Fixed capacitors. Types of capacitors, their classification

Electrical capacitors are used to store electricity. The simplest capacitor consists of two metal plates - plates and a dielectric located between them. If you connect a power source to the capacitor, then opposite charges will appear on the plates and an electric field will appear, attracting them towards each other. These charges remain after the power source is turned off, the energy is stored in the electric field between the plates.

In ceramic capacitors, the dielectric is high-quality ceramics: ultraporcelain, tikond, ultrasteatite, etc. The lining is a layer of silver applied to the surface. Ceramic capacitors are used in isolation circuits of high-frequency amplifiers.

In electrolytic polar capacitors, the dielectric is an oxide layer deposited on a metal foil. The other lining is formed from paper tape impregnated with electrolyte.

In solid oxide capacitors, the liquid dielectric is replaced with a special conductive polymer. This allows you to increase service life (and reliability). The disadvantages of solid oxide capacitors are higher price and voltage limitations (up to 35 V).

Oxide electrolytic and solid-state capacitors are characterized by high capacitance, with relatively small sizes. This feature is determined by the fact that the thickness of the oxide - dielectric is very small.

When connecting oxide capacitors into a circuit, polarity must be observed. In case of polarity violation, electrolytic capacitors explode, solid-state capacitors simply fail. To completely avoid the possibility of explosion (for electrolytic capacitors), some models are equipped with safety valves (not available for solid-state capacitors). The scope of application of oxide (electrolytic and solid-state) capacitors is separating circuits of audio amplifiers, smoothing filters of DC power supplies.

Capacitors based on metallized film are used in high-voltage power supplies.

Table 2.
Characteristics of mica capacitors and capacitors based on polyester and polypropylene.

Mica capacitors are made by laying mica plates between foil plates, or vice versa - by metallizing mica plates. Mica capacitors are used in sound-reproducing devices, high-frequency noise filters and generators. Polyester based capacitors are general purpose capacitors, while polypropylene based capacitors are used in high voltage DC circuits.

Table 3.
Characteristics of mica capacitors based on polycarbonate, polystyrene and tantalum.

Polycarbonate-based capacitors are used in filters, generators and timing circuits. Capacitors based on polystyrene and tantalum are also used in timing and separating circuits. They are considered general purpose capacitors.
In general-purpose metal-paper capacitors, the plates are made by spraying metal onto paper impregnated with a special composition and coated with a thin layer of varnish.

2. The second option - marking is done not in pico, but in microfarads, and the letter µ is placed instead of the decimal point.

3. Third option.

Soviet capacitors used “p” instead of the Latin “r”.

The permissible deviation of the nominal capacity is marked with a letter, often the letter follows the code defining the capacity (the same line).

Capacitors with a linear dependence on temperature.

Next comes the voltage in volts, most often in the form of a regular number.
For example, the capacitor in this picture is marked with two lines. The first (104J) means that its capacitance is 0.1 μF (104), the permissible deviation of the capacitance does not exceed ± 5% (J). The second (100V) is the voltage in volts.

Marking SMD capacitors.

The dimensions of SMD capacitors are small, so their marking is done very succinctly. The operating voltage is often coded with a letter (2nd and 3rd options in the figure below) in accordance with (option 2 in the figure), or using a two-digit alphanumeric code (option 1 in the figure). When using the latter, you can still find two (and not one letter) with one number on the case (option 3 in the figure).

The first letter can be either a manufacturer's code (which is not always interesting) or indicate the rated operating voltage (more useful information), the second can be an encoded value in picoFarads (mantissa). The number is an exponent (indicates how many zeros need to be added to the mantissa).
For example, EA3 may mean that the rated voltage of the capacitor is 16V(E) and the capacitance is 1.0 * 1000 = 1 nanofarad, BF5, respectively, the voltage is 6.3V(V), the capacitance is 1.6 * 100000 = 0.1 microfarad and. etc.

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A capacitor is an element of an electrical circuit that serves as a charge storage device.

There are now many areas of application for this device, which explains their wide range. They differ in the materials from which they are made, purpose, and range of the main parameter. But the main characteristic of a capacitor is its capacity.

Operating principle of a capacitor

Design

In the diagrams, the capacitor is indicated as two parallel lines that are not interconnected:

This corresponds to its simplest design - two plates (plates) separated by a dielectric. The actual design of this product most often consists of plates wrapped in a roll with a layer of dielectric or other fancy shapes, but the essence remains the same.

