Polar and non-polar capacitors - what is the difference. What is a capacitor? The properties of a capacitor of this design are used

After the division of bodies into conductors and non-conductors was established, and experiments with electrostatic machines became widespread, it was completely natural to try to “accumulate” electric charges in some kind of glass vessel that could store them. Among the many physicists who engaged in such experiments, the most famous was the Dutch professor from Leiden, Muschenbroek (Muschenbreck) (1692-1761).

Knowing that glass does not conduct electricity, he (in 1745) took a glass jar (flask) filled with water, dropped a copper wire hanging on the conductor of an electric machine into it, and, taking the jar in his right hand, asked his assistant to rotate the ball cars. At the same time, he correctly assumed that the charges coming from the conductor would accumulate in a glass jar.

After he felt that a sufficient number of charges had accumulated in the jar, he decided to disconnect the copper wire with his left hand. At the same time, he felt a strong blow, it seemed to him that “the end had come.” In a letter to Reaumur in Paris (in 1746), he wrote that “I advise you not to repeat this new and terrible experience” and that “even for the sake of the crown of France he will not agree to undergo such a terrible shock.”

This is how the Leyden jar (named after the city of Leiden) was invented, and soon the first simple capacitor, one of the most common electrical devices.

Muschenbruck's experiment created a genuine sensation not only among physicists, but also among many amateurs interested in electrical experiments.

Independently of Muschenbruck, in the same 1745, the German scientist E.G. also came to the creation of the Leyden jar. Kleist. Physicists from different countries began to carry out experiments with the Leyden jar, and in 1746-1747. The first theories of the Leyden jar were developed by the famous American scientist B. Franklin and the keeper of the physics cabinet, the Englishman W. Watson. It is interesting to note that Watson sought to determine the speed of propagation of electricity by “making” it “run” 12,000 feet.

One of the most important consequences of the invention of the Leyden jar was the establishment of the influence of electrical discharges on the human body, which led to the birth of electromedicine.

The Muschenbruck experiment was repeated in the presence of the French king by Abbot Nollet. He formed a chain of 180 guardsmen holding hands, with the first holding a can in his hand, and the last touching the wire, drawing a spark. “The blow was felt by everyone at one moment; it was curious to see the variety of gestures and hear the instant scream of dozens of people.” From this chain of soldiers the term “electric circuit” originated.

Gradually, the design of the Leyden jar was improved: water was replaced with shot, and then the outer surface was covered with thin lead plates; Later, the inner and outer surfaces began to be covered with tin foil, and the can acquired its modern appearance.

When conducting research with the jar, it was established (in 1746 by the Englishman B. Wilson) that the amount of electricity collected in the jar is proportional to the size of the linings and inversely proportional to the thickness of the insulating column. In the 70s XVIII century metal plates began to be separated not by glass, but by an air gap - thus, the simplest capacitor appeared.

according to materials.

Capacitors, like resistors, are among the most numerous elements of radio engineering devices. About some properties of a capacitor- "storage" I have already talked about electric charges. At the same time he said that 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 F, named after the English physicist M. Faraday. However, 1 F - This is a very large capacity. The globe, for example, has a capacitance of less than 1 F. In electrical and radio engineering, a unit of capacitance equal to a millionth of a farad is used, which is called a microfarad (abbreviated μF). There are 1,000,000 microfarads in one farad, i.e. 1 microfarad = 0.000001 F. But this unit of capacitance is often too large. Therefore, there is an even smaller unit of capacitance called the picofarad (abbreviated pF), which is a millionth of a microfarad, i.e. 0.000001 µF; 1 µF = 1,000,000 pF. All capacitors, whether constant or variable, are characterized primarily by their capacitances, expressed in picofarads and microfarads, respectively.

On circuit diagrams, the capacitance of capacitors from 1 to 9999 pF is indicated by integers corresponding to their capacitances in these units without the designation pF, and the capacitance of capacitors from 0.01 μF (10000 pF) and more— in fractions of a microfarad or microfarads without the designation μF. If the capacitance of the capacitor is equal to an integer number of microfarads, then, in contrast to the designation of capacitance in picofarads, a comma and a zero are placed after the last significant digit. Examples of designation of capacitor capacities in the 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.

