Capacitor

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

Left - capacitors for surface mount; on the right - capacitors for volumetric mounting; top - ceramic; bottom - electrolytic.

The device built by von Kleist consisted of a medicine bottle partially filled with water and sealed with a cork. The nail was pushed through the cork and into the water. Holding the bottle in one hand, the nail was then brought into contact with the terminal of an electrostatic machine, allowing some charging. As von Kleist reached for the nail to pull it out of the cork while still holding the bottle, the individual charges were able to recombine as they flowed through his body. Van Mussenbroek's device and experience was much the same as von Kleist's, with three major exceptions.

Various capacitors for bulk mounting

Capacitor Properties

A capacitor in a DC circuit can conduct current at the moment it is connected to the circuit (the capacitor is being charged or recharged), at the end of the transient process, the current does not flow through the capacitor, since its plates are separated by a dielectric. In an alternating current circuit, it conducts alternating current oscillations by cyclically recharging a capacitor.

First, a visiting student made the shocking discovery that it was not van Mussenbroek; secondly, he made many improvements to the device; and third, he wrote to his colleagues to tell them all about it. But here are some of the highlights. It is enough for a man to stand directly on the ground; that whoever holds the globe must draw a spark; The effect is small if two men are involved, one grabbing the globe and the other pulling the sparks. Soon, scientists on the continent created their own new and improved devices for storing electrical charges.

Design Refinement: Filled with water, ink, vinegar, melted butter, wine or beer, and finally nothing. With the connivance of the abbot of the Grand Monastery of the Carthusians in Paris, he assembled 200 monks in a long line, with each monk holding the ends of eight meters of wire, to form a chain about one mile long. Without warning, he connected the Leiden Jar to the ends of the line, giving the unsuspecting monks a massive electrical shock, and noted with satisfaction that all the monks began cursing and flexing as they reacted to the shock. The king was impressed and surprised when the soldiers jumped at the same time when the circuit was completed. An informal definition of capacitance.

where is the imaginary unit, is the frequency of the flowing sinusoidal current, is the capacitance of the capacitor. It also follows that the reactance of the capacitor is: . For DC, the frequency is zero, hence the reactance of a capacitor is infinite (ideally).

Formal definition of capacity. The capacitance of an electrostatic system is the ratio of the amount of charge separated from the potential difference used. One farad is usually considered large capacity. Henry Cavendish identified factors that affect capacitance.

Directly proportional to the area of one plate, inversely proportional to the distance between the plates and directly proportional to the permittivity of the material between the plates. More advanced: cylindrical. And spherical. And the sphere's own capacity.

More about dielectrics in the next section. Two examples: in power supplies, a condenser microphone. Typically, they are used to smooth the power supply to eliminate spikes or dropouts. Large capacitors - Family portrait. Just because an electrical device is unplugged doesn't mean it can open it and work inside. Heavy appliances like this one microwave oven, often contain capacitors capable of storing a significant amount of electrical energy.

On electric 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 capacitance of not more than 0.01 μF, the capacitance of the capacitor is indicated in picofarads, while it is permissible not to indicate the unit of measurement, i.e. the postfix "pF" is omitted. When designating the nominal capacity in other units, indicate the unit of measurement (picoFarad). For electrolytic capacitors, as well as for high-voltage capacitors on the diagrams, after designating the capacitance rating, indicate their maximum operating voltage in volts (V) or kilovolts (kV). For example: "10 microns x 10 V". For indicate the range of change in capacitance, for example: "10 - 180". Currently, capacitors are manufactured with nominal capacities from 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.

Accidental and rapid discharge can cause serious injury or death. The word "capacitor" is now an almost obsolete term meaning "capacitor". What is a reverse condenser microphone? A condenser microphone is basically a condenser with one fixed plate and one very light, thin, loose plate called a diaphragm. This second plate is so light that the sound waves are strong enough to make it vibrate. This leads to a change in the distance between the fixed and fixed plates.

