PASSIVE ELECTRONIC COMPONENTS
Here we are, getting close to the study of electronics itself. Prior to this, we mainly studied elements of electrical engineering, which are very important to us. This lesson will also be very information-dense and will require your special perseverance and understanding. Most importantly, after a couple of serious lessons like this one, you will confidently begin assembling and soldering your first simple devices. This should certainly cheer you up and give you the incentive to go through and master all the lessons to the end. Believe me, if your first project works the very first moment you apply power to it, it will primarily be your merit and the fruit of your careful study and analysis of the lessons.
This lesson, like the previous one, is designed for memorization. Everything highlighted in the special blocks or in bold must be learned, even if you don't understand everything yet. The formulas provided must also be memorized (I think there is no need to explain why). There will be no practical assignment in this lesson, as it is designed, like the previous one, for thorough study and analysis. Our interesting practical work will begin from the 8th lesson onwards. By that time, I believe you will have enough knowledge.
RESISTORS
These are perhaps the most frequently used components. In a medium-complexity transistor receiver, for example, there can be 20-25 of them.
They are used to limit current in circuits, to create voltage drops in specific sections of circuits, to separate pulsating current into its components, to adjust volume, tone, etc.
For resistors of relatively low resistance, designed for currents of several tens of milliamperes, a thin wire made of constantan, nichrome, and some other metal alloys is used. These are wire-wound resistors. For resistors of high resistance, designed for relatively small currents, various metal alloys and carbon are used, which are applied in thin layers onto insulating materials. These resistors are called non-wire-wound (metal-film) resistors.
Both wire-wound and non-wire-wound resistors can be fixed, i.e., with constant resistances, and variable, whose resistances can be changed during operation from certain minimum to maximum values.
The main characteristics of a resistor are: its nominal resistance (indicated on its body), its nominal power dissipation, and the maximum possible deviation of the actual resistance from the nominal value (indicated as a percentage, also known as tolerance).
Power dissipation is the maximum current power that a resistor can withstand for a long time and dissipate as heat without damaging its operation. If, for example, a current of 0.1 A flows through a 100 Ohm resistor, it dissipates a power of 1 W (you already know how to calculate this from the previous lesson). If the resistor is not designed for such power, it can quickly burn out. The nominal power dissipation is essentially a characteristic of the electrical durability of the resistor.
Our industry produces fixed and variable resistors of various designs and nominal values: from a few ohms to tens and hundreds of megohms. Among the fixed ones, the most common are metal-film resistors of the MLT type (Metallized, Lacquered, Heat-resistant). The construction of this type of resistor is shown in (Fig. 1, a). Its base is a ceramic tube, on the surface of which a layer of a special alloy is applied, forming a conductive film 0.1 μm thick. In high-resistance resistors, this layer may have the shape of a spiral. Metal caps are pressed onto the ends of the rod with the conductive coating, to which the contact leads of the resistor are welded. The outside of the resistor body is covered with moisture-resistant colored enamel.
MLT resistors are manufactured for power dissipation of 2, 1, 0.5, 0.25, and 0.125 W. They are designated respectively: MLT-2, MLT-1, MLT-0.5, MLT-0.25, and MLT-0.125.
The appearance of these resistors and the standard graphical symbols representing their power dissipation on schematic diagrams are shown in (Fig. 1, b and c).
Over time, you will learn to recognize the power dissipation of resistors by their appearance. The maximum possible deviation of a resistor's actual resistance from its nominal value is expressed as a percentage. If, for example, the nominal value of a resistor is 100 kOhm with a tolerance of 10%, this means that its actual resistance can range from 90 to 110 kOhm.
A variable non-wire-wound resistor is constructed like this (in Fig. 2, an SP-1 resistor is shown without a protective cover): a horseshoe-shaped arc made of getinax (phenolic paper) is glued to a round plastic base, covered with a thin layer of carbon black (resistive layer) mixed with varnish. This layer, possessing resistance, is the actual resistor. Leads are made from both ends of the layer. A bushing is pressed into the center of the base. An axle rotates in it, and a shaped getinax plate rotates with the axle. A wiper brush (slider) made of several springy wires is mounted on the outer end of the plate, which is connected to the middle terminal lug. When the axle rotates, the brush moves along the carbon layer on the arc, resulting in a change in resistance between the middle and extreme leads. On top, the resistor is covered with a metal lid protecting it from damage.