Electrical capacity is the ability of a conductor to accumulate electrical charges. The more charge a conductor can hold at a given potential difference, the greater the capacitance. The relationship between charge Q and potential φ is expressed by the formula:

where Q is the charge in coulombs (C), φ is the potential in volts (V).

Capacitance is measured in farads (F), which you remember from physics lessons. In practice, smaller units are more common: millifarad (mF), microfarad (µF), nanofarad (nF), picofarad (pF).

The storage capacity depends on the geometric parameters of the conductor and the dielectric constant of the medium where it is located. So, for a sphere made of conductive material it will be expressed by the formula:

C=4πεε0R

where ε0-8.854·10^−12 F/m is the electrical constant, and ε is the dielectric constant of the medium (tabular value for each substance).

In real life, we often have to deal not with one conductor, but with systems of such. So, in a regular flat capacitor, the capacitance will be directly proportional to the area of ​​the plates and inversely to the distance between them:

C=εε0S/d

ε here is the dielectric constant of the spacer between the plates.

Capacity of parallel and serial systems

A parallel connection of capacitors represents one large capacitor with the same dielectric layer and the total area of ​​the plates, so the total capacitance of the system is the sum of those of each of the elements. The voltage in a parallel connection will be the same, and the charge will be distributed between the circuit elements.​

C=C1+C2+C3

A series connection of capacitors is characterized by a common charge and distributed voltage between the elements. Therefore, it is not the capacity that is summed up, but its inverse:

1/C=1/С1+1/С2+1/С3

From the formula for the capacitance of a single capacitor, it can be concluded that with identical elements connected in series, they can be represented as one large one with the same plate area, but with the total thickness of the dielectric.

Reactance

A capacitor cannot conduct direct current, as can be seen from its design. In such a circuit it can only charge. But in AC circuits it works great, constantly recharging. If not for the limitations emanating from the properties of the dielectric (it can be broken if the voltage limit is exceeded), this element would be charged indefinitely (the so-called ideal capacitor, something like a black body and an ideal gas) in a direct current circuit, and the current through it will not pass. Simply put, the resistance of a capacitor in a DC circuit is infinite.

With alternating current the situation is different: the higher the frequency in the circuit, the lower the resistance of the element. This resistance is called reactance, and it is inversely proportional to frequency and capacitance:

Z=1/2πfC

where f is the frequency in hertz.

Energy storage

The energy stored by a charged capacitor can be expressed by the formula:

E=(CU^2)/2=(q^2)/2C

where U is the voltage between the plates, and q is the accumulated charge.

Capacitor in an oscillating circuit

In a closed loop containing a coil and a capacitor, alternating current can be generated.

After charging the capacitor, it will begin to self-discharge, giving an increasing current. The energy of a discharged capacitor will become zero, but the magnetic energy of the coil will be maximum. A change in the current value causes the self-inductive emf of the coil, and by inertia it will pass current towards the second plate until it is completely charged. In the ideal case, such oscillations are endless, but in reality they quickly die out. The oscillation frequency depends on the parameters of both the coil and the capacitor:

where L is the inductance of the coil.

A capacitor may have its own inductance, which can be observed as the frequency of the current in the circuit increases. In the ideal case, this value is insignificant and can be neglected, but in reality, when the plates are rolled up plates, this parameter cannot be ignored, especially when it comes to high frequencies. In such cases, the capacitor combines two functions and represents a kind of oscillatory circuit with its own resonant frequency.

Performance characteristics

In addition to the above-mentioned capacitance, self-inductance and energy intensity, real capacitors (and not ideal ones) have a number of properties that must be taken into account when choosing this element for the circuit. These include:

To understand where the losses come from, it is necessary to explain what the graphs of sinusoidal current and voltage in this element are. When the capacitor is charged to its maximum, the current in its plates is zero. Accordingly, when the current is maximum, there is no voltage. That is, the voltage and current are out of phase by an angle of 90 degrees. Ideally, a capacitor has only reactive power:

Q=UIsin 90

In reality, the capacitor plates have their own resistance, and part of the energy is spent on heating the dielectric, which causes its losses. Most often they are insignificant, but sometimes they cannot be neglected. The main characteristic of this phenomenon is the dielectric loss tangent, which is the ratio of active power (provided by low losses in the dielectric) and reactive power. This value can be measured theoretically by presenting the real capacity in the form of an equivalent equivalent circuit - parallel or series.