Characteristics of capacitors

main parameters

Capacity

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

When the plate separation changes, the capacitance changes. The plates are charged to a constant value when they are in use, and a change in capacitance results in a change in voltage. Sound, as you remember, is a longitudinal wave; a series of alternating areas of high and low pressure, called compressions and rarefactions, which propagate through a medium such as air. Compression under high pressure the microphone pushes the diaphragm inward, reducing plate separation, increasing capacitance, and decreasing voltage.

Low pressure refraction pushes out the diaphragm, increasing plate separation, decreasing capacitance, and increasing voltage. Pressure and voltage must be directly proportional. Condenser microphone equations. The voltage is small and the change is even smaller, so an amplifier circuit is needed to bring the signal up to an acceptable level.

Capacity 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 permittivity of the medium filling the space between the plates (this formula is valid only when much less than the linear dimensions of the plates).

Capacitive proximity sensors for capacitive tanks capacitive tanks. . We are surrounded by tiny little capacitors. Dynamic RAM. Acceleration is determined by the differential capacitance of adjacent capacitors. To detect sudden vehicle braking as well as seat belt intentional seat belts or airbag deployment as needed to determine speed, position and attitude for time navigation and flight data systems to monitor vibrations in vehicles and detect unusual conditions such as asymmetric loading or potential failure determine if a mechanical device has been disabled and then activate safe mode to detect forced entry and then activate an alarm to detect strong earthquakes and then turn off pipelines or plumbing to measure changes in slope and warn of possible building collapse or simply determine if the surface conforms level or true to determine the orientation of the portable display and enter landscape or portrait mode depending on how to track motion in cameras, and then stabilize the image recorded for control i gaming systems, to analyze movement in professional and collegiate sports, to count the number of steps taken by a person walking or running, to measure the strength of the local gravitational field for geological research. Consider two conductors carrying charges of equal magnitude but opposite sign, as shown in the figure.

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

This combination of two conductors is called a capacitor. Conductors are called plates. Since the unit of potential difference is the volt, the potential difference is often referred to as voltage. We will use this term to describe the potential difference across a circuit element or between two points in space. What determines how much charge is on the plates of a capacitor at a given voltage?

The constant of proportionality depends on the shape and separation of the conductors. Therefore, capacitance is a measure of a capacitor's ability to store charge and electrical potential energy. Farad, which was named after Michael Faraday: Farad is a very large unit of capacitance. In practice, typical devices have capacitances ranging from microfarads to picofarads. Consider a capacitor formed from a pair of parallel plates as shown in the figure. Each plate is connected to one terminal of the battery, which acts as a source of potential difference.

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

At serial connection capacitors, the charges of all capacitors are the same. Total battery capacity successively connected capacitors is

If the capacitor is not initially charged, the battery sets up an electric field at connecting wires when connected. Let's focus on the plate connected to the negative battery terminal. Electric field applies a force to the electrons in the wire directly behind that plate; this force causes the electrons to move onto the plate. This movement continues until the plate, wire and terminal are at the same electrical potential. Once this equilibrium point is reached, the potential difference no longer exists between the terminal and the plate, and as a result there is no electric field, and the movement of electrons stops.

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

The plate now carries a negative charge. A similar process occurs on the other plate of the capacitor as electrons move from the plate to the wire, leaving the plate positively charged. In this final configuration, the potential difference across the capacitor plates is the same as between the battery terminals. Let's say we have a 4 pF capacitor. If a 9-volt battery is connected across this capacitor, one of the conductors ends with a net charge of -36 pC and the other ends with a net charge of 36 pC.

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 at the minimum thickness of the dielectric, however, its breakdown voltage decreases.

What is the meaning of capacitance in a capacitor? When a capacitor is connected to a voltage source, charge carriers enter the capacitor plates. The number of charge carriers on a capacitor depends on the applied voltage. If the voltage doubles, additional charge carriers enter the capacitor, so there are twice as many charge carriers on the capacitor.

Capacitor capacity

Therefore, the number of charge carriers and hence the charge on the capacitor is proportional to the applied voltage. Proportional means that the ratio of charge and applied voltage is constant. The ratio of charge and voltage is called the capacitance of the capacitor.