This is exactly or roughly how almost all variable resistors are designed, including types like SP (Variable Resistance), SPO (Variable Volumetric Resistance), and VK. TK resistors differ from VK resistors only in that switches used to turn on power sources are mounted on their covers. Miniature disk variable resistors, for example, type SP3-3v, are fundamentally designed the same way.
Variable non-wire-wound resistors are manufactured with nominal resistances starting from 47 Ohms, with tolerance deviations from the nominal value of 20, 25, and 30%. On schematic diagrams, in order not to clutter them, an abbreviated resistance notation system is used, where the names of their resistance units (Ohm, kOhm, MOhm) are not placed next to the numbers.
Resistances from 1 to 999 Ohms are indicated on schematic diagrams by whole numbers corresponding to Ohms; resistances from 1 to 999 kOhm are indicated by numbers representing kilohms with the letter (k). Large resistances are indicated in megohms with the letter (M).
Here are some examples of marking resistor values on diagrams: R1 270 corresponds to 270 Ohms; R2 6.8k — 6800 Ohms; R3 56k — 56 kOhm (56,000 Ohms); R4 220k — 220 kOhm (0.22 MOhm); R5 1.5M — 1.5 MOhm.
I will make a reservation right away: for the vast majority of amateur radio designs, a deviation from the resistor values indicated on the diagrams by up to 10-15%, and sometimes more, is permissible without detrimental effects on their operation. This means that a resistor with a resistance of, for example, 5.1 kOhm can be replaced by a resistor of the nearest nominal value, i.e., 4.7 or 5.6 kOhm.
Imagine this situation. You need a resistor of a certain resistance. You don't have exactly that one, but you have resistors of other values. Is it possible to combine them to make a resistor of the required value? Yes, of course, if you know the elementary calculation for series and parallel connections of electrical circuits and resistors.
When resistors are connected in series (Fig. 3, a), their total resistance R_total is equal to the sum of the resistances of all the resistors connected in this chain.
R_total = R1 + R2 + R3, etc. For example, if R1 = 15 kOhm and R2 = 33 kOhm, then their total resistance is R_total = 15 + 33 = 48 kOhm (closest standard values are 47 and 51 kOhm).
When resistors are connected in parallel (Fig. 3, b), their total resistance R_total decreases and is always less than the resistance of the smallest individual resistor in the parallel group.
The resulting resistance of a circuit consisting of two parallel-connected resistors is calculated using the following formula: R_total = (R1 * R2) / (R1 + R2), i.e., the product of the two resistors divided by their sum. If both resistors have the same value, then the nominal value of one of the resistors is simply divided by 2. Suppose R1 = 20 kOhm and R2 = 30 kOhm. The total resistance of the circuit section consisting of these two resistors is: R_total = (20 * 30) / (20 + 30) = 12 kOhm. It should be mentioned that when connecting more than two resistors in parallel, R_total is calculated using the formula below:
CAPACITORS
Capacitors, like resistors, are among the most numerous components in radio engineering devices.
The main property of capacitors is their ability to accumulate (store) an electrical charge. The main parameter of a capacitor is its capacitance.
The capacitance of a capacitor will be greater the larger the area of its plates and the thinner the dielectric layer between them. The basic unit of electrical capacitance is the Farad (abbreviated as F), named in honor of the English physicist M. Faraday.
However, 1 F is a very large capacitance. The Earth, for example, has a capacitance of less than 1 F. In electrical and radio engineering, a unit of capacitance equal to one-millionth of a farad is used, which is called a microfarad (abbreviated as μF). There are 1,000,000 μF in one farad, i.e., 1 μF = 0.000001 F. But even this unit of capacitance is often too large. Therefore, there is an even smaller unit of capacitance called a picofarad (abbreviated as pF), which is a millionth of a microfarad, i.e., 0.000001 μF; 1 μF = 1,000,000 pF.
All capacitors, whether fixed or variable, are characterized primarily by their capacitance, expressed respectively in picofarads, nanofarads, and microfarads. On schematic diagrams, the capacitance of capacitors from 1 to 9999 pF is indicated by integers corresponding to their capacitance in these units without the pF designation, and the capacitance of capacitors from 0.01 μF (10000 pF) and more — in fractions of a microfarad or in microfarads without the μF designation. If the capacitance is equal to an integer number of microfarads, then unlike the designation in picofarads, a comma and a zero are placed after the last significant digit.