A capacitor in its simplest form consists of two plates separated by a dielectric. If a capacitor is connected to a DC circuit, the current in this circuit will stop. Yes, this is understandable: direct current cannot flow through the 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 capacitor charging current appears in the circuit). This is not how a capacitor behaves in an alternating current circuit. Remember: the polarity of the voltage at the terminals of the AC source changes periodically. This means that if you include a capacitor in a circuit powered by such a current source, its plates will be alternately recharged at the frequency of this current. As a result, alternating current will flow in the circuit.

A capacitor, like a resistor and a coil, provides 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 be almost an insulator for low frequency currents. Radio amateurs, for example, sometimes use electrical lighting network wires instead of external antennas, connecting receivers to them through a capacitor with a capacity of 220– 510 pF. Was this capacitor chosen by chance? No, not by chance. A capacitor of such a capacity passes high-frequency currents well, which are necessary for the operation of the receiver, but has great resistance to alternating current with a frequency of 50 Hz flowing in the network. In this case, the capacitor becomes a kind of filter, passing high-frequency current and blocking low-frequency current.

The capacitance of a capacitor to alternating current depends on its capacitance and current frequency: the greater the capacitance of the capacitor and the frequency of the current, the lower its capacitance. This capacitor resistance can be determined with sufficient accuracy using the following simplified formula

RC = 1/6fC
π (more precisely 6.28, sinceπ = 3.14).

where RC is the capacitance of the capacitor, Ohm; f - current frequency, Hz; C is the capacitance of this capacitor, F; digit 6 - value 2 rounded to whole unitsπ (more precisely 6.28, sinceπ = 3.14).

Using this formula, let's find out how a capacitor behaves in relation to alternating currents if we use power wires as an antenna. Let's say that the capacitance of this capacitor is 500 pF (500 pF = 0.0000000005 F). Mains frequency 50 Hz. Let's take 1 MHz (1,000,000 Hz) as the average carrier frequency of the radio station, which corresponds to a wave length of 300 m. What resistance does this capacitor have to the radio frequency?

Rc = = 1/(6·1000000·0.0000000005) ~=300 Ohm.

What about alternating current?

Rc = 1/(6·50·0.0000000005) ~= 7 MOhm.

And here is the result: a capacitor with a capacity of 500 pF provides 20,000 times less resistance to high-frequency current than to low-frequency current. Earnestly? A capacitor of smaller capacity provides even greater resistance to the alternating current of the network.

The capacitance of a capacitor to alternating current decreases with an increase in its capacitance and current frequency, and vice versa, increases with a decrease in its capacitance and current frequency.

The property of a capacitor not to pass direct current and to conduct alternating currents of different frequencies in different ways is used to separate pulsating currents into their components, retain currents of some frequencies and pass currents of other frequencies.

How are constant capacitors constructed?

All capacitors of constant capacity have conductive plates, and between them - ceramics, mica, paper or some other solid dielectric. Based on the type of dielectric used, capacitors are called ceramic, mica, or paper, respectively. The appearance of some ceramic constant capacitors is shown in Fig. 1

Rice. 1. Ceramic constant capacitance capacitors

They use special ceramics as a dielectric, with plates— thin layers of silver-plated metal deposited on the surface of ceramics, and the leads are brass silver-plated wires or strips soldered to the plates. The capacitor housings are covered with enamel on top.

The most common ceramic capacitors are the KDK (Ceramic Disc Capacitor) and KTK (Ceramic Tubular Capacitor) types: For a KTK type capacitor, one plate is applied to the inner and the second to 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 capacitances - up to several thousand picofarads. They are placed in those circuits in which high-frequency current flows (antenna circuit, oscillatory circuit) for communication between them.