Determination of dielectric loss tangent

In a parallel connection, the amount of losses is determined by the ratio of currents:

tgδ = Ir/Ic = 1/(ωCR)

In the case of a series connection, the angle is calculated by the voltage ratio:

tgδ = Ur/Uc = ωCR

In reality, to measure tgδ, they use a device assembled using a bridge circuit. It is used to diagnose insulation losses in high-voltage equipment. Using measuring bridges, you can also measure other network parameters.

Rated voltage

This parameter is indicated on the label. It shows the maximum voltage that can be applied to the plates. Exceeding the nominal value can lead to breakdown of the capacitor and its failure. This parameter depends on the properties of the dielectric and its thickness.

Polarity

Some capacitors have polarity, that is, it must be connected to the circuit in a strictly defined way. This is due to the fact that some kind of electrolyte is used as one of the plates, and the oxide film on the other electrode serves as the dielectric. When the polarity changes, the electrolyte simply destroys the film and the capacitor stops working.

Capacitance temperature coefficient

It is expressed by the ratio ΔC/CΔT where ΔT is the change in ambient temperature. Most often, this dependence is linear and insignificant, but for capacitors operating in aggressive conditions, TKE is indicated in the form of a graph.

Capacitor failure is due to two main reasons - breakdown and overheating. And if in the event of a breakdown some of their types are capable of self-healing, then overheating leads to destruction over time.

Overheating is caused by both external reasons (heating of neighboring circuit elements) and internal ones, in particular, the series equivalent resistance of the plates. In electrolytic capacitors it leads to evaporation of the electrolyte, and in oxide semiconductor capacitors it leads to breakdown and a chemical reaction between tantalum and manganese oxide.

The danger of destruction is that it often occurs with the probability explosion housings.

Technical design of capacitors

Capacitors can be classified into several groups. So, depending on the ability to regulate the capacity, they are divided into constant, variable and adjustable. In shape they can be cylindrical, spherical and flat. You can divide them according to purpose. But the most common classification is according to the type of dielectric.

Paper capacitors

Paper is used as a dielectric, very often oiled paper. As a rule, such capacitors are large in size, but there were also small versions without oiling. They are used as stabilizing and storage devices, and are gradually being replaced from consumer electronics by more modern film models.

In the absence of oiling, they have a significant drawback - they react to air humidity even with sealed packaging. Wet paper increases energy loss.

Dielectric in the form of organic films

Films can be made of organic polymers, such as:

• polyethylene terephthalate;
• polyamide;
• polycarbonate;
• polysulfone;
• polypropylene;
• polystyrene;
• fluoroplastic (polytetrafluoroethylene).

Compared to the previous ones, such capacitors are more compact in size and do not increase dielectric losses with increasing humidity, but many of them are at risk of failure due to overheating, and those that do not have this disadvantage are more expensive.

Solid inorganic dielectric

This can be mica, glass and ceramics.

The advantage of these capacitors is their stability and linearity of the dependence of capacitance on temperature, applied voltage, and in some cases even on radiation. But sometimes such dependence itself becomes a problem, and the less pronounced it is, the more expensive the product.

Oxide dielectric

Aluminum, solid-state and tantalum capacitors are produced with it. They have polarity, so they fail if connected incorrectly and the voltage rating is exceeded. But at the same time they have good capacity, are compact and stable in operation. With proper operation, they can work for about 50 thousand hours.

Vacuum

Such devices are a glass or ceramic flask with two electrodes from which air is pumped out. They have virtually no losses, but their low capacity and fragility limit their scope of application to radio stations, where the size of the capacitance is not so important, but resistance to heating is of fundamental importance.

Electric double layer

If you look at what a capacitor is needed for, you can understand that this type is not exactly it. Rather, it is an additional or backup power source, which is what they are used for. Some categories of such devices - ionistors - contain activated carbon and an electrolyte layer, others operate on lithium ions. The capacity of these devices can be up to hundreds of farads. Their disadvantages include high cost and active resistance with leakage currents.

Whatever the capacitor, there are two mandatory parameters that must be reflected in the marking - these are its capacitance and rated voltage.

In addition, on most of them there is a numerical and alphabetic designation of its characteristics. In accordance with Russian standards, capacitors are marked with four signs.

The first letter K means “capacitor”, the next number is the type of dielectric, followed by a destination indicator in the form of a letter; the last icon can mean both the design type and the development number, this already depends on the manufacturer. The third point is often missed. Such markings are used on products large enough to accommodate them. According to GOST, the decoding will look like this:

First letters:

1. K is a constant capacitor.
2. CT is a trimmer.
3. KP is a variable capacitor.

The second group is the type of dielectric:

All this cannot be placed on small capacitors, so abbreviated markings are used, which, if you are unaccustomed to it, may even require a calculator, and sometimes a magnifying glass. This marking encodes the capacitance, voltage rating and deviations from the main parameter. There is no point in recording the remaining parameters: these are, as a rule, ceramic capacitors.

