Magnetron fail & empty oven detection

[Mark rainess]

I am building a machine that uses a microwave oven. I need a simple and
cheap way to reliably determine 1) that the magnetron is operating, and
2) that the oven cavity is not empty.

My first attempt was a diode RF sensor on a board mounted outside the
cavity with a short probe projecting inside through a hole. The food
item being heated moves relative to the probe and causes the response to
increase and decrease as it acts on the E field. When I see the changing
response I know the magnetron is working and the oven is not empty. This
works fine if the food is by itself; but, the food rests on ceramic
which moves. The ceramic material influences the E field to a large
degree and makes it impossible to detect the food item. I can't
eliminate the ceramic or change to another material. It seems I need an
entirely different approach.

If I put two RF probes in the waveguide spaced one-quarter wavelength
apart, I think the phase relationship between the two probes will change
when the oven is empty.

I want to avoid the expense of adding a secondary waveguide in order to
make a directional coupler.

Does anyone have an idea for a simple cheap way to detect that the oven
is empty.

Mark

 

[Robert Baer]

I am building a machine that uses a microwave oven. I need a simple and
cheap way to reliably determine 1) that the magnetron is operating, and
2) that the oven cavity is not empty.

My first attempt was a diode RF sensor on a board mounted outside the
cavity with a short probe projecting inside through a hole. The food
item being heated moves relative to the probe and causes the response to
increase and decrease as it acts on the E field. When I see the changing
response I know the magnetron is working and the oven is not empty. This
works fine if the food is by itself; but, the food rests on ceramic
which moves. The ceramic material influences the E field to a large
degree and makes it impossible to detect the food item. I can't
eliminate the ceramic or change to another material. It seems I need an
entirely different approach.

If I put two RF probes in the waveguide spaced one-quarter wavelength
apart, I think the phase relationship between the two probes will change
when the oven is empty.

I want to avoid the expense of adding a secondary waveguide in order to
make a directional coupler.

Does anyone have an idea for a simple cheap way to detect that the oven
is empty.

Mark

Try adding a low value resistor in series with the maggie supply to
sense load current.
If the maggie is not working at all, the current will be zero.
The load current, in most cases, will be rather low with an empty
oven, and much higher (and possibly varying due to rotating items) if
the oven is not empty.

 

[Jasen]

I am building a machine that uses a microwave oven. I need a simple and
cheap way to reliably determine 1) that the magnetron is operating, and
2) that the oven cavity is not empty.

My first attempt was a diode RF sensor on a board mounted outside the
cavity with a short probe projecting inside through a hole. The food
item being heated moves relative to the probe and causes the response to
increase and decrease as it acts on the E field. When I see the changing
response I know the magnetron is working and the oven is not empty. This
works fine if the food is by itself; but, the food rests on ceramic
which moves. The ceramic material influences the E field to a large
degree and makes it impossible to detect the food item. I can't
eliminate the ceramic or change to another material. It seems I need an
entirely different approach.

ceramic is something, therfore the cavity is not empty,

what do you really want?

If I put two RF probes in the waveguide spaced one-quarter wavelength
apart, I think the phase relationship between the two probes will change
when the oven is empty.

I want to avoid the expense of adding a secondary waveguide in order to
make a directional coupler.

probably won't work either.

Does anyone have an idea for a simple cheap way to detect that the oven
is empty.

define empty.

Bye.
Jasen

 

[neon]

use thermocouple wire and monitor the output

 

[Graham]

Try adding a low value resistor in series with the maggie supply to
sense load current.
If the maggie is not working at all, the current will be zero.
The load current, in most cases, will be rather low with an empty oven,
and much higher (and possibly varying due to rotating items) if the oven
is not empty.

I feel the above might be a fruitful approach.
I would be inclined to put the series resistor in the
transformer secondary rather than the Magnetron
cathode thus avoiding the voltage doubling effect
of the hv rectifier/capacitor/magnetron configuration.

I doubt if monitoring the transformer primary would
work too well, as oven transformers work near
saturation.

FFS be careful.

I remember doing something similar years ago and
used a torch-bulb and ORP12 arrangement as
a HV opto-coupler.

Graham.
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