To obtain a capacitor of small size, but with a relatively large capacity, it is made not from two, but from several plates, stacked and separated from each other by a dielectric (Fig. 2). In this case, each pair of adjacent plates forms a capacitor. By connecting these pairs of plates in parallel, a capacitor of significant capacity is obtained.

Rice. 2. Mica capacitors

This is how all capacitors with a mica dielectric are designed. Their plates— The plates are sheets of aluminum foil or layers of silver deposited directly on mica, and the leads are pieces of silver-plated wire. Such capacitors are molded with plastic. These are KSO capacitors. Their name contains a number characterizing the shape and size of the capacitors, for example: KSO-1, KSO-5. The higher the number, the larger the size of the capacitor. Some mica capacitors are produced in ceramic, waterproof cases. They are called SGM type capacitors. The capacitance 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 interlocking and for communication between high-frequency circuits.

In paper capacitors (Fig. 3), the dielectric is paraffin-impregnated thin paper, and the plates are foil. Strips of paper together with the covers are rolled into a roll and placed in a cardboard or metal case. The wider and longer the plates, the greater the capacitance of the capacitor.

Rice. 3. Paper and metal-paper capacitors of constant capacity

Paper capacitors are used mainly in low-frequency circuits, as well as for blocking power supplies. There are many types of capacitors with paper dielectric. And all of them have the letter B (Paper) in their designation. Capacitors of the BM type (Small Paper) are enclosed in metal tubes, filled at the ends with a special resin.

KB capacitors have cardboard cylindrical cases. Capacitors of the KBG-I type are placed in porcelain cases with metal end caps connected to plates from which narrow lead petals extend.

Capacitors with a capacity of up to several microfarads are produced in metal cases. These include capacitors of the KBG-MP, KBG-MN, KBGT types. There may be two or three of them in one building.

The dielectric of capacitors of the MBM type (Metal-paper Small-sized) is varnished capacitor paper, and the plates are layers of metal less than a micron thick deposited on one side of the paper. A characteristic feature of capacitors of this type— the ability to self-heal after electrical breakdown of a dielectric.

A special group of constant-capacity capacitors are electrolytic ones (Fig. 4).

Rice. 4. Electrolytic capacitors

In terms of its internal structure, an electrolytic capacitor is somewhat reminiscent of a paper capacitor. It contains two aluminum foil strips. The surface of one of them is covered with a thin layer of oxide. Between the aluminum strips there is a strip of porous paper impregnated with a special thick liquid.— electrolyte. This four-layer strip is rolled up and placed in an aluminum cylindrical cup or cartridge.

The dielectric of the capacitor is an oxide layer. The positive plate (anode) is the tape that has an oxide layer. It is connected to a petal isolated from the body. The second, negative plate (cathode) paper, impregnated with electrolyte through a tape on which there is no oxide layer, is connected to the metal body. Thus, the body is a negative terminal, and the petal isolated from it is the terminal of the positive plate of the electrolytic capacitor. This is how, in particular, capacitors of the KE and K50-3 types are designed. KE-2 capacitors differ from KE type capacitors only in the plastic bushing with thread and nut for mounting on the panel. Aluminum housings of K50-3 capacitors have the shape of a cartridge with a diameter of 4.5– 6 and length 15-20 mm. conclusions— wire Capacitors of type K50-6 are designed similarly. But their electrode leads (plates) are isolated from the housings.

On circuit diagrams, electrolytic capacitors are depicted in the same way as other capacitors of constant capacitance - with two " dashes, but put a sign near the positive facing« + » .

Electrolytic capacitors have large capacitances— from fractions to several thousand microfarads. They are designed for use in circuits with pulsating currents, such as AC rectifier filters, for coupling between low frequency circuits. In this case, the negative electrode of the capacitor is connected to the negative pole of the circuit, and the positive— with its positive pole. If the switching polarity is not observed, the electrolytic capacitor may fail.

The nominal capacitances of electrolytic capacitors are written on their cases. The actual capacity may be significantly greater than the nominal capacity.