Marking of ceramic capacitors

Sometimes everything is simple with them - the capacity is marked with a number and units: pF - picofarad, nF - nanofarad, μF - microfarad, mF - millifarad. That is, the 100nF inscription can be read directly. The denomination is, respectively, the number and the letter V. But sometimes even this does not fit, so abbreviations are used. So, often the capacity fits into three digits (103, 109, etc.), where the last one means the number of zeros, and the first two mean the capacity in picofarads. If there is a 9 at the end, then there are no zeros, and a comma is placed between the first two. When the number 8 is at the end, the comma is moved back one more place.

For example, the designation 109 stands for 1 picofarad, and 100–10 picofarads; 681–680 picofarads, or 0.68 nanofarads, and 104–100 thousand pF or 100nF

You can often find the first letter of the unit of measurement as a comma: p50–0.5 pF, 1n5–1.5 nF, 15μ – 15 µF, 15m – 15 mF. Sometimes R is written instead of p.

After three numbers there may be a letter indicating the spread of the capacity parameter:

If you calculate the characteristics of a circuit in SI units, then in order to find the capacitance in farads, you need to remember the exponents of the number 10:

1. -3 - millifarads;
2. -6 - microfarads;
3. -9 - nanofarads;
4. -12 is picofarads.

Thus, 01 pF is 0.1 *10^-12 F.

On SMD devices, the capacitance in picofarads is indicated by a letter, and the number after it is the power of 10 by which this value must be multiplied.

The rated operating voltage can be marked with a letter in the same way, if it is problematic to write it completely. The following standard for the letter designation of denominations has been adopted in Russia:

Despite the lists and tables, it is still better to study the encoding of a specific manufacturer - they may differ in different countries.

Some capacitors come with a more detailed description of their characteristics.

Capacitor - this is an element of an electrical circuit capable, with a small size, of accumulating electrical charges of a sufficiently large magnitude. The simplest model of a capacitor is two electrodes, between which there is any dielectric. The role of the dielectric in it is played by paper, air, mica and other insulating materials, the task of which is to prevent the contact of the plates.

Properties

Capacity. This is the main property of a capacitor. Measured in Farads and calculated using the following formula (for a parallel-plate capacitor):

where C, q, U are respectively the capacitance, charge, voltage between the plates, S is the area of ​​the plates, d is the distance between them, is the dielectric constant, is the dielectric constant equal to 8.854*10^-12 F/m..

Capacitor Polarity;

Rated voltage;

Specific capacitance and others.

The capacitance value of the capacitor depends on

Plate area. This is clear from the formula: capacity is directly proportional to charge. Naturally, by increasing the area of ​​the plates, we obtain a larger amount of charge.

Distances between plates. The closer they are located, the greater the intensity of the resulting electric field.

Capacitor device

The most common capacitors are flat and cylindrical. Flat ones consist of plates spaced apart from each other
friend at a short distance. Cylindrical, assembled using cylinders of equal length and different diameters. All capacitors are basically the same. The difference is mainly in what material is used as the dielectric. Capacitors are classified according to the type of dielectric medium, which can be liquid, vacuum, solid, or air.

How is a capacitor charged and discharged?

When connected to a direct current source, the plates of the capacitor are charged, one acquires a positive potential, and the other a negative one. Between the plates, electric charges of opposite sign, but equal in value, create an electric field. When the voltages become the same on both the plates and the source of the supplied current, the movement of electrons will stop and charging of the capacitor will end. For a certain period of time, the capacitor retains charges and functions as an autonomous source of electricity. It can remain in this state for quite a long time. If instead of a source, you include a resistor in the circuit, the capacitor will discharge onto it.

Processes occurring in a capacitor

When the device is connected to alternating or direct current, different processes will occur in it. Direct current will not flow through the circuit with a capacitor. Since there is a dielectric between its plates, the circuit is actually open.

Alternating current, due to the fact that it periodically changes direction, can pass through a capacitor. In this case, a periodic discharge and charge of the capacitor occurs. During the first quarter of the period, the charge goes to the maximum, electricity is stored in it, in the next quarter the capacitor is discharged and the electrical energy is returned back to the network. In an alternating current circuit, a capacitor has, in addition to active resistance, also a reactive component. In addition, in a capacitor, the current leads the voltage by 90 degrees, this is important to take into account when constructing vector diagrams.

