Principle of operation and circuit diagram of the magnetron of the microwave oven

Microwave ovens have become a common feature in households and are now considered an essential appliance in any apartment. These devices offer the convenience of quickly heating or cooking food using invisible radiation. However, in order to understand the source and safety of this radiation for humans, it is important to grasp the workings of the microwave oven’s magnetron – the generator of high-frequency waves.

By comprehending the inner workings and principle of operation of the microwave oven magnetron, individuals can gain knowledge about the origin and potential risks associated with the radiation emitted by this household appliance.

What are microwaves and how they heat food

Microwaves are a form of electromagnetic radiation characterized by a wavelength ranging from 1 mm to 1 m. This particular type of radiation serves various purposes, not limited to domestic use, but also finds applications in navigation and radar systems, as well as cellular communications and satellite television.

Microwaves can occur both naturally, such as those emitted by the Sun, and artificially. They are also known as ultra-high frequency radiation, or UHF.

All household microwave ovens operate at a specific emission frequency of 2450 MHz, which is an internationally recognized standard. Manufacturers of household appliances must comply with this frequency to ensure that their products do not interfere with other microwave devices.

American physicist Percy Spencer discovered the thermal effect of microwave radiation in 1942. He obtained a patent for a microwave-generating device that could be used for cooking, which marked the beginning of using microwave ovens in households.

Over the following years, this technology underwent improvements, enabling the mass production of affordable and efficient devices for quickly heating food.

To effectively heat any substance in a microwave oven, it must contain dipole molecules – molecules that possess opposite electric charges at their ends.

The primary source of water in food is the main contributor to these processes. When exposed to ultra-high frequency radiation, these molecules align themselves with the electromagnetic field, altering their orientation approximately 5 billion times per second. This constant movement creates friction between the molecules, resulting in the generation of heat that effectively cooks the food.

Nevertheless, the penetration depth of microwaves is limited to approximately 2-3 cm from the surface of the food item. Consequently, anything below this layer relies on heat conduction from the heated regions for cooking.

Using a microwave to heat food

The device of the magnetron and its application

The magnetron is the primary source of ultra-high-frequency oscillations in most types of microwave equipment. Other devices, such as klystrons and platinotrons, which operate on a similar principle, have not gained as much widespread use. The magnetron was first employed in microwave ovens in 1960. The multiresonator magnetron is the most commonly utilized type in technology, consisting of several components:

1. The anode, a copper cylinder that is divided into sectors with thick metal walls. These volumetric cavities serve as resonators, forming a ring system of oscillations. The anode is subjected to a voltage of approximately 4000 volts.
2. The cathode is situated in the center of the magnetron and takes the form of a cylindrical structure that contains a filament. This component is responsible for electron emission, achieved by applying a 3-volt voltage to the heater (filament).
3. Ring magnets, which can be either electromagnets or high-power permanent magnets, are positioned at the ends of the device. These magnets are crucial for creating a magnetic field that runs parallel to the magnetron’s axis, enabling the movement of electrons in the same direction.
4. A wire loop is connected to the cathode, anchored within the resonator, and extends to the emitter antenna. This loop’s purpose is to guide the ultra-high-frequency radiation into the waveguide, allowing it to pass directly into the microwave chamber.

Due to their simple design and low cost, magnetrons have found numerous applications in various fields, with the most prevalent being:

• Microwave ovens. In addition to their ability to quickly cook and defrost food in household ovens, magnetrons also facilitate industrial tasks. An industrial microwave oven can be used for heating, drying, melting, roasting, and more. It is important to note that microwaves should not be operated empty, as the radiation will not be absorbed and could damage the waveguide.
• When it comes to radar, there is a vital component that plays a significant role in the functioning of the system – the radar antenna. Connected to a waveguide, this antenna takes the form of a conical irradiator and works in tandem with a parabolic reflector, often referred to as a dish. Performing a crucial task, the antenna assists in the transmission and reception of signals.

Magnetrons in radar

Principle of magnetron operation

The magnetron operates by converting electrical energy into extremely high-frequency electromagnetic radiation, which causes motion in the water molecules of the food. The dipole molecules, which constantly change direction, generate heat, enabling fast heating of food while maintaining its beneficial properties. The magnetron is the device responsible for generating microwaves.

A magnetron is a type of electric valve that uses the process of thermoelectron emission. This process occurs when the emitter or cathode’s surface is heated, causing the most active electrons to leave the surface. However, this only happens when a voltage is applied to the anode. When this voltage is applied, an electric field is created, and the electrons start moving towards the anode along its lines of force. If the electrons are impacted by a magnetic field, their paths will be deflected in the direction of the magnetic field lines.