The most important characteristic of any capacitor, in addition to capacitance, is also its rated voltage, i.e. the voltage at which the capacitor can operate for a long time without losing its properties. This voltage depends on the properties and thickness of the dielectric layer of the capacitor. Ceramic, mica, paper and metal-paper capacitors of various types are designed for rated voltages from 150 to 1000 V or more.

Electrolytic capacitors are produced at rated voltages from several volts to 30– 50 V and from 150 to 450 – 500 V. In this regard, 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 designs, always pay attention to their rated voltages. In a circuit with a voltage lower than the rated one, capacitors can be turned on, but in a circuit with a voltage higher than the rated voltage, they cannot be turned on. If there is a voltage on the capacitor plates that exceeds its rated voltage, the dielectric will break through. A broken capacitor is unusable.

Now about variable capacitors.

The structure of the simplest variable capacitor is shown in Fig. 5. One of its lining - the stator is stationary. Second rotor— attached to the axle. When the axis rotates, the overlap area of ​​the plates, and with it the capacitance of the capacitor, changes.

Rice. 5. The simplest variable capacitor

Variable capacitors used in tuned oscillating circuits of receivers consist of two groups of plates (Fig. 6, a) made of sheet aluminum or brass. The rotor plates are connected by an axis. The stator plates are also connected and isolated from the rotor. When the axis rotates, the plates of the stator group gradually enter the air gaps between the plates of the rotor group, causing the capacitance of the capacitor to smoothly change. When the rotor plates are completely removed from the gaps between the stator plates, the capacitance of the capacitor is smallest; it is called the initial capacitance of the capacitor. When the rotor plates are fully inserted between the stator plates, the capacitance of the capacitor will be greatest, i.e., maximum for a given capacitor. The maximum capacitance of the capacitor will be greater, the more plates it contains and the smaller the distance between the moving and stationary plates.

In the capacitors shown in Fig. 5 and 6, a, the dielectric is air. In small-sized variable capacitors (Fig. 6, b), the dielectric can be paper, plastic films, or ceramics. Such capacitors are called solid dielectric variable capacitors. With smaller dimensions than air dielectric capacitors, they can have significant maximum capacitances. It is these capacitors that are used to tune the oscillatory circuits of small-sized transistor receivers.

Rice. 7. One of the designs of a block of variable capacitors

Single capacitors and blocks of variable capacitors with an air dielectric require careful handling. Even slight bending or other damage to the plates leads to a short circuit between them. Correction of the same capacitor plates- it's a complicated matter.

Capacitors with a solid dielectric also include tuning capacitors, which are a type of variable capacitor. Most often, such capacitors are used to tune circuits to resonance, which is why they are called tuning capacitors. The designs of the most common tuning capacitors are shown in Fig. 8. Each of them consists of a relatively massive ceramic base and a thin ceramic disk. On the surface of the base (under the disk) and on the disk, metal layers are applied in the form of sectors, which are the plates of the capacitor. When the disk rotates around its axis, the overlap area of ​​the sectors-plates changes, and the capacitance of the capacitor changes.

The capacity of tuning capacitors is indicated on their cases in the form of a fractional number, where the numerator is the smallest and the denominator is the largest capacity of the given capacitor. If, for example, 6/30 is indicated on a capacitor, this means that its smallest capacitance is 6 pF, and its largest is 30 pF. Trimmer capacitors usually have the smallest capacitance 2 – 5 pF, and the highest up to 100– 150 pF. Some of them, such as KPK-2, can be used as variable capacitors to configure simple single-circuit receivers.

Capacitors, like resistors, can be connected in parallel or in series. Connecting capacitors is most often resorted to in cases where there is no capacitor of the required value at hand, but there are others from which the required capacitance can be made. If you connect capacitors in parallel (Fig. 8, a), then their total capacitance will be equal to the sum of the capacitances of all connected capacitors, i.e.

Commun = C1 + C2 + C3, etc.

