The principle of operation of a microwave oven is based on the process of converting an electromagnetic microwave field. The electromagnetic field turns into heat and heats the product placed in the chamber in a non-contact manner. This is the main difference between a microwave oven and other devices - electric ovens, gas ovens, etc.
In microwave ovens, heating occurs directly within the heated product, which is why the process takes a few minutes. And in traditional stoves, heat is directed to the surface of the object, and spreads further, depending on the heat-conducting properties of the product. Of course, this method is less effective than microwave waves, and it also requires much more knowledge in the field of cooking.
Even with the passage of time and the development of technology, the microwave device remains unchanged. Yes, the design improved and became more comfortable, the functionality developed, new ways of adjusting the time and power appeared. But the principle of work has not changed to this day.
Many people confuse concepts such as "microwave oven" and "microwave oven", but these are just different names for the same appliance.
Even ten years ago, microwaves were considered an unaffordable luxury and a simple addition to the interior of the kitchen. It was all the fault of the high cost of such devices, and not everyone could afford such an innovation. But over time, it became clear that this is not a luxury, but a necessity, especially in times of urbanization and the incessant race for money.
How does the heating occur?
The principle of operation of a microwave is based on an element such as a magnetron. This concept is widely known to everyone who is familiar with the world of radar devices. It is thanks to the magnetron that mankind has received the most efficient and easy-to-use device for cooking and heating food. This is one of the most striking examples of how the developments of the heavy industry began to be used in everyday life with overwhelming success.
The device of a microwave oven can be briefly conveyed in such words - a magnetron generates microwave energy, which is converted into heat. The power sources for these elements are special anode-heating transformers-stabilizers, which are the reason for such a high price for microwave ovens. These stabilizers are the most expensive and important element of the furnace.
Studying the principle of operation of a microwave oven, we will pay special attention to the design of a magnetic circuit equipped with magnetic shunts. This device makes it possible to change the overvoltage within only two percent, under conditions of 10% mains voltage fluctuation. The magnetic circuit and the substantial inductive dissipation force of the high voltage winding are the main attractions of the microwave oven transformer. In words, everything seems quite complicated, but in fact, such a device has shown itself to be simple and extremely effective.
But at the first stages of development, the creators faced such a problem as increased noise during the operation of the stove. Excessive noise has always been a problem for many household appliances, but manufacturers have easily bypassed this pitfall. To reduce the noise level, some parts of the magnetic circuit are connected by welding.
The standard microwave housing is a rectangular chamber that plays an essential role in heating. The generated waves are not just directed to the heated object - they are reflected from the walls of the case.
An additional element, thanks to which it is possible to ensure high-quality and uniform heating, is a rotating saucer in the chamber. As it rotates, it allows the waves to strike the surface evenly. The principle of operation is such that the generated waves are by no means uniform - they can be with beams, nodes, etc. The magnetron transmits power to the emitter through a rectangular waveguide.
The door is an important element of the microwave oven
In the manufacture of microwave ovens, the greatest attention is paid to the door. Everyone knows that in every microwave oven, the door plays the role of a kind of fuse - as soon as it opens, the oven stops working. The device of the door is quite complex, as it is directly related to the level of safety during operation. The principle of the door is as follows:
- An ideal door and enclosure shape is required to keep the gap as small as possible. The door protects the environment from microwave radiation and must therefore be designed with the utmost responsibility.
- The perimeter of the door is equipped with a high frequency throttle shutter, which is responsible for reducing microwave radiation to the required level.
- In the production process, a special type of plastic is used that helps to absorb radiation.
The control panel is nothing complicated. Usually these are two knobs that regulate the power and cooking time. The timer can be either mechanical or in the form of an electronic dial.
Many modern models these days are equipped with panels with a choice of different modes, but in reality these are just minor additions - the basic principle of operation does not change.
Much has been said over the years of microwave ovens about their harmful effects on health. In fact, microwave ovens do not emit radioactive radiation, and even allow you to cook healthier food, preserving up to 75% of vitamins in products (which cannot be said about traditional cooking methods). Subject to safety precautions, there will be no harm to your health from the microwave.
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In order to understand whether a microwave oven is harmful, you need to have an idea of what microwaves are. For this we turn not to rumors, but to the scientific data of physics, which explains the nature and properties of all physical phenomena.
What are microwaves and their place in the spectrum of electromagnetic radiation.
Microwave- this is one of the types of electromagnetic radiation. And, as you know, the electromagnetic radiation of the Sun is the main source of energy for life on Earth. It consists of visible and invisible radiation.
All the colors that we see are the visible part of the radiation. Invisible is radio waves, infrared (thermal), ultraviolet, X-ray and gamma radiation. All these waves are manifestations of the same phenomenon - electromagnetic radiation, but they differ in wavelength and frequency of oscillations. The longer the wavelength, the lower the frequency of their vibrations. These parameters determine the properties of a particular type of radiation.
The entire spectrum of electromagnetic waves can be sequentially arranged as the wavelength decreases (and, accordingly, the oscillation frequency increases) in the following order:
- Radio waves- electromagnetic waves with a wavelength of more than 1mm. These include: a) Long waves - wavelengths from 10 km to 1 km (frequency 30 kHz - 300 kHz);
b) Medium waves - wavelength from 1 km to 100 m (frequency 300 kHz -3 MHz);
c) Short waves - wavelength from 100 m to 10 m (frequency 3 - 30 MHz);
d) Ultra-short waves with a wavelength less than 10 m (frequency 30 MHz - 300 GHz). Ultrashort waves, in turn, are divided into:
meter, centimeter (including microwaves), millimeter waves.
Microwave Is a form of electromagnetic energy located in the frequency scale between radio waves and infrared radiation. Therefore, they have some of the properties of their neighbors. Microwave or ultra-high frequency (microwave) waves are short electromagnetic radio waves with a wavelength of 1 mm - 1 m (frequency less than 300 MHz). It is called ultrahigh frequency (microwave) radiation because it has the highest frequency in the radio range. The physical nature of microwave radiation is the same as that of radio waves. They are used for telephone communications, the Internet, the transmission of television programs, in microwave ovens. - Infrared radiation- electromagnetic waves with a wavelength of 1 mm - 780 nm (frequency 300 GHz - 429 THz). It is also called "thermal" radiation, as it is perceived by the human skin as a sensation of warmth.
- Visible radiation- electromagnetic waves with a wavelength of 780-380 nm (frequency 429 THz - 750 THz).
