Electronics inside the microwave.

[Ian Stirling]

I was wondering about auto-simmer on my microwave (a feature it lacks).
I'd like a thing I can throw in the food, and it'll regulate the
temperature to 99C or so.

I was considering something really simple, but would like component
and general suggestions.

First, very simple idea.
Diode, shorted by a bimetallic normally open stat.
Pick up the 5.8Ghz signal generated.

Any fundemental problems with this?

 

[Rene Tschaggelar]

I was wondering about auto-simmer on my microwave (a feature it lacks).
I'd like a thing I can throw in the food, and it'll regulate the
temperature to 99C or so.

I was considering something really simple, but would like component
and general suggestions.

First, very simple idea.
Diode, shorted by a bimetallic normally open stat.
Pick up the 5.8Ghz signal generated.

Any fundemental problems with this?

Yes, that cooking temperature is dependent on pressure or height.
At 3000m above sealevel, water cooks some degrees lower than
those 99DegC.

Rene

 

[Ian Stirling]

Yes, that cooking temperature is dependent on pressure or height.
At 3000m above sealevel, water cooks some degrees lower than
those 99DegC.

True;.
I'm kind of making the assumption that my home will not suddenly lift
several thousand meters in the air.

However, if this happened, I think the microwave malfunctioning
would not be at the top of my list of priorities

 

[Peter Bennett]

I was wondering about auto-simmer on my microwave (a feature it lacks).
I'd like a thing I can throw in the food, and it'll regulate the
temperature to 99C or so.

I was considering something really simple, but would like component
and general suggestions.

First, very simple idea.
Diode, shorted by a bimetallic normally open stat.
Pick up the 5.8Ghz signal generated.

Any fundemental problems with this?

Yes - a microwave oven heats the food, not the air in the oven, so you
would need to put the temperature sensor in the food.

Also, placing electrical conductors in a microwave oven is generally a
Bad Idea - the microwave energy will induce voltages in the
conductors, with unpredictable results on any circuits connected to
them.

--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca

 

[Scott Stephens]

I was wondering about auto-simmer on my microwave (a feature it lacks).
I'd like a thing I can throw in the food, and it'll regulate the
temperature to 99C or so.

I was considering something really simple, but would like component
and general suggestions.

First, very simple idea.
Diode, shorted by a bimetallic normally open stat.
Pick up the 5.8Ghz signal generated.

Any fundemental problems with this?

I don't think microwave "ovens" are designed to withstand constantly
high temperatures. The box would probably not insulate its contents and
dissipate heat (away from electronics) properly.

BTW the magnetrons are 2.5 GHz, IIRC.

Perhaps an infrared sensor aimed at the food? That takes a couple
chopped IR sensors operating at different wavelengths, say 10uM & 1.5
uM, piezoceramic and PbS, for instance. Probably a pain.

Don't some ovens have a temperature probe (thermistor/thermocouple/LM35,
et.) that sticks in the food to regulate power?

--
Scott

**********************************

DIY Piezo-Gyro, PCB Drill Bot & More Soon!

http://home.comcast.net/~scottxs/

Those who sow excuses shall reap excuses

**********************************

 

[Mac]

I was wondering about auto-simmer on my microwave (a feature it lacks).
I'd like a thing I can throw in the food, and it'll regulate the
temperature to 99C or so.

I was considering something really simple, but would like component
and general suggestions.

First, very simple idea.
Diode, shorted by a bimetallic normally open stat.
Pick up the 5.8Ghz signal generated.

Any fundemental problems with this?

I think it will be hard to get any kind of signal out of a microwave oven
via wires. The wires will experience very high voltages and self-destruct
spectacularly.

Likewise, any metallic item in the microwave is apt to do the same thing
unless it is much smaller than the microwave wavelength. The fundamental
is around 2.5 GHz, I think. I don't know how far down the harmonics are,
but they might still be pretty strong.

However, I think you could embed some kind of temperature sensor in the
food (even a metallic or semiconductor one), because the food would then
protect the sensor from the microwave exposure. But how to get the signal
out? IR? Ultrasound?

Practically speaking, I think you can just give up now and save yourself a
lot of trouble. ;-)

--Mac

 

[Dan Major]

I think it will be hard to get any kind of signal out of a microwave
oven via wires. The wires will experience very high voltages and
self-destruct spectacularly.

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.

 

[Ian Stirling]

Yes - a microwave oven heats the food, not the air in the oven, so you
would need to put the temperature sensor in the food.

"that I can throw in the food"

Also, placing electrical conductors in a microwave oven is generally a
Bad Idea - the microwave energy will induce voltages in the
conductors, with unpredictable results on any circuits connected to
them.

Well, yes.
With proper component selection and design, it should be possible.
It's never going to be very trivial.

 

[Ian Stirling]

I don't think microwave "ovens" are designed to withstand constantly
high temperatures. The box would probably not insulate its contents and
dissipate heat (away from electronics) properly.

BTW the magnetrons are 2.5 GHz, IIRC.

But the generaterd signal would be double this.

Perhaps an infrared sensor aimed at the food? That takes a couple
chopped IR sensors operating at different wavelengths, say 10uM & 1.5
uM, piezoceramic and PbS, for instance. Probably a pain.

Especially with steam/fat/... condensing on it.

Don't some ovens have a temperature probe (thermistor/thermocouple/LM35,
et.) that sticks in the food to regulate power?