Examples of capacitor values on diagrams: C1 47 corresponds to 47 pF; C2 3300 corresponds to 3300 pF; C3 0.47 corresponds to 0.047 μF (47000 pF); C4 0.1 corresponds to 0.1 μF; C5 20.0 corresponds to 20 μF. There is also an intermediate capacitance value — the nanofarad (nF), which is one-thousandth of a microfarad. For example: 1000 pF = 1 nF or 0.01 μF = 10 nF. How to convert a large physical quantity to a smaller one and vice versa, I think you have already guessed; it's pure mathematics.
I already mentioned that a capacitor in its simplest form consists of two plates separated by a dielectric. If a capacitor is connected to a direct current (DC) circuit, the current in this circuit will stop. And this is understandable: direct current cannot flow through an insulator, which is the dielectric of the capacitor. Including a capacitor in a DC circuit is equivalent to breaking it (we do not take into account the moment of switching on, when a short-term charging current of the capacitor appears in the circuit). A capacitor behaves differently in an alternating current (AC) circuit.
Let's remember: the polarity of the voltage at the terminals of an AC source changes periodically. This means that if you connect a capacitor to a circuit powered by such a current source, its plates will alternately recharge with the frequency of this current. As a result, an alternating current will flow in the circuit. A capacitor, like a resistor and an inductor, offers resistance to alternating current, but it is different for currents of different frequencies. It can pass high-frequency currents well and at the same time act almost as an insulator for low-frequency currents.
Amateur radio operators, for example, sometimes use the wires of the electrical lighting network instead of outdoor antennas, connecting receivers to them through a capacitor with a capacity of 220-510 pF. Is such a capacitor value chosen by chance? No, not by chance. A capacitor of this capacity easily passes high-frequency currents necessary for the receiver's operation but offers high resistance to the 50 Hz alternating current flowing in the mains. In this case, the capacitor becomes a kind of filter that passes high-frequency current and blocks low-frequency current.
The capacitive reactance of a capacitor to an alternating current depends on its capacitance and the frequency of the current: the greater the capacitance and the current frequency, the lower its capacitive reactance.
This resistance can be determined with sufficient accuracy using this simplified formula: R_c = 1 / (6 * F * C), where R_c is the capacitive reactance in Ohms; F is the current frequency in Hz; C is the capacitance in Farads; the number 6 is the rounded value of 2π (more accurately 6.28, since π = 3.14).
Using this formula, let's find out how a capacitor behaves in relation to alternating currents if we use the mains wires as an antenna. Let's assume that the capacity of this capacitor is 500 pF (500 pF = 0.0000000005 F). The mains current frequency is 50 Hz. Let's take 1 MHz (1,000,000 Hz) as the average carrier frequency of a radio station, which corresponds to a wavelength of 300 m. What resistance does this capacitor offer to the radio frequency? R_c = 1 / (6 * 1000000 * 0.0000000005) = 300 Ohms. And to the AC mains current? R_c = 1 / (6 * 50 * 0.0000000005) ≈ 7 MOhm. And here is the result: a 500 pF capacitor offers 20,000 times less resistance to a high-frequency current than to a low-frequency one. Convincing? A smaller capacitor offers even greater resistance to the alternating mains current.
You must remember: the capacitive reactance of a capacitor to an AC current decreases with an increase in its capacitance and current frequency, and vice versa, it increases with a decrease in its capacitance and current frequency. This property of a capacitor—blocking DC and conducting AC currents of different frequencies differently—is used to separate pulsating currents into their components, block currents of some frequencies, and pass currents of other frequencies (capacitive filters).
You will frequently use this property of capacitors in your experiments and designs.
How are fixed capacitors constructed? All fixed capacitors have conductive plates with ceramic, mica, paper, or some other solid dielectric between them. Depending on the type of dielectric used, capacitors are called ceramic, mica, or paper, respectively. The appearance of some ceramic fixed capacitors is shown in Fig. 4. Their dielectric is a special ceramic, their plates are thin layers of silvered metal deposited on the surface of the ceramic, and their leads are silvered brass wires or strips soldered to the plates. The capacitor bodies are coated with enamel on the outside.
The most common ceramic capacitors are types KDK (Ceramic Disk Capacitor), KTK (Ceramic Tubular Capacitor), and KM. In a KTK type capacitor, one plate is deposited on the inner surface and the other on the outer surface of a thin-walled ceramic tube. Sometimes tubular capacitors are placed in sealed porcelain cases with metal caps at the ends. These are KGK type capacitors.
Ceramic capacitors have relatively small capacities — up to a few thousand picofarads. They are placed in circuits where high-frequency current flows (antenna circuit, oscillatory circuit) to couple them together.