Application

Capacitors are used in radio engineering, electronics, and automation. A capacitor is an irreplaceable element that is used in many branches of electrical engineering, in enterprises, and in scientific research. As an example, if necessary, acts as a current separator: alternating and direct, used in capacitor installations, if necessary

In all radio engineering and electronic devices, in addition to transistors and microcircuits, capacitors are used. Some circuits have more of them, others have less, but there is practically no electronic circuit without capacitors.

At the same time, capacitors can perform a variety of tasks in devices. First of all, these are capacitances in the filters of rectifiers and stabilizers. Using capacitors, a signal is transmitted between amplifier stages, low- and high-pass filters are built, time intervals are set in time delays, and the oscillation frequency in various generators is selected.

Capacitors trace their origins back to , which was used by the Dutch scientist Pieter van Musschenbroeck in his experiments in the mid-18th century. He lived in the city of Leiden, so it’s not hard to guess why this jar was called that.

Actually, it was an ordinary glass jar, lined inside and outside with tin foil - staniol. It was used for the same purposes as modern aluminum, but aluminum had not yet been discovered.

The only source of electricity in those days was an electrophore machine, capable of developing voltages up to several hundred kilovolts. This is where the Leyden jar was charged. Physics textbooks describe a case when Muschenbroek discharged his can through a chain of ten guardsmen holding hands.

At that time, no one knew that the consequences could be tragic. The blow was quite sensitive, but not fatal. It didn’t come to this, because the capacity of the Leyden jar was insignificant, the pulse was very short-lived, so the discharge power was low.

How does a capacitor work?

The design of a capacitor is practically no different from a Leyden jar: the same two plates separated by a dielectric. This is exactly how capacitors are depicted on modern electrical diagrams. Figure 1 shows the schematic structure of a flat-plate capacitor and the formula for its calculation.

Figure 1. Design of a parallel-plate capacitor

Here S is the area of ​​the plates in square meters, d is the distance between the plates in meters, C is the capacitance in farads, ε is the dielectric constant of the medium. All quantities included in the formula are indicated in the SI system. This formula is valid for the simplest flat capacitor: you can simply place two metal plates next to each other, from which conclusions are drawn. Air can serve as a dielectric.

From this formula it can be understood that the larger the area of ​​the plates and the smaller the distance between them, the greater the capacitance of the capacitor. For capacitors with a different geometry, the formula may be different, for example, for the capacitance of a single conductor or. But the dependence of the capacitance on the area of ​​the plates and the distance between them is the same as that of a flat capacitor: the larger the area and the smaller the distance, the greater the capacitance.

In fact, the plates are not always made flat. For many capacitors, for example metal-paper capacitors, the plates are aluminum foil rolled together with a paper dielectric into a tight ball, shaped like a metal case.

To increase the electrical strength, thin capacitor paper is impregnated with insulating compounds, most often transformer oil. This design makes it possible to make capacitors with a capacity of up to several hundred microfarads. Capacitors work in much the same way with other dielectrics.

The formula does not contain any restrictions on the area of ​​the plates S and the distance between the plates d. If we assume that the plates can be spaced very far apart, and at the same time the area of ​​the plates can be made very small, then some capacity, albeit small, will still remain. Such reasoning suggests that even just two conductors located next to each other have electrical capacitance.

This circumstance is widely used in high-frequency technology: in some cases, capacitors are made simply in the form of printed circuit tracks, or even just two wires twisted together in polyethylene insulation. An ordinary noodle wire or cable also has a capacitance, and it increases with increasing length.

In addition to capacitance C, any cable also has a resistance R. Both of these physical properties are distributed along the length of the cable, and when transmitting pulse signals they work as an integrating RC chain, shown in Figure 2.

Figure 2.

In the figure, everything is simple: here is the circuit, here is the input signal, and here is the output signal. The impulse is distorted beyond recognition, but this is done on purpose, which is why the circuit was assembled. In the meantime, we are talking about the effect of cable capacitance on the pulse signal. Instead of a pulse, a “bell” like this will appear at the other end of the cable, and if the pulse is short, then it may not reach the other end of the cable at all, it may completely disappear.

Historical fact

Here it is quite appropriate to recall the story of how the transatlantic cable was laid. The first attempt in 1857 failed: telegraph dots and dashes (rectangular pulses) were distorted so that nothing could be made out at the other end of a 4,000 km long line.

