Working Principles of Microwave Oven

Magnetron

Magnetron, also known as microwave generator and magnetron microwave tube, is a kind of electronic tube, mainly composed of two parts: tube core and magnet. The magnetron is the heart of the microwave oven. From the outside, it consists of a microwave transmitter (waveguide), a radiator, two pins of the filament, and a magnet. From the inside, there is a cylindrical cathode surrounded by an anode made of high-conductivity oxygen-free copper, which is used to receive electrons emitted by the cathode.

Magnet

The function of the magnet is to supply a strong magnetic field parallel to the axis of the anode. Generally, it adopts a simple structure and is made of permanent magnets.


Die

The die consists of a filament, a cathode, an anode and a microwave energy exporter.

Filament: The thorium tungsten wire or pure tungsten wire is wound into a spiral shape, and its function is to heat the cathode.

Cathode: The cathode is made of a material with a strong ability to emit electrons. It is divided into two types: direct heating (the cathode and filament are integrated) and indirect heating (the cathode is made into a cylinder, and the filament is installed in the cylinder). When the cathode is heated, its surface quickly emits enough electrons to maintain the current required for the magnetron to function properly.

Anode: There are several resonant cavities on the anode, most of which are in the form of slot and fan, and are made of oxygen-free copper to receive the emitted electrons. They are frequency-selective resonant circuits that generate high-frequency oscillations. The resonant frequency is mainly determined by the size of the cavity. The upper cavity of the anode block faces the cathode, and there are generally even numbers. For safe use and convenient installation, the anode is grounded, and the cathode is connected to a negative high voltage, so that a radial DC electric field is formed between the anode and the cathode.

Microwave energy output device: The microwave energy generated by the magnetron is coupled out and sent to the load to heat the food.

How the magnetron works

A constant DC voltage is applied between the anode and the cathode, and the electrons emitted by the cathode are affected by the positive potential of the anode and fly to the anode. A strong magnet makes a magnetic field with a direction perpendicular to the electric field in space, so the electrons make a pendulum motion under the action of the magnetic field force and the electric field force. Due to the existence of a high-frequency electric field in the anode resonant cavity, an "electron cloud" is formed that rotates around the anode; when the rotation speed is synchronized with the high-frequency magnetic field, the electrons give all the energy to the high-frequency magnetic field, thereby maintaining the high-frequency oscillation. This high-frequency energy is output through the microwave energy output device, and is transmitted to the microwave oven cavity by the waveguide to heat the food.

Magnetron Fault Detection

After shutdown, discharge the high voltage capacitor and unplug the two plugs of the magnetron filament. Use a multimeter × 1Ω resistance gear to measure the two filaments, which should be less than 1Ω. Using the ×10k file to measure any filament to the ground (metal casing) is 'infinitely great'. Otherwise, it's broken.

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