- Ultraviolet radiation e - electromagnetic waves with a wavelength of 380 - 10 nm (frequency 7.5 1014 Hz - 3 1016 Hz).
- X-ray radiation- electromagnetic waves with a wavelength of 10 nm - 5 pm (frequency 3 1016 - 6 1019 Hz).
- Gamma rays- electromagnetic waves with a wavelength of less than 5 pm (frequency more than 6 1019 Hz).
The amount of energy that it carries depends on the wavelength and frequency. Long wavelength, low frequency waves carry little energy. Waves with a short wavelength and a high frequency are many. The more energy the radiation possesses, the more destructive effect it has on a person.
According to the ability to cause such an effect as ionization of a substance, all of the above types of electromagnetic radiation are divided into 2 categories: ionizing and non-ionizing.
These 2 types of radiation differ in the amount of energy they carry.
1. Ionizing radiation otherwise called radioactive. It includes X-rays, gamma rays, and in some cases ultraviolet radiation.
Ionizing radiation It is characterized by high energy, capable of ionizing substances, and causes such changes in cells that disrupt the course of biological reactions in the body and pose a health hazard.
The maximum energy is inherent in gamma radiation. As a result of its effects, food becomes radioactive, and radiation sickness develops in a person. That is why the effect of all ionizing radiation is very dangerous for a living organism.
2. Non-ionizing radiation - radio waves, infrared, visible radiation.
These types of radiation have insufficient energy to ionize matter, therefore they cannot change the structure of atoms and molecules. The boundary between non-ionizing and ionizing radiation is usually considered a wavelength of about 100 nanometers.
The energy of long radio waves is not even enough to heat anything - they will simply pass through any food. The energy of infrared radiation (thermal) is absorbed by all objects, including food, therefore it is successfully used, for example, in toasters. Microwaves occupy the middle position of them and therefore also have low energy.
Microwaves used in microwave ovens.
Household microwave ovens use microwaves with a radiation frequency of 2450 MHz (2.45 GHz) and a wavelength of approximately 12 cm. These values are significantly lower than the frequencies of X-rays and gamma rays, which cause an ionizing effect and are dangerous to humans. Microwaves are located between radio and infrared waves, i.e. they have insufficient energy to ionize atoms and molecules.
In working microwave ovens, microwaves do not directly affect a person. They are absorbed by food, causing a heat generating effect.
Microwaves do not create ionizing radiation and do not emit radioactive particles, therefore they do not have a radioactive effect on living organisms and food. They generate radio waves, which, according to all the laws of physics, cannot change the atomic-molecular structure of matter, they can only heat it up.
So microwaves are a kind of radio waves. Being in the frequency scale between radio waves and infrared radiation, they have properties in common with them.
However, neither the heat nor the radio waves that surround us everywhere do not affect food in any way, and therefore there is no particular reason to expect this from microwaves.
V. KOLYADA. The material was prepared by the editors of "We buy from A to Z" at the request of the journal "Science and Life".
Science and Life // Illustrations
Rice. 1. Scale of electromagnetic radiation.
Rice. 2. Dipole molecules: a - in the absence of an electric field; b - in a constant electric field; c - in an alternating electric field.
Rice. 3. Penetration of microwaves deep into a piece of meat.
Rice. 4. Labeling of dishes.
Rice. 5. Attenuation of the energy of microwave radiation in the atmosphere: on each next line, with distance from the furnace, the radiation power is 10 times less than on the previous one.
Rice. 6. The main elements of the microwave oven.
Rice. 7. Microwave oven door.
Rice. 8. Furnace with a dissector (a) and a turntable (b).
In the second half of the twentieth century, ovens entered our everyday life, in which food is heated by invisible rays - microwaves.
Like many other discoveries that have significantly influenced the daily life of people, the discovery of the thermal effects of microwaves happened by accident. In 1942, the American physicist Percy Spencer worked in the Raytheon laboratory with a device that emitted microwave waves. Different sources describe the events that happened that day in the laboratory in different ways. According to one version, Spencer put his sandwich on the device, and after taking it off after a few minutes, he found that the sandwich had warmed up to the middle. According to another version, the chocolate that Spencer had in his pocket when he worked near his installation warmed up and melted, and, overshadowed by a happy guess, the inventor rushed to the buffet for raw corn kernels. The popcorn brought to the installation soon began to burst with a bang ...
One way or another, the effect was discovered. In 1945, Spencer received a patent for the use of microwaves for cooking, and in 1947, the first appliances for cooking food using microwaves appeared in the kitchens of hospitals and military canteens, where the requirements for food quality were not so high. These human-height products from Raytheon weighed 340 kg and cost $ 3,000 each.
It took a decade and a half to "bring to mind" the oven, in which food is cooked using invisible waves. In 1962, the Japanese company Sharp launched the first commercial microwave oven, which, however, at first did not cause consumer fuss. The same company developed a revolving table in 1966, first applied a microprocessor-based oven control system in 1979, and in 1999 developed the first microwave oven with Internet access.
Today, dozens of firms produce household microwave ovens. In the United States alone, 12.6 million microwave ovens were sold in 2000, not counting combo ovens with a built-in microwave source.
The experience of using millions of microwave ovens in many countries over the past decades has proven the indisputable convenience of this method of cooking - fast, economical, and easy to use. The very mechanism of cooking with the help of microwaves, which we will introduce you below, predetermines the preservation of the molecular structure, and hence the taste of the products.
What are microwaves
Microwave, or ultra-high-frequency (UHF) radiation is electromagnetic waves from one millimeter to one meter long, which are used not only in microwave ovens, but also in radar, radio navigation, satellite TV systems, cellular telephony, etc. Microwaves exist in nature, they are emitted by the Sun.
The place of microwaves on the scale of electromagnetic radiation is shown in Fig. one.
Household microwave ovens use microwaves with a frequency f of 2450 MHz. This frequency is established for microwave ovens by special international agreements so as not to interfere with the operation of radars and other devices using microwaves.
Knowing that electromagnetic waves travel at the speed of light With equal to 300,000 km / s, it is easy to calculate what the wavelength is equal to L microwave radiation of a given frequency:
L = c/f= 12.25 cm.
To understand how a microwave oven works, you need to remember one more fact from a school physics course: a wave is a combination of alternating fields - electric and magnetic. The products we eat are not magnetic, so we can forget about the magnetic field. But the changes in the electric field that the wave carries with it are very useful for us ...