So, for example, if C1 = 33 pF and C2 = 47 pF, then the total capacitance of these two capacitors will be: Total = 33 + 47 = 80 pF. When capacitors are connected in series (Fig. 8, b), their total capacitance is always less than the smallest capacitance included in the chain. It is calculated by the formula

Comm = C1 · C2/(C1 + C2)

For example, let's say that C1 = 220 pF and C2 = 330 pF; then Total = 220 · 330/(220 + 330) = 132 pF. When two capacitors of the same capacitance are connected in series, their total capacitance will be half the capacitance of each of them.

Rice. 8. Parallel (a) and series (b) connections of capacitors

Capacitor

The basis of the capacitor design is two conductive plates, between which there is a dielectric

On the left are surface mount capacitors; on the right - capacitors for volumetric installation; on top - ceramic; below - electrolytic.

Various capacitors for volumetric mounting

Capacitor properties

A capacitor in a DC circuit can conduct current at the moment it is connected to the circuit (charging or recharging of the capacitor occurs); at the end of the transient process, no current flows through the capacitor, since its plates are separated by a dielectric. In an alternating current circuit, it conducts alternating current oscillations through cyclic recharging of the capacitor.

where is the imaginary unit, is the frequency of the flowing sinusoidal current, and is the capacitance of the capacitor. It also follows that the reactance of the capacitor is equal to: . For direct current, the frequency is zero, therefore the reactance of the capacitor is infinite (ideally).

On electrical circuit diagrams, the nominal capacitance of capacitors is usually indicated in microfarads (1 µF = 10 6 pF) and picofarads, but often in nanofarads. With a capacity of no more than 0.01 µF, the capacitance of the capacitor is indicated in picofarads, but it is permissible not to indicate the unit of measurement, i.e. the postfix “pF” is omitted. When indicating the nominal value of a capacity in other units, indicate the unit of measurement (picoFarad). For, as well as for high-voltage capacitors in the diagrams, after the designation of the capacitance rating, their maximum operating voltage is indicated in volts (V) or kilovolts (kV). For example: “10 microns x 10 V”. For indicate the range of change in capacity, for example: “10 – 180”. Currently, capacitors are manufactured with nominal capacities from the decimal logarithmic series of values ​​E3, E6, E12, E24, i.e. there are 3, 6, 12, 24 values ​​per decade, so that the values ​​with the appropriate tolerance (scatter) cover the entire decade.

Characteristics of capacitors

Main settings

Capacity

The main characteristic of a capacitor is its capacity. The designation of a capacitor indicates the value of the nominal capacitance, while the actual capacitance can vary significantly depending on many factors. The actual capacitance of a capacitor determines its electrical properties. Thus, according to the definition of capacitance, the charge on the plate is proportional to the voltage between the plates ( q = CU ). Typical capacitance values ​​range from units of picofarads to hundreds of microfarads. However, there are capacitors with a capacity of up to tens of farads.

The capacitance of a flat capacitor, consisting of two parallel metal plates with an area each, located at a distance from each other, in the SI system is expressed by the formula: , where is the relative dielectric constant of the medium filling the space between the plates (this formula is valid only when much less than the linear dimensions of the plates ).

To obtain large capacities, capacitors are connected in parallel. In this case, the voltage between the plates of all capacitors is the same. Total battery capacity parallel of connected capacitors is equal to the sum of the capacitances of all capacitors included in the battery.

If all parallel-connected capacitors have the same distance between the plates and the same dielectric properties, then these capacitors can be represented as one large capacitor, divided into fragments of a smaller area.

When capacitors are connected in series, the charges on all capacitors are equal. Total battery capacity sequentially connected capacitors is equal to

or

This capacity is always less than the minimum capacity of the capacitor included in the battery. However, with a series connection, the possibility of breakdown of capacitors is reduced, since each capacitor accounts for only part of the potential difference of the voltage source.

If the area of ​​the plates of all capacitors connected in series is the same, then these capacitors can be represented as one large capacitor, between the plates of which there is a stack of dielectric plates of all the capacitors that make it up.

Specific capacity

Capacitors are also characterized by specific capacitance - the ratio of capacitance to the volume (or mass) of the dielectric. The maximum value of specific capacitance is achieved with a minimum thickness of the dielectric, but at the same time its breakdown voltage decreases.