To obtain a capacitor of small dimensions but with a relatively large capacity, it is made not from two, but from several plates stacked together and separated from each other by a dielectric (Fig. 5). In this case, each pair of adjacent plates forms a capacitor. Connecting these pairs of plates in parallel yields a capacitor of significant capacity. This is how most mica-dielectric capacitors are made. Their plates are sheets of aluminum foil or layers of silver applied directly to the mica, and the leads are pieces of silvered wire. Such capacitors are molded in plastic.
These are KSO capacitors. Their designation contains a number characterizing the shape and dimensions of the capacitors, for example: KSO-1, KSO-5. The larger the number, the larger the dimensions of the capacitor. Some mica capacitors are produced in moisture-proof ceramic cases. They are called SGM type capacitors. The capacity of mica capacitors ranges from 47 to 50,000 pF (0.05 μF). Like ceramic ones, they are intended for high-frequency circuits, as well as for use as blocking capacitors and for coupling between high-frequency circuits.
In paper capacitors (Fig. 5), the dielectric is thin paper soaked in paraffin, and the plates are foil. Paper strips together with the plates are rolled into a tight roll and placed in a cardboard or metal housing. The wider and longer the plates, the greater the capacity of the capacitor. Paper capacitors are used mainly in low-frequency circuits, as well as for bypassing power sources. There are many varieties of capacitors with a paper dielectric. And all of them have the letter B (Paper - "Бумажные" in Russian) in their designation.
BM (Paper Miniature) type capacitors are enclosed in metal tubes sealed at the ends with special resin (older style). KB capacitors have cylindrical cardboard housings. KBG-I type capacitors are placed in porcelain housings with metal end caps connected to the plates, from which narrow lead lugs extend. Capacitors with a capacity of up to several microfarads are produced in metal housings. These include capacitor types KBG-MP, KBG-MN, KBGT. There can be two or three of them in a single housing.
Fig. 5 Mica capacitors. Paper and metal-paper fixed capacitors.
The dielectric of MBM (Metal-Paper Miniature) capacitors is lacquered capacitor paper, and the plates are metal layers less than a micron thick, applied to one side of the paper. A characteristic feature of this type of capacitor is the ability to self-heal after electrical breakdown of the dielectric.
A special group of fixed capacitors are electrolytic capacitors (Fig. 6). In its internal structure, an electrolytic capacitor is somewhat similar to a paper one. It has two ribbons of aluminum foil. The surface of one of them is covered with a very thin oxide layer. A ribbon of porous paper soaked in a special thick liquid — an electrolyte — is laid between the aluminum ribbons. This four-layer strip is rolled up and placed in a cylindrical aluminum cup or cartridge. The dielectric of the capacitor is the oxide layer. The positive plate is the ribbon that has the oxide layer. It connects to a terminal lug insulated from the case. The second, negative plate — the paper soaked in electrolyte transferring connection through the non-oxidized ribbon — is connected to the metal housing.
Thus, the housing is the terminal of the negative plate, and the lug insulated from it is the terminal of the positive plate of the electrolytic capacitor. This is exactly how KE and K50-3 type capacitors are constructed. KE-2 capacitors differ from KE types only by a plastic bushing with a thread and a nut for chassis mounting. The aluminum bodies of K50-3 capacitors are shaped like cartridges with a diameter of 4.5-6 mm and a length of 15-20 mm. The leads are wire type. K50-6 capacitors are constructed similarly. But their electrode (plate) leads are isolated from the housing. On schematic diagrams, electrolytic capacitors are depicted in the same way as other fixed capacitors — with two parallel lines — but a "+" sign is placed next to the positive plate.
Electrolytic capacitors have large capacities — from fractions of a microfarad to several thousand microfarads. They are designed to work in circuits with pulsating currents, for example, in AC rectifier filters, and for coupling low-frequency circuits. In this case, the negative electrode of the capacitor is connected to the negative pole of the circuit, and the positive one — to its positive pole. If the polarity is reversed, the electrolytic capacitor will fail.
The nominal capacities of electrolytic capacitors are written on their bodies. The actual capacity can differ significantly from the nominal one. The most important characteristic of any capacitor, besides its capacity, is also its nominal voltage, i.e., the voltage at which the capacitor can work for a long time without losing its properties. This voltage depends on the properties and thickness of the capacitor's dielectric layer. Ceramic, mica, paper, and metal-paper capacitors of various types are designed for nominal voltages from 150 to 1000 V and more. Electrolytic capacitors are produced for nominal voltages ranging from a few volts to 30-50 V and from 150 to 450-500 V.