How do microwaves heat food?
The composition of food includes many substances: mineral salts, fats, sugar, water. To heat food with microwaves, you need the presence of dipole molecules, that is, those with a positive electrical charge at one end and a negative one at the other. Fortunately, there are plenty of such molecules in food - these are molecules of both fats and sugars, but the main thing is that the dipole is a molecule of water - the most widespread substance in nature.
Each piece of vegetables, meat, fish, fruits contains millions of dipole molecules.
In the absence of an electric field, the molecules are arranged randomly (Fig. 2, a).
In an electric field, they line up strictly in the direction of the field lines, with "plus" in one direction, "minus" in the other. As soon as the field changes its direction to the opposite, the molecules immediately turn over by 180 ° (Fig. 2, b).
Now let's remember that the frequency of microwaves is 2450 MHz. One hertz is one oscillation per second, megahertz is one million oscillations per second. During one period of the wave, the field changes its direction twice: there was a "plus", there was a "minus", and the original "plus" returned again. This means that the field in which our molecules are located changes polarity 4,900,000,000 times per second! Under the action of microwave radiation, the molecules tumble with a frantic frequency and literally rub against one another during flips (Fig. 2, c). The heat released at the same time serves as the reason for heating the food.
Microwaves heat food in much the same way our palms heat up when we quickly rub them together. There is one more similarity: when we rub the skin of one hand against the skin of the other, heat penetrates deep into the muscle tissue. So microwaves: they work only in a relatively small surface layer of food, not penetrating deeper than 1-3 cm (Fig. 3). Therefore, the heating of products occurs due to two physical mechanisms - heating by microwaves of the surface layer and the subsequent penetration of heat into the depth of the product due to thermal conductivity.
Hence, the recommendation immediately follows: if you need to cook in the microwave, for example, a large piece of meat, it is better not to turn on the oven at full power, but to work at medium power, but increase the residence time of the piece in the oven. Then the heat from the outer layer will have time to penetrate deep into the meat and bake the inside of the piece well, but the piece will not burn on the outside.
For the same reason, liquid foods such as soups are best stirred occasionally by removing the saucepan from the oven from time to time. By doing this, you will help the penetration of heat into the depths of the bowl with the soup.
Microwave utensils
Different materials behave differently with respect to microwaves, and not all utensils are suitable for microwave ovens. Metal reflects microwave radiation, so the inner walls of the oven cavity are made of metal to reflect the waves towards the food. Accordingly, metal utensils for microwaves are not suitable.
An exception is low, open metal dishes (eg aluminum food trays). Such dishes can be placed in a microwave oven, but, firstly, only downward, to the very bottom, and not on the second highest level (some microwave ovens allow a "two-story" placement of trays); secondly, it is necessary that the oven does not operate at maximum power (it is better to increase the operating time), and the edges of the tray are at least 2 cm from the walls of the chamber so that an electric discharge does not form.
Glass, porcelain, dry cardboard and paper allow microwaves to pass through them (wet cardboard will begin to heat up and will not allow microwaves to pass through until it dries). Glassware can be used in the microwave, but only if it can withstand the high heating temperature. For microwave ovens, cookware made of special glass (for example, Pyrex) with a low coefficient of thermal expansion, resistant to heat, is produced.
Recently, many manufacturers have been providing microwave-safe markings to cookware (Figure 4). Pay attention to the markings before using the cookware.
Please note that, for example, heat-resistant plastic food containers are excellent for microwaves, but they may not withstand high temperatures if you turn on the grill in addition to the microwaves.
Microwaves are absorbed by food. Clay and porous ceramics behave the same way, which are not recommended for use in microwave ovens. Porous dishes trap moisture and heat up on their own instead of passing microwaves to the food. As a result, food gets less microwave energy, and you risk getting burned when removing the dishes from the oven.
Here are three main rules on the topic: which should not be placed in the microwave.
1. Do not place utensils with gold or other metal rims in the microwave. The fact is that the alternating electric field of microwave radiation leads to the appearance of induced currents in metal objects. By themselves, these currents do not represent anything terrible, but in a thin conductive layer, which is the layer of decorative metal coating on the dishes, the density of the induced currents can be so high that the rim, and with it the dishes, overheat and collapse.
In general, metal objects with sharp edges, pointed ends (for example, forks) do not belong in the microwave: the high density of the induced current on the sharp edges of the conductor can cause the metal to melt or cause an electric discharge.
2. In no case should you put tightly closed containers in the microwave: bottles, cans, containers with food, etc., and eggs(it doesn't matter raw or boiled). All of these items may burst when heated and render the oven unusable.
Items that can burst when heated include food that has a skin or shell, such as tomatoes, sausages, wieners, sausages, etc. To avoid explosive expansion of such foods, pierce the shell or skin with a fork before placing them in the oven. Then the steam generated inside during heating will be able to calmly come out and not break the tomato or sausage.
3. And the last thing: it is impossible for the microwave to be ... empty. In other words, you can not turn on an empty oven, without a single object to absorb microwaves. A simple and understandable unit is adopted as the minimum load of the furnace at any time it is switched on (for example, when checking the operability): a glass of water (200 ml).
Turning on an empty microwave oven can seriously damage it. Without encountering any obstacles in their path, the microwaves will be repeatedly reflected from the inner walls of the oven cavity, and the concentrated radiation energy can damage the oven.
By the way, if you want to bring water in a glass or other tall narrow vessel to a boil, do not forget to dip a teaspoon into it before placing the glass in the oven. The fact is that the boiling of water under the action of microwaves does not happen in the same way as, for example, in a kettle, where heat is supplied to the water only from below, from the bottom side. Microwave heating goes on from all sides, and if the glass is narrow - practically over the entire volume of water. In the kettle, the water boils when it boils, as bubbles of air dissolved in the water rise from the bottom. In the microwave, the water will reach the boiling point, but there will be no bubbles - this is called the boil delay effect. But when you take the glass out of the oven, stirring it up at the same time, the water in the glass will seethe belatedly, and the boiling water can scald your hands.
If you do not know what material the cookware is made of, follow a simple experiment that will allow you to determine whether it is suitable for this purpose or not. Of course, we are not talking about metal: it is not difficult to identify it. Place the empty dishes in the oven next to a glass filled with water (don't forget the spoon!). Switch on the oven and let it run for one minute at maximum power. If the dishes remain cold after this, it means that they are made of a material transparent to microwaves and can be used. If the cookware is hot, then it is made of material that absorbs microwaves and you are unlikely to be able to cook food in it.