Yes.

 

[Spajky]

I was wondering about auto-simmer on my microwave (a feature it lacks).
I'd like a thing I can throw in the food, and it'll regulate the
temperature to 99C or so.

I was considering something really simple, but would like component
and general suggestions.

maybe an infrared sensor diode mounted in a hole on that MW owen´s
door (possibly on the outer side, that MW do not fry it) poited to the
food on the plate & connected to some circuity to pilot (on/off) the
magnetron thru owens electronics could do the job?

 

[Bill Schuh]

I think it will be hard to get any kind of signal out of a microwave oven
via wires. The wires will experience very high voltages and self-destruct
spectacularly.

Likewise, any metallic item in the microwave is apt to do the same thing
unless it is much smaller than the microwave wavelength. The fundamental
is around 2.5 GHz, I think. I don't know how far down the harmonics are,
but they might still be pretty strong.

However, I think you could embed some kind of temperature sensor in the
food (even a metallic or semiconductor one), because the food would then
protect the sensor from the microwave exposure. But how to get the signal
out? IR? Ultrasound?

Practically speaking, I think you can just give up now and save yourself a
lot of trouble. ;-)

--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

 

[N. Thornton]

True;.
I'm kind of making the assumption that my home will not suddenly lift
several thousand meters in the air.

However, if this happened, I think the microwave malfunctioning
would not be at the top of my list of priorities

lol. It might be a problem for news2020, alien abductees and so on.


I also think a simmer function would be an advantage, and I came up
with a way to do it. It was less elegant, but worked well.

There is just one issue. Food tends to heat unevenly, and steam is a
good way to blow heat from hotter spots to cooler, and make sure it
all gets cooked. Thus your approach may result in less even cooking,
and thus a need for longer cooking times.


Regards, NT

 

[Ian Stirling]

lol. It might be a problem for news2020, alien abductees and so on.

I also think a simmer function would be an advantage, and I came up
with a way to do it. It was less elegant, but worked well.

There is just one issue. Food tends to heat unevenly, and steam is a
good way to blow heat from hotter spots to cooler, and make sure it
all gets cooked. Thus your approach may result in less even cooking,
and thus a need for longer cooking times.

True.
It depends on what it is.
It'd be nice if the sensor was a bit smarter than a diode and a
bimetallic disk.
However, for some things, like rice, which could do with full power
for 5 minutes, stir, then cover and maintain at 100C, it might be nice.

 

[Rich Grise]

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.

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. ;-)

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

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

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

 

[Jonathan Kirwan]

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. ;-)

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

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

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.

I'm probably the resident expert on actual experiments done and microwave ovens
destroyed...

My daughter is profoundly autistic, yet knows how to push buttons to get a
microwave oven running (and/or learns quickly what works.) I've had some 4
microwave ovens destroyed just in the last 16-18 months, when we moved into our
newer home here. (These were destroyed by something catching fire inside and
then starting the plastic burning, etc.)

She places tin pans of various sizes, frying pans, etc., into the oven. 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.)

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'.

Jon

 

[Jonathan Kirwan]

I was wondering about auto-simmer on my microwave (a feature it lacks).
I'd like a thing I can throw in the food, and it'll regulate the
temperature to 99C or so.

I was considering something really simple, but would like component
and general suggestions.

First, very simple idea.
Diode, shorted by a bimetallic normally open stat.
Pick up the 5.8Ghz signal generated.

Any fundemental problems with this?

Well, you are expecting an interaction with the microwaves used to heat the food
to now generate your externally sensed signal. In other words, your device is
by definition going to need to be directly exposed to and interact with the
microwaves, themselves. Seems to me that this will cause your device to spark
and otherwise self-destruct over time and also read more of its own reaction
than that of the surrounding food unless you have a precise plan how to overcome
it. On the other hand, if you imagine that the food will absorb the energy then
how will your device interact with, then generate, and then propagate to the
outside the signal you want to observe?

I'd say... just try it without the detection part (which makes it very easy and
cheap, since those parts are easily obtained) and see how things proceed. If it
doesn't spark or get tanned or get much hotter than the surrounding food itself
(check quickly) then maybe the rest is worth some experimenting.

Jon

 

[Ian Stirling]

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'.

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

 

[Jonathan Kirwan]

I was wondering about auto-simmer on my microwave (a feature it lacks).
I'd like a thing I can throw in the food, and it'll regulate the
temperature to 99C or so.

I was considering something really simple, but would like component
and general suggestions.

First, very simple idea.
Diode, shorted by a bimetallic normally open stat.
Pick up the 5.8Ghz signal generated.

An optical technique could use a rare-earth target material at the tip of an
optical fiber (say, 100 micron to 1000 micron as convenient choices) that is
optically pulsed, with the returned decay curve of light observed. That curve
is temperature dependent and none of the sensor system interacts with the
microwaves, directly. You can actually use common, commercially available
phosphors used in lighting (such as those phosphors used in compact fluorescent
bulbs, for example) bought by the gram or kg. So you don't have to make it.
(In fact, if you ask for a sample you might get all you'll ever need.) You'd
need to use some kind of epoxy to tack it to a fiber tip. And you'd need to
select an LED and a detector, etc., for the other end.

Jon

 

[ChrisGibboGibson]

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

 

[Jonathan Kirwan]

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

Ah! I didn't recall reading that!

Jon