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 the 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 must not exceed the nominal voltage. For many types of capacitors, the allowable voltage decreases with increasing temperature.

The capacitance is the size of the component. Thus, it depends only on the physical capacitor variables, and not from the current voltage. This equation is very often required in electrical engineering. Therefore, you should remember them best. This formula is very well remembered by the cow cow.

After we saw in the last episode what the capacitance of a capacitor is, this article is about calculating the capacitance of a capacitor. As mentioned in the last article, the capacitance of a capacitor is the size of the component. That is, the capacitance of the capacitor is only from the capacitor itself, and not from the applied voltage or the like. dependent.

Polarity

Capacitors destroyed without explosion due to temperature and voltage not suitable for working.

Many oxide dielectric (electrolytic) capacitors function only with the correct voltage polarity due to the chemical nature of the interaction of the electrolyte with the dielectric. With a reverse voltage polarity, electrolytic capacitors usually fail due to the chemical destruction of the dielectric, followed by an increase in current, boiling of the electrolyte inside and, as a result, with the likelihood of an explosion of the case.

A capacitor is known to consist of two plates, even if in practice these plates are rather rolled films. The plates are spaced apart. So don't get confused. In the case of rolled capacitor films, it additionally contains an insulating film between the conductive films, which also have other positive properties, as we will soon see.

Finally, many charge carriers fit on these plates. The distance between the plates is vice versa. A long distance causes a small capacitance, a small distance corresponds to a large capacitance. In addition, there is a constant electric field constant ε0.

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

Capacitor insulation electrical resistance - r

Insulation resistance is the resistance of a capacitor direct current, determined by the ratio r = U / I ut, where U is the voltage applied to the capacitor, I ut- leakage current.

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1 farad [F] = 1000000 microfarad [uF]

Initial value

Converted value

farad exafarad petafarad terafarad gigafarad megafarad kilofarad hectofarad decafarad decifarad centifarad millifarad microfarad nanofarad picofarad femtofarad attofarad coulomb per volt abfarad CGSM capacitance unit statfarad CGSE capacitance unit

More about electrical capacitance

General information

Electrical capacitance is a value that characterizes the ability of a conductor to accumulate a charge, equal to the ratio of the electric charge to the potential difference between the conductors:

C = Q/∆φ

Here Q- electric charge, measured in coulombs (C), - potential difference, measured in volts (V).

In the SI system, electrical capacitance is measured in farads (F). This unit of measurement is named after the English physicist Michael Faraday.

A farad is a very large capacitance for an insulated conductor. So, a metal solitary ball with a radius of 13 solar radii would have a capacity of 1 farad. And the capacitance of a metal ball the size of the Earth would be about 710 microfarads (uF).

Since 1 farad is a very large capacitance, smaller values are used, such as: microfarad (uF), equal to one millionth of a farad; nanofarad (nF), equal to one billionth; picofarad (pF), equal to one trillion farad.

In the CGSE system, the basic unit of capacitance is the centimeter (cm). 1 centimeter of capacitance is the electric capacitance of a sphere with a radius of 1 centimeter placed in a vacuum. CGSE is an extended system of CGS for electrodynamics, that is, a system of units in which the centimeter, gram, and second are taken as base units for calculating length, mass, and time, respectively. In the extended CGS, including the CGSE, some physical constants are taken as unity in order to simplify the formulas and make calculations easier.

Capacity usage

Capacitors - devices for storing charge in electronic equipment

The concept of electric capacitance applies not only to the conductor, but also to the capacitor. A capacitor is a system of two conductors separated by a dielectric or vacuum. In the simplest version, the design of the capacitor consists of two electrodes in the form of plates (plates). Capacitor (from Latin condensare - “to condense”, “to thicken”) - a two-electrode device for accumulating charge and energy of an electromagnetic field, in the simplest case, it consists of two conductors separated by some kind of insulator. For example, sometimes radio amateurs, in the absence of finished parts, make tuning capacitors for their circuits from pieces of wire of different diameters, insulated with a varnish coating, while a thinner wire is wound on a thicker one. By adjusting the number of turns, radio amateurs fine-tune the equipment circuits to the desired frequency. Examples of images of capacitors on electrical diagrams shown in the figure.