Are microwaves dangerous?
A number of misconceptions are associated with microwave ovens, which are explained by a lack of understanding of the nature of this type of electromagnetic waves and the mechanism of microwave heating. We hope that our story will help overcome such prejudices.
Microwaves are radioactive or make food radioactive. This is not true: microwaves are classified as non-ionizing radiation. They do not have any radioactive effect on substances, biological tissues and food.
Microwaves alter the molecular structure of foods or make foods carcinogenic.
This is also not true. Microwaves work differently from X-rays or ionizing radiation, and they cannot make foods carcinogenic. On the contrary, since cooking with microwaves requires very little fat, the finished meal contains less burnt fat with a modified molecular structure during cooking. Therefore, cooking with microwaves is healthier and does not pose any danger to humans.
Microwaves emit hazardous radiation.
This is not true. Although direct exposure to microwaves can cause thermal damage to tissues, there is no risk when using a working microwave oven. The design of the oven provides for strict measures to prevent the emission of radiation outside: there are duplicated devices for blocking the microwave source when the oven door is opened, and the door itself prevents microwaves from leaving the cavity. Neither the casing, nor any other part of the oven, nor food placed in the oven accumulates microwave electromagnetic radiation. As soon as the oven is switched off, the emission of microwaves is stopped.
Those who are afraid to even get close to a microwave oven should be aware that microwaves are damped very quickly in the atmosphere. To illustrate, let’s give an example: the permissible Western standards for microwave power at a distance of 5 cm from a new, just purchased oven is 5 milliwatts per square centimeter. Already at a distance of half a meter from the microwave, the radiation becomes 100 times weaker (see Fig. 5).
As a result of such a strong attenuation, the contribution of microwaves to the general background of the electromagnetic radiation surrounding us is not higher than, say, from a TV set in front of which we are ready to sit for hours without any fear, or a mobile phone, which we so often hold to our temples. Just don't lean your elbow on a working microwave oven or lean your face against the door trying to see what's going on in the cavity. It is enough to move away from the stove at arm's length and you can feel completely safe.
Where do microwaves come from?
The source of microwave radiation is a high-voltage vacuum device - magnetron... In order for the magnetron antenna to emit microwaves, a high voltage (about 3-4 kW) must be applied to the magnetron filament. Therefore, the mains supply voltage (220 V) is not enough for the magnetron, and it is powered through a special high-voltage transformer(fig. 6).
The magnetron power of modern microwave ovens is 700-850 W. This is enough to bring water to a boil in a 200-gram glass in a few minutes. To cool the magnetron, there is a fan next to it that continuously blows air over it.
Microwaves generated by the magnetron enter the furnace cavity through waveguide- a channel with metal walls reflecting microwave radiation. In some microwave ovens, waves enter the cavity through only one hole (as a rule, under the “ceiling” of the cavity), in others, through two holes: at the “ceiling” and at the “bottom”. If you look into the cavity of the oven, you can see mica plates that close the holes for the introduction of microwaves. The plates do not allow splashes of fat to enter the waveguide, and they do not interfere with the passage of microwaves at all, since mica is transparent to radiation. Over time, mica plates become saturated with fat, become loose, and must be replaced with new ones. You can cut a new plate from a piece of mica yourself in the shape of the old one, but it is better to buy a new plate in a service center that serves the equipment of this brand, since it is inexpensive.
The cavity of the microwave is made of metal, which may have one or another coating. In the cheapest models of microwave ovens, the inner surface of the cavity walls is coated with enamel-like paint. This coating is not resistant to high temperatures, therefore it is not used in models where, in addition to microwaves, food is heated by the grill.
Coating the walls of the cavity with enamel or special ceramics is more resistant. Walls with this coating are easy to clean and can withstand high temperatures. The disadvantage of enamel and ceramics is their fragility in relation to impacts. When placing the dishes in the microwave cavity, it is not difficult to accidentally touch the wall, and this can damage the coating applied to it. Therefore, if you have purchased a microwave oven with enamel or ceramic walls, handle it with care.
The strongest and most impact-resistant walls are stainless steel. The advantage of this material is the excellent reflection of microwaves. The downside is that if the hostess does not pay too much attention to cleaning the inner cavity of the microwave oven, then splashes of fat and food that are not removed in time can leave traces on the stainless surface.
The volume of the microwave oven cavity is one of the important consumer characteristics. Compact ovens with a cavity volume of 8.5-15 liters are used for defrosting or cooking small portions of food. They are ideal for single people or for special tasks such as warming up a bottle of baby food. Furnaces with a cavity volume of 16-19 liters are suitable for a couple. A small chicken can be placed in such an oven. Medium-sized ovens have a cavity volume of 20-35 liters and are suitable for a family of three to four people. Finally, for a large family (five to six people), you need a CB oven with a cavity of 36-45 liters, allowing you to bake a goose, turkey or a large pie.
A very important element of the microwave oven is the door. It should make it possible to see what is happening in the cavity, and at the same time exclude the exit of microwaves to the outside. The door is a multi-layer cake made of glass or plastic plates (fig. 7).
In addition, there is always a mesh of perforated metal sheet between the plates. The metal reflects the microwaves back into the oven cavity, and the perforations that make it transparent for viewing are no more than 3 mm in diameter. Recall that the wavelength of microwave radiation is 12.25 cm. It is clear that such a wave cannot pass through three-millimeter holes.
To prevent radiation from finding a loophole where the door adjoins the cut of the cavity, a sealant made of dielectric material. It fits snugly against the front end of the microwave oven body when the door is closed. The thickness of the seal is on the order of a quarter of the microwave wavelength. Here, a calculation based on the physics of waves is used: as you know, waves in antiphase cancel each other out. Due to the precisely matched thickness of the seal, the so-called negative interference of the wave that has penetrated into the inside of the seal material and the reflected wave emerging from the seal to the outside is ensured. Thanks to this, the seal serves as a trap, reliably quenching radiation.
To completely exclude the possibility of generating microwaves when the chamber door is open, a set of several redundant independent switches is used. These switches are closed by contact pins on the oven door and break the power supply circuit of the magnetron even if the door is slightly closed.