A second attempt was made in 1865. By this time, the English physicist W. Thompson had developed a theory of data transmission over long lines. In light of this theory, the cable laying turned out to be more successful; signals were received.

For this scientific feat, Queen Victoria awarded the scientist a knighthood and the title of Lord Kelvin. This was the name of a small town on the coast of Ireland where the cable laying began. But this is just a word, and now let’s return to the last letter in the formula, namely, the dielectric constant of the medium ε.

A little about dielectrics

This ε is in the denominator of the formula, therefore, its increase will entail an increase in capacity. For most dielectrics used, such as air, lavsan, polyethylene, fluoroplastic, this constant is almost the same as that of vacuum. But at the same time, there are many substances whose dielectric constant is much higher. If an air condenser is filled with acetone or alcohol, its capacity will increase by 15...20 times.

But such substances, in addition to high ε, also have a fairly high conductivity, so such a capacitor will not hold a charge well; it will quickly discharge through itself. This harmful phenomenon is called leakage current. Therefore, special materials are being developed for dielectrics, which make it possible to provide acceptable leakage currents with high specific capacitance of capacitors. This is precisely what explains such a variety of types and types of capacitors, each of which is designed for specific conditions.

They have the highest specific capacity (capacity/volume ratio). The capacity of the “electrolytes” reaches up to 100,000 uF, operating voltage up to 600V. Such capacitors work well only at low frequencies, most often in power supply filters. Electrolytic capacitors are connected with correct polarity.

The electrodes in such capacitors are a thin film of metal oxide, which is why these capacitors are often called oxide capacitors. A thin layer of air between such electrodes is not a very reliable insulator, so a layer of electrolyte is introduced between the oxide plates. Most often these are concentrated solutions of acids or alkalis.

Figure 3 shows one such capacitor.

Figure 3. Electrolytic capacitor

To estimate the size of the capacitor, a simple matchbox was photographed next to it. In addition to the fairly large capacity, in the figure you can also see the tolerance as a percentage: no less than 70% of the nominal.

In those days when computers were large and were called computers, such capacitors were in disk drives (in modern HDD). The information capacity of such drives can now only cause a smile: 5 megabytes of information were stored on two disks with a diameter of 350 mm, and the device itself weighed 54 kg.

The main purpose of the supercapacitors shown in the figure was to remove magnetic heads from the working area of ​​the disk during a sudden power outage. Such capacitors could store a charge for several years, which was tested in practice.

Below, we will suggest doing a few simple experiments with electrolytic capacitors to understand what a capacitor can do.

Non-polar electrolytic capacitors are produced for operation in alternating current circuits, but for some reason they are very difficult to obtain. To somehow get around this problem, conventional polar “electrolytes” are switched on counter-sequentially: plus-minus-minus-plus.

If a polar electrolytic capacitor is connected to an alternating current circuit, it will first heat up, and then there will be an explosion. Old domestic capacitors scattered in all directions, while imported ones have a special device that allows them to avoid loud shots. This is, as a rule, either a cross notch on the bottom of the capacitor, or a hole with a rubber stopper located there.

They really don't like high voltage electrolytic capacitors, even if the polarity is correct. Therefore, you should never put “electrolytes” in a circuit where a voltage close to the maximum for a given capacitor is expected.

Sometimes in some, even reputable forums, beginners ask the question: “The diagram shows a 470µF * 16V capacitor, but I have a 470µF * 50V, can I install it?” Yes, of course you can, but reverse replacement is unacceptable.

The capacitor can store energy

A simple diagram shown in Figure 4 will help you understand this statement.

Figure 4. Circuit with capacitor

The main character of this circuit is an electrolytic capacitor C of a sufficiently large capacity so that the charge and discharge processes proceed slowly, and even very clearly. This makes it possible to observe the operation of the circuit visually using a regular flashlight light bulb. These flashlights have long given way to modern LED ones, but light bulbs for them are still sold. Therefore, it is very simple to assemble a circuit and conduct simple experiments.

Maybe someone will say: “Why? After all, everything is obvious, but if you also read the description...” There seems to be nothing to object to here, but any, even the simplest thing, remains in the head for a long time if its understanding came through the hands.

So, the circuit is assembled. How does it work?

In the position of the switch SA shown in the diagram, the capacitor C is charged from the power source GB through the resistor R in the circuit: +GB __ R __ SA __ C __ -GB. The charging current in the diagram is shown by an arrow with the index iз. The capacitor charging process is shown in Figure 5.