Rated voltage

Another, no less important characteristic of capacitors is the rated voltage - the voltage value indicated on the capacitor at which it can operate under specified conditions during its service life while maintaining parameters within acceptable limits.

The rated voltage depends on the design of the capacitor and the properties of the materials used. During operation, the voltage on the capacitor should not exceed the rated voltage. For many types of capacitors, the permissible voltage decreases as temperature increases.

Polarity

Capacitors that collapsed without an explosion due to temperature and voltage not suitable for operating conditions.

Many oxide dielectric (electrolytic) capacitors function only when the voltage polarity is correct due to the chemical characteristics of the interaction of the electrolyte with the dielectric. When the voltage polarity is reversed, electrolytic capacitors usually fail due to chemical destruction of the dielectric with a subsequent increase in current, boiling of the electrolyte inside and, as a result, the possibility of explosion of the housing.

Explosions of electrolytic capacitors are a fairly common occurrence. The main cause of explosions is overheating of the capacitor, caused in most cases by leakage or an increase in equivalent series resistance due to aging (relevant for pulsed devices). To reduce damage to other parts and injury to personnel, modern large-capacity capacitors install a valve or make a notch on the body (you can often see it in the shape of the letter X, K or T at the end). When the internal pressure increases, the valve opens or the housing is destroyed along the notch, the evaporated electrolyte comes out in the form of a corrosive gas, and the pressure drops without explosion or fragments.

Real capacitors, in addition to capacitance, also have their own resistance and inductance. With a high degree of accuracy, the equivalent circuit of a real capacitor can be represented as follows:

Electrical insulation resistance of the capacitor - r

Insulation resistance is the capacitor's resistance to direct current, given by r = U / I ut, Where U- voltage applied to the capacitor, I ut- leakage current.

Equivalent series resistance - R

Equivalent series resistance (ESR, English. ESR) is caused mainly by the electrical resistance of the material of the plates and leads of the capacitor and the contact(s) between them, as well as losses in the dielectric. Typically, the ESR increases with increasing frequency of the current flowing through the capacitor.

In most cases, this parameter can be neglected, but sometimes (for example, in the case of using electrolytic capacitors in switching power supply filters) a sufficiently small value can be vital for the reliability of the device (see, for example, Capacitor plague) .

Equivalent series inductance - L

The equivalent series inductance is mainly due to the intrinsic inductance of the capacitor plates and leads. At low frequencies (up to a few kilohertz) it is usually not taken into account due to its insignificance.

Loss tangent

Loss tangent is the ratio of the imaginary and real parts of the complex dielectric constant.

Temperature coefficient of capacity (TKE)

TKE - relative change in capacitance when the ambient temperature changes by one degree Celsius (Kelvin). Thus, the value of capacitance versus temperature is represented by the linear formula:

,

where Δ T- temperature increase in °C or °K relative to the normal conditions under which the capacitance value is specified. TKE is used to characterize capacitors with a significant linear dependence of capacitance on temperature. However, TKE is not determined for all types of capacitors. Capacitors that have a nonlinear dependence of capacitance on temperature, and capacitors with large changes in capacitance from the influence of ambient temperature, have an indication in the designation of the relative change in capacitance in the operating temperature range.

Dielectric absorption

If a charged capacitor is quickly discharged to zero voltage by connecting a low-resistance load, and then remove the load and observe the voltage at the capacitor terminals, we will see that the voltage slowly rises. This phenomenon is called dielectric absorption or

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 (tabulated 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 through when the voltage limit is exceeded), this element would be charged indefinitely (the so-called ideal capacitor, something like an absolutely 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 energy 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

It 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 the number 9 is 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.

letter	C	letter	C	letter	C	letter	C
A	1	J	2,2	S	4,7	a	2,5
B	1,1	K	2,4	T	5,1	b	3,5
C	1,2	L	2,7	U	5,6	d	4
D	1,3	M	3	V	6,2	e	4,5
E	1,5	N	3,3	W	6,8	f	5
F	1,6	P	3,6	X	7,5	m	6
G	1,8	Q	3,9	Y	8,2	n	7
Y	2	R	4,3	Z	9,1	t	8

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:

letter	V	letter	V
I	1	K	63
R	1,6	L	80
M	2,5	N	100
A	3,2	P	125
C	4	Q	160
B	6,3	Z	200
D	10	W	250
E	16	X	315
F	20	T	350
G	25	Y	400
H	32	U	450
S	40	V	500
J	50

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.