Taking a closer look at the microwave ovens on display in the sales area of a large home appliance store, you will notice that they differ in the direction of opening the door: for some ovens, the door opens to the side (usually to the left), while for others, it opens towards you, forming a small shelf. Although the latter option is less common, it gives additional convenience when using the oven: the horizontal plane of the open door serves as a support when loading dishes into the oven cavity or when removing a finished dish. You just need not to overload the door with unnecessary weight and do not lean on it.
How to "stir" microwaves
Microwaves entering the cavity of the oven through the waveguide are chaotically reflected from the walls and sooner or later fall on the food placed in the oven. At the same time, waves come from various directions to each point, say, of a chicken carcass that we want to defrost or fry. The trouble is that the interference we have already mentioned can work both in "plus" and in "minus": waves arriving in the phase will strengthen one another and warm up the area they hit, and those arriving in antiphase will extinguish each other, and they won't be of any use.
In order for the waves to penetrate the products evenly, they must be "mixed" in the oven cavity, as it were. It is better for the products themselves to literally rotate in the cavities, substituting different sides for the radiation flux. So it appeared in microwave ovens Rotary table- a dish resting on small rollers and driven by an electric motor (Fig. 8, b).
There are several ways to "stir" microwaves. The simplest and most straightforward solution is to hang an agitator under the "ceiling" of the cavity: a rotating impeller with metal blades that reflect microwaves. Such a mixer is called a dissector (Fig. 8, a). It is good for its simplicity and, as a result, low cost. But, unfortunately, the high uniformity of the wave field of a microwave oven with a mechanical microwave reflector does not differ.
The combination of a rotary dissector and a rotary product table sometimes has a special name. So, in microwave ovens Mielе it is called the Duplomatic system.
Some microwave ovens (eg models Y82, Y87, ET6 from "Moulinex") have two turntables, one above the other. This system is called DUO and allows you to cook two meals at the same time. Each table has a separate drive through a socket on the back wall of the oven cavity.
A more subtle but effective way to achieve a uniform wave field is to carefully work on the geometry of the internal cavity of the furnace and create optimal conditions for the reflection of waves from its walls. Such "advanced" microwave distribution systems have their own "brand" name for each oven manufacturer.
Magnetron operating hours
Any microwave oven allows the owner to set the power required to perform a function, from the minimum power needed to keep food warm to the full power needed to cook food in a loaded oven.
A feature of magnetrons used in most microwave ovens is that they cannot "burn up". Therefore, in order for the furnace to work not at full, but at a reduced power, you can only periodically turn off the magnetron, stopping the generation of microwaves for a while.
When the oven is operating at minimum power (let it be 90 W, while the food in the oven cavity is kept warm), the magnetron turns on for 4 seconds, then turns off for 17 seconds, and these on-off cycles alternate all the time.
Let's increase the power to, say, 160 W if we need to defrost food. The magnetron now turns on for 6 seconds and turns off for 15 seconds. Let's add power: at 360 W, the duration of the on and off cycles is almost equal - these are 10 s and 11 s, respectively.
Note that the total duration of the magnetron on and off cycles remains constant (4 + 17, 6 + 15, 10 + 11) and is 21 s.
Finally, if the oven is turned on at full power (in our example it is 1000 W), the magnetron works continuously without shutting down.
In recent years, models of microwave ovens have appeared on the domestic market, in which the magnetron is powered through a device called an "inverter". The manufacturers of these ovens (Panasonic, Siemens) emphasize such advantages of the inverter circuit as the compactness of the microwaves emitting unit, which makes it possible to increase the volume of the cavity with unchanged external dimensions of the oven and to more efficiently convert the consumed electricity into microwave energy.
Inverter power systems are widely used, for example, in air conditioners and can smoothly change their power. In microwave ovens, inverter power systems make it possible to smoothly change the power of the radiation source, instead of turning it off every few seconds.
Due to the smooth change in the power of the microwave emitter in ovens with an inverter, the temperature also changes smoothly, in contrast to traditional ovens, where the radiation supply is interrupted from time to time due to the periodic switching off of the magnetron. However, let's be fair to traditional ovens: these temperature fluctuations are not so strong and hardly affect the quality of cooked food.
As with air conditioners, inverter-powered microwaves are more expensive than traditional microwaves.
Did you know …
that any milk can be heated in a microwave oven without compromising its nutritional value? The only exception is freshly expressed breast milk: under the influence of microwaves, it loses the components it contains, which are vital for the baby.
that sometimes it is better to cancel the table rotation. This will allow you to cook large-volume dishes (salmon, turkey, etc.), which simply do not turn in the cavity without touching its walls. Use the undo function if your microwave has one.
Microwaves have become so closely embedded in our everyday life that today it is difficult to imagine an apartment or house in whose kitchen this useful appliance would not be. The functionality of microwave ovens allows you to perform a wide range of work related to the thermal processing of food: defrosting, heating and even preparing. You may not have known this, but they can also be used to disinfect metal-free rags and sponges. In this article, we will look at how microwave ovens are arranged and what is the principle of their work.
As is often the case with useful and ingenious inventions of mankind, the use of microwave waves for domestic use was discovered quite by accident. This happened in 1942 at the Raytheon Company, where the physicist Percy Spencer was studying the properties of microwave devices.
According to one of the versions, the scientist accidentally put the sandwich on the installation and discovered after a couple of minutes that it had warmed up throughout its entire thickness. Another version says that a bar of chocolate melted in Spencer's pocket and, happy with his discovery, he immediately ran to the store - after a while, the scientist observed how microwave waves in a couple of seconds turn the fresh corn he bought into popcorn.
In 1945, Percy Spencer patented the technology of using microwave waves in the food industry, and two years later, the first devices similar to modern microwave ovens appeared in American military hospitals and canteens. It should be said separately that these units, unlike modern ones, really resembled ovens, since they weighed about 340 kg.
In the future, the development and launch of microwave ovens on the home appliance market was undertaken by Sharp, who owns the main revolutionary achievements in this industry:
- in 1962, they put into mass production the first household microwave;
- in 1966 - they began to equip the furnaces with a rotating support-table;
- 1979 - released the first microprocessor-controlled device;
- 1999 - created the first microwave oven with Internet access.
Today on the home appliance market there is a huge variety of microwave ovens with a wide variety of functionality, but they are all simple, economical and safe for health.
How it works?
Surely many of us at least once in our life were interested in what the principle of the microwave oven is based on and how it turns out that the products placed in it warm up rather quickly.
The fact is that the products we use for cooking contain water, fat, mineral components, and sugar in various proportions. All these substances contain dipolar molecules in their structure, which means that one of their ends is positively charged, and the other is negatively. Meat, cereals, vegetables, and in general any foodstuffs contain a huge amount of such molecules.
