;-)

--Mac

 

[Terry Given]

This is pretty far from anything I have any real practical experience
with, so I can't argue with you about it. ;-)




Again, I don't know what the properties are of a bimetallic thermometer,
so I'll just have to let this go.




500 Watts / 500 mW = 1000.

I am pretty sure that 20 *log (1000) = 60, so I will have to stick to my
guns on this one. ;-)

--Mac

oops. 20*log(ratio of voltages) or 10*log(ratio of powers). P = V^2/R
which is where the 10 becomes a 20..... so 10*log(500W/500uW) = 60dB

Cheers
Terry

 

[Terry Given]

Also try compact disks. I recommend that you convince your friends to do
it rather than risk doing any slight damage to your own microwave.

One or two seconds is plenty of time to see the effect.

--Mac

CDs are also fun when you have a dremel. but wear safety goggles.....

Cheers
Terry

 

[Ian Stirling]

CDs are also fun when you have a dremel. but wear safety goggles.....

I'd say "just say no", unless wearing full body armour and face protection.

I tried this once, and ended up with shards of CD 2cm deep in the
plasterboard.

 

[Terry Given]

I'd say "just say no", unless wearing full body armour and face protection.

I tried this once, and ended up with shards of CD 2cm deep in the
plasterboard.

As did I. except it was thru 20mm Gib board (antipodean for plasterboard
. I figured the rest of me (and the 5 guys watching) would heal....I
was wearing a face shield (safety goggles = double glazing = instant
headache

If you Nukrowave the CD first, it self-destructs at much lower speeds.

Cheers
Terry

 

[Mac]

Mac wrote: [snip]

500 Watts / 500 mW = 1000.

I am pretty sure that 20 *log (1000) = 60, so I will have to stick to my
guns on this one. ;-)

--Mac

oops. 20*log(ratio of voltages) or 10*log(ratio of powers). P = V^2/R
which is where the 10 becomes a 20..... so 10*log(500W/500uW) = 60dB

Cheers
Terry

Sigh. Thanks for the correction. As you surmised, I was thinking of
Voltage. I am in the habit of dealing with Voltage, so I always use 20
log(x) to convert x into dB.

--Mac