The magnetron’s anode is designed as a cylinder that contains cavities, or resonators, housing a cathode with a filament. Positioned at the anode’s edges, two ring magnets generate an internal magnetic field. This field alters the path of electrons, causing them to orbit around the cathode instead of moving directly to the anode. As electrons pass by the resonators, they transfer some of their energy to induce the creation of a robust ultrahigh-frequency field within the cavities. This field is then emitted to the outside world via a wire loop connected to an antenna-emitter.

In order to activate the magnetron, an anode needs to be supplied with a high voltage of approximately 3-4 thousand volts. Hence, the magnetron is connected to the household power supply using a high-voltage transformer. Moreover, the microwave oven’s circuitry includes a waveguide for transmitting radiation within the chamber, a switching circuit, a control unit, as well as protective and cooling components. Additionally, the chamber’s inner walls and a thin metal mesh on the door of the device serve to contain the radiation within its confines.

An illustration of the magnetron’s schematic diagram

How the magnetron affects the microwave power

Many contemporary microwave oven manufacturers now provide the option to select the power of the device. The mode of operation (defrosting or heating) and the speed at which food is heated depend on this parameter. Unfortunately, due to the design characteristics of the magnetron, its power cannot be reduced. Therefore, to decrease the intensity of heating, power is supplied to the magnetron at specific intervals. These pauses in the magnetron’s operation can be observed by turning on the microwave at medium power and listening to its sound.

Recently, several manufacturers of household appliances made the announcement that they have introduced several models of microwave ovens that utilize an inverter power supply system. The implementation of this system not only allows for an increase in the usable space within the chamber by reducing the size of the emitter, but it also helps to lower the overall power consumption of the device. In contrast to traditional models, inverter-type ovens smoothly adjust the heating temperature, but their price is significantly higher.

Cooling and protection of the magnetron

During operation, the magnetron generates a significant amount of heat, necessitating the installation of a radiator on its body to dissipate the heat. Since overheating is the primary cause of magnetron failure, additional measures are implemented to protect it:

1. Thermostat relay. This device safeguards both the magnetron and the grill (if present in the model). Equipped with a bimetallic plate, the thermostat relay can be adjusted to a specific temperature. If the temperature exceeds this threshold, the plate bends, opening the power circuit.
2. Fan. Besides cooling the magnetron radiator, the fan also fulfills other beneficial functions, such as cooling the electronic components of the device, promoting air circulation within the chamber during grill operation, and expelling hot steam through dedicated vents.
3. Locking mechanism. The microwave door is equipped with multiple microswitches that ensure the magnetron cannot be activated while the door is open.

Can the magnetron be replaced

One of the key benefits of modern magnetrons used in household microwave ovens is their interchangeability. Magnetrons produced by different companies can be used on various models of microwaves, allowing for easy replacement if necessary. The only requirement in such cases is ensuring that the power specifications are compatible. To purchase a magnetron, one can visit numerous electronics stores; however, it is important to understand the parameters and labeling in order to make the right choice. The following models of magnetrons are commonly found in microwaves:

• 2M 213 (with a nominal power of 600 W and 700 W under load);
• 2M 214 (with a power of 1000 W);
• 2M 246 (with the highest power of 1150 W).

It is not advisable to replace the magnetron at home even after thoroughly studying all the essential parameters of the device. Firstly, removing it independently could prove quite challenging, and secondly, ensuring its safe operation after installation can only be done by a qualified specialist.

An average setup of the magnetron

Diagnosis of faults and causes of their occurrence

To ensure that you’re not wasting money on a new device, it’s important to diagnose the old one before investing in a replacement magnetron. This can be done at home using a regular tester to check for any defects. To do this, follow these steps:

1. First, disconnect the microwave from the power supply.
2. Next, remove the protective cover and visually inspect the part.
3. Using a tester or multimeter, test the main elements of the circuit board.
4. Don’t forget to inspect the thermorelay as well.

After completing the diagnosis, conclusions can be made regarding the failure of specific components. The main causes of magnetron failure can be attributed to the following:

• Failure of the vacuum tube cap. You can replace it yourself by finding a similar cap from another magnetron. The cap seats have a standard configuration.
• Breakage of the heater. When the microwave is turned on without anything inside or with incorrect loading, the magnetron can overheat, causing excessive heating of the filament and eventual breakage. To diagnose this issue, the resistance between the legs of the capacitor needs to be measured. If the value is between 5-7 ohms, the heater is still functional.
• Breakdown of the pass capacitor. If the tester does not show an "infinite" resistance value between its contacts, the capacitor needs to be replaced.