Let's remember physics - in the absence of an electric field, dipolar molecules are in a disordered position. As soon as the substance is under the influence of an electric field, the molecules rearrange and "fit" according to the direction of the lines of force: the positively charged ends - towards the "plus" pole, and the negative ends - toward the "minus". Accordingly, when the polarity of the electric field changes, the molecules will begin to "unfold" by 180˚. The principle of operation of the microwave oven is based on this phenomenon.
So, on average, the operating frequency of a microwave oven is 2.45 GHz. This means that 1,000,000 oscillations (switchings) are performed in one second. During one such switching, the polarity of the electric field inside the microwave changes twice - from "plus" to "minus" and vice versa. A simple mathematical calculation tells us that in one second, the electric field in which we place food changes polarity 4.9 million times. That is why these devices are called microwave ovens - the decoding of the abbreviation reveals the word "microwave". In fact, we make the molecules rotate at a very high speed, as a result of the friction of which from each other, heat is released. The upper 1-3 cm of substances fall under the influence of the electric field, from which the heat spreads inward. That is why for cooking some food in the microwave it is recommended not to turn it on "at full", but to select medium power and increase the duration of processing.
How does a microwave oven work?
A household microwave oven consists of the following functional parts:
- a metal chamber with a metal door, into which food for heating is placed;
- magnetron - a device that emits microwave waves;
- transformer for powering the magnetron;
- control and switching circuits;
- a waveguide that transmits radiation generated by the magnetron to the chamber.
In addition, the design of the furnace includes the following components that do not affect the heating process itself, but serve to improve the operation of the device:
- rotating stand for uniform action of waves;
- a circuit that ensures the operation of the timer;
- a safety circuit that blocks the operation of the device in various situations (for example, when the door is opened);
- a fan necessary to ventilate the stone and cool the magnetron.
How does a magnetron work?
The magnetron is the main device of the microwave oven, which emits the necessary microwave waves to heat up. In fact, it is a high-frequency electric vacuum diode with a cylindrical copper anode. From the inner wall, this anode is divided into several sectors with copper walls.
The rod-shaped cathode forms the center of this structure (see drawing) Thanks to the filament inside it, electrons are emitted. On the front sides of the device there are powerful ring-shaped magnets. With the help of the magnetic field created by them inside the magnetron, microwave radiation is generated.
During operation, a voltage of 4 kV is applied to the anode, and only 3 V to the filament. This provokes the emission of electrons, which are picked up by a high-voltage electric field. The generation frequency of the magnetron is determined by the magnitude of the anode voltage and the geometry of the resonator chambers.
To remove energy, a special wire loop is used, which runs from the cathode to a radiator in the form of an antenna. From it, through the waveguide, microwave waves enter the main chamber. The output power of magnetrons installed in household microwave ovens is typically 800 watts.
To reduce the intensity of the magnetron's operation, the control circuit turns it on for short time intervals, with pauses between them. So, for example, in order for the output power of the magnetron to be 50% (400 W), it is necessary to alternately turn it on and off every 5 seconds. This control principle is called pulse width modulation.
The operation of the magnetron is accompanied by the release of a large amount of heat, therefore, to prevent overheating, it is equipped with a plate radiator, to which air is constantly supplied from the fan.
Thermal fuse
Thermal overload (overheating) is the most common reason for magnetron failure, therefore thermal fuses or thermal relays are installed on them. It is of particular importance for microwave ovens with functions that require long-term operation of the magnetron, for example, a grill.
A specific device is selected based on the nominal temperature indicators that are applied to their case.
The principle of their operation is quite simple:
- The thermostat has an aluminum housing and is installed in the place, the temperature of which must be controlled, using a flange connection, providing direct thermal contact.
- The bimetallic plate located inside the fuse is preset to operate at a given temperature.
- As soon as the device heats up to the specified limit, the plate bends and with the help of a pusher breaks the connection on the plates of the contact group, as a result of which the power supply to the microwave oven is cut off.
- As the device cools down, the bimetallic plate takes on its previous shape, the pusher returns to its initial position and the contact group closes again, supplying power.
The importance of the fan
One of the most important parts of any microwave oven is the built-in fan. Thanks to it, not only the magnetron is cooled, but the rest of the circuit as well.
Using a thermostat alone to prevent overheating of microwave components is not enough. Firstly, this will lead to the constant operation of the thermal fuse and the switching on and off of the magnetron, which will negatively affect not only it, but also other devices. Secondly, in some microwave ovens, the thermostat simply cannot cope with the heating - in appliances with a grill function, you cannot do without a fan.
During operation of a microwave oven, a large amount of heat is emitted not only by the components that make up its design, but also by heated foods. And since the main "target" of microwave waves is water, heating is also accompanied by the release of steam. The fan allows you to get rid of excess humid air by pumping fresh air into the chamber. This allows escaping steam to escape through the ventilation openings.
In microwave ovens with one fan located at the back of the device, special air ducts run from it, carrying fresh air first to the magnetron, and then to the inside of the chamber.
Protection system
The powerful high-frequency radiation of the magnetron can cause irreparable damage to human (and not only) health, therefore, special attention in modern models of microwave ovens is paid to prevent this danger.
To protect the user and other living beings from harmful radiation, microwave cameras are made of shielded metal. And since the camera itself is also placed in a metal case, a two-level isolation of microwave radiation is actually carried out.
You can ask a quite reasonable question: is the glass door, through which we observe the readiness of the dish, dangerous? Isn't it a "hole" in this defense? Rest assured - the glass is covered with a special fine-mesh metal mesh that reflects the radiation emitted by the magnetron at a frequency of up to 2.45 GHz (with a wavelength of up to 122 mm) back into the camera.
It is also important how tightly the door closes and adheres to the body. The gap between its groove and the body is specially measured at the factory (it is equal to ¼ of the wavelength, for example, 122/4 = 30.5mm) and must be maintained throughout the entire service life. This distance contributes to the formation of a standing electromagnetic wave, which does not go beyond the device, because the amplitude value at the point of contact between the case and the door is zero. Such a simple and effective scheme for organizing radiation protection is known as a microwave choke.
What happens when the door is opened?