Figure 5. Capacitor charging process

The figure shows that the voltage across the capacitor increases along a curved line, called an exponential in mathematics. The charge current directly mirrors the charge voltage. As the voltage across the capacitor increases, the charging current becomes less. And only at the initial moment it corresponds to the formula shown in the figure.

After some time, the capacitor will charge from 0V to the voltage of the power source, in our circuit up to 4.5V. The whole question is how to determine this time, how long to wait, when will the capacitor charge?

Time constant "tau" τ = R*C

This formula simply multiplies the resistance and capacitance of a series-connected resistor and capacitor. If, without neglecting the SI system, we substitute the resistance in Ohms and the capacitance in Farads, then the result will be obtained in seconds. This is the time required for the capacitor to charge to 36.8% of the power source voltage. Accordingly, charging to almost 100% will require a time of 5* τ.

Often, neglecting the SI system, they substitute resistance in Ohms and capacitance in microfarads into the formula, then the time will be in microseconds. In our case, it is more convenient to obtain the result in seconds, for which you simply have to multiply microseconds by a million, or, more simply, move the decimal point six places to the left.

For the circuit shown in Figure 4, with a capacitor capacity of 2000 μF and a resistor resistance of 500 Ω, the time constant will be τ = R*C = 500 * 2000 = 1,000,000 microseconds or exactly one second. Thus, you will have to wait approximately 5 seconds until the capacitor is fully charged.

If, after the specified time, the switch SA is moved to the right position, the capacitor C will discharge through the light bulb EL. At this moment there will be a short flash, the capacitor will discharge and the light will go out. The direction of capacitor discharge is shown by an arrow with the index ip. The discharge time is also determined by the time constant τ. The discharge graph is shown in Figure 6.

Figure 6. Capacitor discharge graph

The capacitor does not pass direct current

An even simpler diagram shown in Figure 7 will help you verify this statement.

Figure 7. Circuit with a capacitor in a DC circuit

If you close switch SA, the light bulb will flash briefly, indicating that capacitor C has charged through the light bulb. The charge graph is also shown here: at the moment the switch is closed, the current is maximum, as the capacitor is charged, it decreases, and after a while it stops completely.

If the capacitor is of good quality, i.e. with a low leakage current (self-discharge), repeated closure of the switch will not lead to a flash. To get another flash, the capacitor will have to be discharged.

Capacitor in power filters

The capacitor is usually placed after the rectifier. Most often, rectifiers are made full-wave. The most common rectifier circuits are shown in Figure 8.

Figure 8. Rectifier circuits

Half-wave rectifiers are also used quite often, as a rule, in cases where the load power is insignificant. The most valuable quality of such rectifiers is their simplicity: just one diode and a transformer winding.

For a full-wave rectifier, the capacitance of the filter capacitor can be calculated using the formula

C = 1000000 * Po / 2*U*f*dU, where C is the capacitance of the capacitor μF, Po is the load power W, U is the voltage at the output of the rectifier V, f is the frequency of the alternating voltage Hz, dU is the amplitude of ripple V.

The large number in the numerator 1,000,000 converts the capacitance of the capacitor from system Farads to microfarads. The two in the denominator represents the number of half-cycles of the rectifier: for a half-wave rectifier, one will appear in its place

C = 1000000 * Po / U*f*dU,

and for a three-phase rectifier the formula will take the form C = 1000000 * Po / 3*U*f*dU.

Supercapacitor - ionistor

Recently, a new class of electrolytic capacitors has appeared, the so-called. In its properties it is similar to a battery, although with several limitations.

The ionistor is charged to the rated voltage within a short time, literally in a few minutes, so it is advisable to use it as a backup power source. In fact, the ionistor is a non-polar device; the only thing that determines its polarity is charging at the manufacturer. To prevent this polarity from being confused in the future, it is indicated with a + sign.

The operating conditions of ionistors play a big role. At a temperature of 70˚C at a voltage of 0.8 of the rated voltage, the guaranteed durability is no more than 500 hours. If the device operates at a voltage of 0.6 of the nominal voltage, and the temperature does not exceed 40 degrees, then proper operation is possible for 40,000 hours or more.

The most common application of an ionistor is in backup power supplies. These are mainly memory chips or electronic watches. In this case, the main parameter of the ionistor is low leakage current, its self-discharge.

