How power is controlled in mocrowave ovens having magnetron

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Hello everyone.
As every body knows that microwave oven has magnetron which is the source of electromagnetic radiations.
I have seen in ovens that we can set power to 300W, 600 etc, means we can vary power.
How this is achieve, how power becomes variable, by selecting few buttons.
What is the basic principle behind it.

 

[shaiko]

I'm not an expert in the field of magnetrons and kitchen appliances - but I do know that PWM is a common method for quantisizing energy delivery...

The higher the setting, the higher the On time (Duty cycle) of the energy delivering device.

 

[xpress_embedo]

Okay thanks for the information.
Can you please provide me a document where i can read more about it or any website.

How pwm is provided to magnetron?
What is the input of magnetron which results in the output.

 

[FvM]

PWM is generated by an electromechanical or electronical timer, switching the input to the high voltage transformer supplying the magnetron.

 

[shaiko]

Again,
I'm not saying: "this is how it a microwave works". It's only my idea...

A microcontroller outputs a PWM wave: a fixed frequency pulse train with a duty cycle that varies according to the desired energy output. This wave controls the gate of a power transistor. The higher the duty cycle (on time vs off time) the more time the transistor conducts and the more time the magnetometer is powered on every cycle.

At maximum output (100% duty cycle), the PWM becomes a flat DC line - and the transistor is always conducting.

 

[magicfoot]

Hi,

CST has some good articles about this. Have a look at:

https://www.cst.com/search?searchQuery=oven

 

[THINKER_KAM_MAN]

just a suggestion.

the magneton use >2.5KV @ >400Ma in a microwave its lethal so
don't mess with it if you don't know what you are doing.
the anode of the magnetron is connected to the case and it is +ve.
the cathode is isolated and the magnetron is like a thermionic tube it has
an heater to produce electrons, the electrons are accelerated and sent
to a tuned cavity then to the inside of the oven which is also a tuned
cavity. there are powerful magnets on a magnetron which direct the electrons beam. the magnatron oscillates at about 2.2 Ghz where the cavity is tuned for, this frequency is absorbed by water and the energy
makes it hot. the power is controlled by switching on/off the magnatron
by the supplied mains power as it would be very difficult to switch the 2.5KV. also the 2.5KV is half wave rectified and uses a 2uf-4Uf high voltage capacitor so be care full if you are messing with them also
they have interlocks on then to stop you from opening the door and
having the magnetron powered up and 'anti tamper screws' to stop you opening the case. there are lots of circuit for microwave ovens on the WWW.

 

[BradtheRad]

When I put the cooking power on 50 percent, I watched what the microwave oven did.

It heated for 10 seconds, then coasted for 10 seconds. Then the cycle repeated.

The food would bubble at the edges for 10 seconds, then it cooled for 10 seconds.

So the duty cycle is measured in seconds, not milli-seconds.
I'm pretty sure the magnetron is either fully on, or fully off.

 

[FvM]

So the duty cycle is measured in seconds, not milli-seconds.
I'm pretty sure the magnetron is either fully on, or fully off.

Most magnetrons up to recent models are still using mechanical relays for switching power supply "pwm", so long periods are a must.

The other point is that the standard design uses a single transformer for heater and anode voltage. Heater start up time is in the range of seconds. An electronical solution with faster pwm periods must use a separate heater transformer. It's said the some products have "continuous" power control, e.g. in the Wikipedia article about microwave ovens, but I haven't seen it yet.