There is an opinion that when the door of the microwave oven is opened at the time of work, dangerous high-frequency radiation falls on the user, which is very harmful to the body. In reality, this is just a myth - the magnetron control circuit includes several switches that react to the state of the door. Their number depends on the specific model, but usually there are at least three of them. One is responsible for turning off the magnetron when the door is opened, the second for the backlight, and the third sends information to the microprocessor about the position of the door. The operation of these switches is organized in such a way that the magnetron can only work when the microwave door is tightly closed.
Control block
The most important role in the operation of any microwave oven is played by the control unit. In fact, it is the brain of the oven, performing two basic functions:
- maintains the power value at a given level;
- shuts down the device after a specified period of time.
The control unit circuit of the old models was a design of two switches, one of which was used to set the timer, and the other to select the intensity of treatment. With the development of technologies, the "filling" of microwaves has also improved - electromechanical control units have been replaced by electronic ones, which have already been superseded by microprocessor ones. Their advantage lies not only in compactness, but also in broader functionality, including:
- power setting using a touch or keypad;
- display of the current power on the display;
- built-in clock;
- numerous presets for preparing various dishes and performing specific tasks (several defrosting modes for different types of food);
- automatic calculations - for example, you enter only the weight of the defrosting piece of meat, and the oven itself selects the power for it;
- a large selection of sound signals for the completion of the working cycle.
A separate power supply is used to power the control circuit. A relay unit serves to transmit signals and commands between the unit, keyboard, magnetron, grill, lamp and fan. To connect other components to the circuit (indication, keyboard), loops are used.
Inverter technology
Many consumers, when choosing a microwave oven, try to find a model with a large chamber volume in order to provide maximum versatility. Unfortunately, functional elements often occupy a large part of the entire volume of the device; accordingly, the size of the oven is large, but its useful volume is small. This problem was one of the main ones before the advent of inverter ovens.
This technology can significantly reduce the space occupied by the magnetron by using smaller components. This allows for larger chambers while keeping the overall dimensions of the entire microwave within accepted standards.
Inverter technology is fundamentally distinguished by the fact that in such microwave ovens the emitter does not have to constantly operate at the maximum power, unlike classical magnetrons. The intensity of processing is regulated by impulse, so that powerful surges of microwave energy do not fall on the food, which affects its quality. Research has shown that inverter-cooked cabbage retains one-third more vitamin C, while pork retains 41% more vitamin B1.
In addition to being compact and useful, inverter microwave ovens are also more economical devices, due to the use of only the amount of electricity that is necessary to maintain the selected power. Fine impulse setting of the operating mode also serves to accelerate the defrosting of products.
Today, the flagship in the application of this technology is Panasonic, which just released the first inverter microwave oven.
Microwave Myths
There are several well-established misconceptions about the harmfulness and dangers of using microwave ovens. In reality, most (usually all) are wrong. Listed below are typical microwave oven myths that aren't really worth worrying about.
- A microwave oven can destroy electrically devices located a few meters away if it is left on for a long time. In fact, modern microwave ovens are completely protected from radiation escaping outside the chamber. The minimum radiation, which may be in the immediate vicinity of the device, does not exceed the radiation of the computer system unit.
- Microwave ovens can cause the user to become allergic to electromagnetic waves. This is simply not possible. There are only a few people in the world with such a rare disease and its causes have nothing to do with the use of a microwave. In general, no household appliance generates such hazardous radiation, since all models are certified for safety.
- Products become radioactive when exposed to microwave waves. This is also not true. Microwave microwave radiation is non-ionizing and therefore does not alter the properties of food. The statement about the carcinogenicity of products subjected to microwave processing is also incorrect - microwaves have a completely different principle of action than X-rays and, once again, cannot affect the properties of products in any way.
- Microwaves pose a high health hazard. This has already been partially said - the direct effect of microwave radiation is really detrimental to tissues, however, if you follow the rules for using the microwave, you will never be exposed to such an effect. In fact, radiation from a microwave oven represents only a small fraction of the total electromagnetic field in which we find ourselves using numerous other household appliances.
Just remember that the oven door must close tightly, and the body must be intact, and also do not touch the working microwave with your hands and other parts of the body and you can rest assured - at a distance of half a meter from the device you are not exposed to more electromagnetic radiation than when watching TV.
The microwave oven has become a part of everyday life and has become one of the indispensable attributes of any apartment. This household appliance allows you to heat or cook food in a matter of minutes using radiation invisible to the eye.
But in order to find out where this radiation comes from and how safe it is for humans, it is necessary to understand the structure and principle of operation of the microwave oven magnetron, which is the generator of high-frequency waves.
Magnetron
What are microwaves and how they heat food
Microwave is called electromagnetic radiation with a wavelength of 1 mm to 1 m. This type of radiation is used not only for domestic purposes, but also in navigation and radar systems, and also ensures the operation of cellular communications and satellite television.
Microwaves can be generated both artificially and naturally (for example, on the sun). Another name for microwaves is ultra-high frequency radiation, or microwave.
All types of household microwave ovens have a single radiation frequency equal to 2450 MHz. This value is an international standard that manufacturers of household appliances must strictly adhere to so that their products do not interfere with the operation of other microwave devices.
Microwave radiation
The thermal effect of microwave radiation was discovered by the American physicist Percy Spencer in 1942. It was he who patented the use of a microwave-generating device for cooking, thereby initiating the use of microwave ovens in everyday life.
Over the next few decades, this technology was perfected, which allowed for mass production of simple and inexpensive devices for fast.
To heat any material in a microwave oven, it requires the presence of dipole molecules, that is, molecules with opposite electric charges at both ends.
Water is the main source of food in food. Under the influence of ultrahigh frequency radiation, these molecules begin to line up along the lines of force of the electromagnetic field, changing their direction about 5 billion times per second. The friction that occurs between them is accompanied by the release of heat, which heats the food.
However, microwaves are not able to penetrate deeper than 2-3 cm from the surface of the product, so everything under this layer heats up due to thermal conductivity from the heated areas.
Heating food with microwave
Magnetron device and its application
In most types of microwave technology, the generator of microwave oscillations is a magnetron. Devices that are similar in their principle of operation - klystrons and platinotrons - are not so widespread. The magnetron was first used in microwave ovens in 1960. The most commonly used in technology is a multi-resonator magnetron, which consists of several components:
- Anode. It is a copper cylinder, divided into sectors with thick metal walls. These volumetric cavities are the resonators that create an annular oscillation system. A voltage of about 4000 volts is applied to the anode.