The use of ionistors in conjunction with solar batteries is quite promising. This is also due to the non-criticality of the charge conditions and the practically unlimited number of charge-discharge cycles. Another valuable property is that the ionistor does not require maintenance.

So far I’ve managed to tell you how and where electrolytic capacitors work, mainly in DC circuits. The operation of capacitors in alternating current circuits will be discussed in another article -.

They are the second most common and widely used component in electronic circuits, after resistors. Indeed, in any electronic device, be it a multivibrator with 2 transistors or a computer motherboard, these radioelements are used in all of them.

A capacitor has the ability to accumulate charge and subsequently release it. The simplest capacitor consists of 2 plates separated by a thin layer of dielectric. The capacitance of a capacitor depends on its capacitance and the frequency of the current. A capacitor conducts alternating current and does not allow direct current to pass through. The capacitance of the capacitor is greater, the larger the area of ​​the plates (plates) of the capacitor, and the greater, the thinner the dielectric layer between them.

The capacitances of parallel connected capacitors add up. The capacitances of series-connected capacitors are calculated using the formula shown in the figure below:

Capacitors come in both fixed and variable capacitance. The latter are called and abbreviated as KPE (variable capacitor). Fixed capacitors can be either polar or non-polar. The figure below shows a schematic representation of a polar capacitor:

Electrolytic capacitors are polar. Tantalum capacitors are also produced, which differ from aluminum electrolytic capacitors in that they are more stable, but are also more expensive. Electrolytic capacitors are subject to faster aging compared to non-polar ones. Polar capacitors have positive and negative electrodes, plus and minus. The photo below shows an electrolytic capacitor:

For Soviet electrolytic capacitors, the polarity was indicated on the body with a plus sign near the positive electrode. For imported capacitors, the negative electrode is indicated with a minus sign. If the operating conditions of electrolytic capacitors are violated, they can swell and even explode. For electrolytic capacitors, in order to avoid explosion, special notches are made on the housing cover during their manufacture:

Electrolytic capacitors can also explode if they are mistakenly applied at a voltage higher than what they were designed for. In the photo of the electrolytic capacitor above, you can see the inscription 33 μF x 100 V., this means its capacity is equal to 33 microfarads and the permissible voltage is up to 100 volts. A non-polar capacitor in the diagrams is designated as follows:

Non-polar capacitor image on the diagram

The photo below shows film and ceramic capacitors:

Film

Ceramic

Capacitors are distinguished by the type of dielectric. There are capacitors with solid, liquid and gaseous dielectric. With a solid dielectric these are: paper, film, ceramic, mica. There are also electrolytic capacitors, which have already been discussed above, and oxide-semiconductor capacitors. These capacitors differ from all others in their high specific capacitance. Many, I think, have seen the following digital designation on imported capacitors:

The figure above shows how you can calculate the value of such a capacitor. For example, if a capacitor is marked 332, this means that it has a capacity of 3300 picofarads or 3.3 nanofarads. Below is a table, by consulting which you can easily calculate the value of any capacitor with this marking:

There are capacitors in SMD design, the most common in amateur radio designs, I think, are types 0805 and 1206. An image of a non-polar SMD capacitor can be seen in the figures below:

The industry also produces so-called solid-state capacitors. Instead of an electrolyte, they have an organic polymer inside.

Variable capacitors

The capacitance in variable capacitors is adjusted by changing the area of ​​parallel capacitor plates. Capacitors are divided into variable capacitors, which have a handle for rotating the shaft, and trimmers, which have a slot for a screwdriver, and also consist of moving and non-moving parts.

In the figure they are designated as rotor and stator. Such capacitors are used in radio receivers to tune to the desired broadcast frequency. The capacity of such capacitors is usually small and equals a few - a maximum of hundreds of picofarads. This is how a variable capacitor is designated in the diagrams:

The following figure shows a trimmer capacitor. The trimmer capacitor is designated in the diagrams as follows:

Such capacitors are usually adjusted only once during the assembly and configuration of electronic equipment.

The following figure shows the structure of a trimmer capacitor:

The capacitance of a capacitor is measured in Farads. But even 1 Farad is a very large capacity, so parts per million Farads, microfarads, as well as even smaller ones, nanofarads and picofarads are usually used for designation. Converting from microfarads to picofarads and back is very easy. 1 microfarad is equal to 1000 nanofarads or 1,000,000 picofarads. Capacitors, among other things, are used in oscillating circuits of radio receivers, in power supplies to smooth out ripples, and also as isolation circuits in amplifiers. Review prepared AKV.

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