In connection with this, they are divided into two groups: low-voltage and high-voltage. Capacitors of the first group are used in circuits with relatively low voltage, and capacitors of the second group — in circuits with relatively high voltage.
When selecting capacitors for your projects, always pay attention to their nominal voltages!
Capacitors can be used in a circuit with a voltage lower than the nominal one, but they must not be used in a circuit with a voltage exceeding the nominal one. If the voltage across the capacitor plates exceeds its nominal voltage, the dielectric will break down (in this case, an electrolytic capacitor can explode rather violently, so be careful!!!). A broken-down capacitor is unfit for operation.
Now about variable capacitors. You can see the structure of a simple variable capacitor in Fig. 7. One of its plates is the stator and is stationary. The second is the rotor and is aligned with the axle. When the axle rotates, the overlapping area of the plates changes, and with it, the capacity of the capacitor changes.
Variable capacitors used in tunable oscillatory circuits of receivers consist of two groups of plates (Fig. 8, a) made of sheet aluminum or brass. The rotor plates are connected by an axle. The stator plates are also connected to each other and insulated from the rotor. When the axle rotates, the plates of the stator group gradually enter the air gaps between the plates of the rotor group, which smoothly changes the capacitor's capacity. When the rotor plates are completely moved out of the gaps between the stator plates, the capacity of the capacitor is the smallest; it is called the initial capacity of the capacitor. When the rotor plates are fully inserted between the stator plates, the capacity of the capacitor will be the greatest, i.e., maximum for the given capacitor.
The maximum capacity of a capacitor will be greater the more plates it has and the smaller the distance between the moving and stationary plates.
In the capacitors shown in Fig. 7 and 8, a, the dielectric is air. In miniature variable capacitors (Fig. 8, b), the dielectric can be paper, plastic films, or ceramics. Such capacitors are called variable capacitors with a solid dielectric. With smaller dimensions than air-dielectric capacitors, they can have significant maximum capacities. These are the capacitors used to tune the oscillatory circuits of miniature transistor receivers. The most common are variable capacitors with an initial capacity of a few picofarads and a maximum of 240-490 pF.
It is quite possible that you have already seen one of these capacitors and used it to tune your first radio receiver. Receivers with two tunable oscillatory circuits use blocks of variable capacitors (ganged capacitors — paired, triple, etc.). In the variable capacitor block shown in Fig. 9, there are two capacitors whose rotors share a common axle. Rotating the axle changes the capacities of both capacitors simultaneously.
Single capacitors and blocks of variable capacitors with an air dielectric require careful handling. Even a slight bending or other damage to the plates leads to a short circuit between them. And straightening capacitor plates is a difficult task.
The category of solid dielectric capacitors also includes trimmer capacitors, which are a variety of variable capacitors.
Most often, such capacitors are used to fine-tune circuits to resonance, which is why they are called trimmers.
The designs of the most common trimmer capacitors are shown in Fig. 10. Each of them consists of a relatively massive ceramic base and a thin ceramic disk. Metal layers in the form of sectors, which act as the capacitor plates, are applied to the surface of the base (under the disk) and onto the disk itself. When the disk rotates around its axis, the overlapping area of the plate sectors changes, altering the capacitor's capacity.
The capacity of trimmer capacitors is indicated on their bodies as a fraction, where the numerator is the minimum and the denominator is the maximum capacity of the given capacitor.
If, for example, 6/30 is indicated on a capacitor, this means that its minimum capacity is 6 pF, and its maximum is 30 pF. Trimmer capacitors usually have a minimum capacity of 2-5 pF and a maximum of up to 100-150 pF. Some of them, for example KPK-2, can be used as main variable tuning capacitors for simple single-circuit receivers.
Capacitors, like resistors, can be connected in parallel or in series to achieve the required capacity. If you connect capacitors in parallel (Fig. 11, a), their total capacity will be equal to the sum of the capacities of all the connected capacitors.
C_total = C1 + C2 + C3, etc. So, for example, if C1 = 33 pF and C2 = 47 pF, then the total capacity of these two capacitors will be: C_total = 33 + 47 = 80 pF.
When connecting capacitors in series (Fig. 11, b), their total capacity is always less than the smallest capacity included in the chain.
It is calculated using the formula C_total = (C1 * C2) / (C1 + C2). For example, suppose C1 = 220 pF and C2 = 330 pF; then C_total = (220 * 330) / (220 + 330) = 132 pF. When two capacitors of the same capacity are connected in series, their total capacity will be half the capacity of either one. An attentive reader should have noticed the absolute similarity with resistor calculations. Yes, indeed, the formulas are the same, with the only difference being that they are applied in reverse depending on the connection type.