- Cathode. It is located in the central part of the magnetron and is a cylinder with a filament inside. In this part of the device, electrons are emitted. The heater (filament) is supplied with a voltage of 3 volts.
- Ring magnets. Electromagnets or high-power permanent magnets located in the end parts of the device are necessary to create a magnetic field directed parallel to the magnetron axis. The movement of electrons is also carried out in this direction.
- Wire loop. It is connected to the cathode, fixed in the resonator and brought out to the radiating antenna. The loop is used to output microwave radiation into the waveguide, after which it enters the microwave chamber.
Magnetron device
Due to the simplicity of design and low cost, magnetrons have found application in many areas, but they are most widely used:
- In microwave ovens. In addition to quickly cooking and defrosting food in domestic ovens, magnetrons also allow you to perform production tasks. An industrial microwave oven can perform heating, drying, melting, roasting, and more. At the same time, it is important to remember that the microwave cannot be turned on when empty, since in this case the radiation will not be absorbed by anything and will return back to the waveguide, which can lead to its breakdown.
- In radar. The radar antenna connected to the waveguide is actually a conical feed and is used in conjunction with a parabolic reflector (dish). The magnetron generates powerful short pulses of energy with a short wavelength, part of which is reflected back to the antenna and then to a sensitive receiver, which processes the signal and displays it on the screen.
Magnetrons in radar
How the magnetron works
A microwave oven works by converting electrical energy into ultra-high frequency electromagnetic radiation, which sets the water molecules in food in motion. Dipole molecules, constantly changing direction, generate heat, which allows you to quickly heat food, while maintaining their beneficial properties. The device that generates microwaves is a magnetron.
A magnetron, in fact, is an electrovacuum diode, which uses the phenomenon of thermionic emission. This phenomenon occurs in the process of heating the surface of the emitter or cathode. Under the influence of high temperature, the most active electrons tend to leave its surface, but this will only happen when a voltage is applied to the anode. In this case, an electric field arises, and the electrons begin to move towards the anode, heading along its lines of force. If electrons find themselves in the zone of action of a magnetic field, then their trajectories deviate in the direction of the direction of the lines of force.
Vacuum diode
The anode of the magnetron has the shape of a cylinder with a system of cavities, or resonators, inside which there is a cathode with a filament. Two ring magnets located at the edges of the anode create a magnetic field inside the anode, due to which the electrons do not move directly from the cathode to the anode, but change their trajectory, rotating around the cathode. Near the resonators, the electrons give them part of their energy, which leads to the formation of a powerful microwave field in their cavities, which is brought out with the help of a wire loop connected to the radiating antenna.
To activate the magnetron, it is necessary to apply a high voltage of the order of 3-4 thousand volts to the anode. Therefore, the magnetron is connected to the household electrical network by means of a high-voltage transformer. In addition, the microwave oven switching circuit includes a waveguide that transmits radiation into the chamber, a switching circuit, a control unit, and protection and cooling elements. In addition, the inner walls of the chamber and the thin metal mesh on the door of the device prevent radiation from escaping.
Magnetron switching circuit
How a magnetron affects microwave power
Most modern microwave oven manufacturers offer a choice of instrument wattage. This parameter, in turn, determines the operating mode (defrosting or heating) and the heating rate of food. However, the design features of the magnetron do not allow to reduce its power, therefore, to reduce the heating intensity, power is supplied to it at regular intervals. These pauses in the operation of the magnetron can be noticed if you turn on the microwave at medium power and listen to the sound of its operation.
Not so long ago, some manufacturers of household appliances announced the appearance of a number of models of microwave ovens with an inverter power supply circuit. The use of this scheme made it possible not only to increase the volume of usable space in the chamber by reducing the dimensions of the emitter, but also to reduce the power consumption of the device. Unlike conventional models, the heating temperature in inverter-type furnaces changes smoothly, but their cost is an order of magnitude higher.
Cooling and protection of the magnetron
During operation, the magnetron emits a large amount of heat, so a radiator is installed on its body. Since overheating is the main reason for the failure of the magnetron, other methods are also used to protect it:
- Thermal relay. This device is used to protect the magnetron, as well as the grill, if available in the model. The thermal fuse is equipped with a bimetallic plate that can be adjusted to a specific temperature. If this value is exceeded, it bends and opens the power circuit.
- Fan. It not only blows cool air over the magnetron radiator, but also performs a number of other useful functions, such as cooling the electronic components of the device, circulating air inside the chamber when the grill is in operation, and removing hot steam outside through special holes.
- Locking system. Several microswitches control the position of the microwave door, preventing the magnetron from turning on when it is open.
Thermal relay
Is it possible to replace the magnetron
The main advantage of modern household microwave magnetrons is their interchangeability. Magnetrons from other manufacturers will be suitable for various models of microwave ovens, so they can be changed if necessary. In this case, the only necessary requirement will be compliance in terms of power. You can buy a magnetron in many electronics stores, but in order to make the right choice, you need to understand its parameters and labeling. Most often, the following models of magnetrons are installed in microwave ovens:
- 2M 213 (600 W nominal power and 700 W under load);
- 2M 214 (1000 W);
- 2M 246 (1150 W - maximum power).
Even having studied all the necessary parameters of this device, it is not recommended to replace the magnetron at home. Firstly, it will be quite difficult to remove it yourself, and secondly, only a qualified specialist can ensure its safe operation after installation.
Standard magnetron configuration
Diagnostics of malfunctions and the reasons for their occurrence
Replacing a magnetron can be quite costly, so before buying a new device, you need to diagnose the old one to make sure that it is really faulty. Checking can be done at home using a conventional tester. This will require:
- Disconnect the microwave from the mains.
- Remove the protective cover and visually inspect the part.
- "Call" the main elements of the printed circuit board using a tester or "multimeter".
- Inspect the thermostat.
Diagnostics
At the end of the diagnosis, conclusions can be drawn about the malfunction of certain parts. The main reasons for the failure of the magnetron include the following:
- Vacuum tube cap defective. You can replace it yourself by simply picking up a similar cap from another magnetron. The seats for such caps have a standard configuration.
- Heater breakage. If loaded or improperly loaded, the magnetron will overheat, which can lead to excessive filament heating and breakage. To diagnose it, it is necessary to measure the resistance between the legs of the capacitor. If its value is within 5-7 Ohm, then the heater is in good working order.
- Breakdown of the bushing capacitor. If the tester does not show an "infinite" resistance value between its contacts, then the capacitor must be replaced.