A capacitor is a common two-pole device used in various electrical circuits. It has a constant or variable capacity and is characterized by low conductivity; it is capable of accumulating a charge of electric current and transmitting it to other elements in the electrical circuit.
The simplest examples consist of two plate electrodes separated by a dielectric and accumulating opposite charges. In practical conditions, we use capacitors with a large number of plates separated by a dielectric.

The capacitor starts charging when the electronic device is connected to the network. When the device is connected, there is a lot of free space on the electrodes of the capacitor, therefore the electric current entering the circuit is of the greatest magnitude. As it is filled, the electric current will decrease and disappear completely when the device’s capacity is completely filled.

In the process of receiving an electric current charge, electrons (particles with a negative charge) are collected on one plate, and ions (particles with a positive charge) are collected on the other. The separator between positively and negatively charged particles is a dielectric, which can be used in various materials.

When an electrical device is connected to a power source, the voltage in the electrical circuit is zero. As the containers are filled, the voltage in the circuit increases and reaches a value equal to the level at the current source.

When the electrical circuit is disconnected from the power source and a load is connected, the capacitor stops receiving charge and transfers the accumulated current to other elements. The load forms a circuit between its plates, so when the power is turned off, positively charged particles will begin to move towards the ions.

The initial current in the circuit when a load is connected will be equal to the voltage across the negatively charged particles divided by the value of the load resistance. In the absence of power, the capacitor will begin to lose charge and as the charge in the capacitors decreases, the voltage level and current in the circuit will decrease. This process will only complete when there is no charge left in the device.

The figure above shows the design of a paper capacitor:
a) winding the section;
b) the device itself.
On this picture:

1. Paper;
2. Foil;
3. Glass insulator;
4. Lid;
5. Frame;
6. Cardboard gasket;
7. Wrapping;
8. Sections.

Capacitor capacity is considered its most important characteristic; the time it takes to fully charge the device when connecting the device to a source of electric current directly depends on it. The discharge time of the device also depends on the capacity, as well as on the load size. The higher the resistance R, the faster the capacitor will empty.

As an example of the operation of a capacitor, consider the operation of an analog transmitter or radio receiver. When the device is connected to the network, the capacitors connected to the inductor will begin to accumulate charge, electrodes will collect on some plates, and ions on others. After the capacity is fully charged, the device will begin to discharge. A complete loss of charge will lead to the start of charging, but in the opposite direction, that is, the plates that had a positive charge this time will receive a negative charge and vice versa.

Purpose and use of capacitors

Currently, they are used in almost all radio engineering and various electronic circuits.
In an alternating current circuit they can act as capacitance. For example, when you connect a capacitor and a light bulb to a battery (direct current), the light bulb will not light. If you connect such a circuit to an alternating current source, the light bulb will glow, and the intensity of the light will directly depend on the value of the capacitance of the capacitor used. Thanks to these features, they are now widely used in circuits as filters that suppress high-frequency and low-frequency interference.

Capacitors are also used in various electromagnetic accelerators, photo flashes and lasers due to their ability to store a large electrical charge and quickly transfer it to other low-resistance network elements, thereby creating a powerful pulse.

In secondary power supplies they are used to smooth out ripples during voltage rectification.

The ability to retain a charge for a long time makes it possible to use them for storing information.

Using a resistor or current generator in a circuit with a capacitor allows you to increase the charging and discharging time of the device’s capacitance, so these circuits can be used to create timing circuits that do not have high requirements for temporal stability.

In various electrical equipment and in higher harmonic filters, this element is used to compensate reactive power.