It is also important to remember that with a series connection, the total capacity decreases, but the maximum allowable operating voltage increases, i.e., it will equal the sum of the voltages of all the capacitors in the series chain.
DETAILS ON THE ABBREVIATED MARKING SYSTEM FOR RESISTOR AND CAPACITOR VALUES
On relatively large resistors and capacitors, their nominal resistances or capacities are marked using generally accepted abbreviated designations of electrical units, and next to them — the possible deviation from the nominal value as a percentage, for example: 1.5 10%, 33 20%.
According to the alphanumeric system, the resistance unit OHM is abbreviated with the letter (E) or (R), kilohm — with the letter (K), megohm — with the letter (M).
Resistances from 100 to 910 Ohms are expressed in fractions of a kilohm, and resistances from 100,000 to 910,000 Ohms — in fractions of a megohm. If the nominal resistance of a resistor is expressed as an integer, the letter designating the unit of measurement is placed after the number, for example, 33E (33 Ohm), 47K (47 kOhm), 1M (1 MOhm). When the resistance is expressed as a decimal fraction less than one, the letter designation is placed before the number, for example, K22 (220 Ohm), M47 (470 kOhm). When expressing a resistance with an integer and a decimal fraction, the integer is placed before the letter, and the decimal fraction after the letter symbolizing the unit (the letter replaces the decimal point). Examples: 1E5 (1.5 Ohm), 2K2 (2.2 kOhm), 1M5 (1.5 MOhm).
Tolerance is indicated after the nominal value designation with the following letters (in Soviet/Russian standards): Tolerance, 10% (C), 5% (И). Suppose a miniature resistor is marked: 1M5И. This means the nominal resistance is 1.5 MOhm, and the tolerance deviation is 5%.
The nominal capacities of capacitors up to 91 pF are expressed in picofarads, using the letter (P) for this unit. Capacities from 100 to 9100 pF are expressed, as mentioned above, in fractions of a nanofarad (1 nF = 1000 pF or 0.001 μF), and from 0.01 to 0.091 μF — in nanofarads, designating the nanofarad with the letter (H or n). Capacities from 0.1 μF and above are expressed in microfarads, using the letter (M or μ) for this unit. If the capacity is expressed as an integer, the letter is placed after the number, for example: 12P (12 pF), 15H (15 nF = 15,000 pF or 0.015 μF), 10M (10 μF). To express a nominal capacity as a decimal fraction, the letter is placed before the number: H15 (0.15 nF = 150 pF), M22 (0.22 μF). For a capacity expressed as an integer with a decimal fraction, the letter is placed between the integer and the decimal fraction, replacing the decimal point, for example: 1P2 (1.2 pF), 4H7 (4.7 nF = 4700 pF), 1M5 (1.5 μF). The tolerance is marked after the nominal capacity designation with numbers representing percentages, e.g., 30%, 20%, 10%, 5%, etc.
BRIEFLY ABOUT THE FUSE
This device is a piece of wire whose thickness is calculated to pass a current of a certain specific value, for example, 0.25 A. It protects the power source from overload. All power grids, sometimes wall sockets, and electronic devices powered by the mains have fuses. The fuse is inserted into a break in the electrical circuit so that all the current consumed by the circuit passes through it. As long as the current does not exceed the permissible limit, the fuse wire is slightly warm or completely cold. But as soon as an unacceptably heavy load or short circuit occurs in the circuit, the current will rise sharply, melt the wire, and the circuit will automatically break.
The cartridge (commonly called a plug - "пробка") of a fuse used in the lighting mains is constructed exactly like a light bulb socket. A porcelain plug (Fig. 12 - left) containing a lead wire is screwed into it.
One end of it is soldered to the metal bottom of the plug, and the other to the metal threaded cylinder with which the fuse is screwed into the socket. The fuse wire of an electronic device (in Fig. 12 - right) is enclosed in a glass tube and its ends are soldered to metal caps acting as contacts. Using these contacts, the fuse is inserted into a special holder or between two metal stands connected to the wires of the network protected from overloads.
You must find and eliminate the cause that made the fuse blow out. Only after doing this, and exercising extreme caution, can you insert a new fuse into the electrical circuit—AND ONLY WHEN THE POWER IS DISCONNECTED!!!
Moving on to the next lesson!
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