History reference

Even 250 years ago, the principles of creating capacitors were known. So, in 1745 in Leiden, the German physicist Ewald Jürgen von Kleist and the Dutch physicist Pieter van Muschenbrook created the first capacitor - the “Leiden jar” - the walls of the glass jar were the dielectric in it, and the water in the vessel and the palm of the experimenter holding the vessel served as the plates. Such a "bank" made it possible to accumulate a charge of the order of a microcoulomb (μC). After it was invented, it was often experimented with and publicly presented. To do this, the jar was first charged with static electricity by rubbing it. After that, one of the participants touched the jar with his hand, and received a small electric shock. It is known that 700 Parisian monks, holding hands, conducted the Leiden experiment. At the moment when the first monk touched the head of the jar, all 700 monks, reduced to one convulsion, screamed in horror.

The “Leyden jar” came to Russia thanks to the Russian Tsar Peter I, who met Mushenbrook while traveling in Europe, and learned more about experiments with “ Leyden jar". Peter I founded the Academy of Sciences in Russia, and ordered various instruments for the Academy of Sciences from Mushenbruk.

In the future, capacitors improved and became smaller, and their capacitance - more. Capacitors are widely used in electronics. For example, a capacitor and an inductor form an oscillatory circuit that can be used to tune a receiver to a desired frequency.

There are several types of capacitors, differing in constant or variable capacitance and dielectric material.

Capacitor examples

The industry produces a large number of types of capacitors for various purposes, but their main characteristics are capacitance and operating voltage.

typical value containers capacitors vary from units of picofarads to hundreds of microfarads, with the exception of ionistors, which have a slightly different character of capacitance formation - due to the double layer at the electrodes - in this they are similar to electrochemical batteries. Nanotube-based supercapacitors have an extremely developed electrode surface. These types of capacitors have typical capacitance values of tens of farads, and in some cases they can replace traditional electrochemical batteries as current sources.

The second most important parameter of capacitors is its operating voltage. Exceeding this parameter can lead to the failure of the capacitor, therefore, when building real circuits, it is customary to use capacitors with a double value of the operating voltage.

To increase the values of capacitance or operating voltage, the method of combining capacitors into batteries is used. When two capacitors of the same type are connected in series, the operating voltage doubles, and the total capacitance is halved. At parallel connection two capacitors of the same type, the operating voltage remains the same, and the total capacitance doubles.

The third most important parameter of capacitors is temperature coefficient of capacitance change (TKE). It gives an idea of the change in capacitance under conditions of temperature changes.

Depending on the purpose of use, capacitors are divided into capacitors general purpose, the requirements for the parameters of which are not critical, and for special-purpose capacitors (high-voltage, precision and with various TKE).

Capacitor marking

Like resistors, depending on the dimensions of the product, a full marking can be used indicating the nominal capacity, derating class and operating voltage. For small-sized capacitors, use code marking of three or four digits, mixed alphanumeric marking and color marking.

The corresponding tables for recalculating markings by face value, operating voltage and TKE can be found on the Internet, but the most effective and practical method for checking the face value and serviceability of a real circuit element remains to directly measure the parameters of a soldered capacitor using a multimeter.

Warning: since capacitors can store a large charge at very high voltage, to avoid injury electric shock Before measuring the parameters of the capacitor, it is necessary to discharge it by shorting its terminals with a wire with a high resistance of external insulation. The standard wires of the measuring device are best suited for this.

Oxide Capacitors: This type of capacitor has a large specific capacitance, that is, the capacitance per unit weight of the capacitor. One plate of such capacitors is usually an aluminum tape coated with a layer of aluminum oxide. The second plate is the electrolyte. Since oxide capacitors have polarity, it is of fundamental importance to include such a capacitor in the circuit strictly in accordance with the voltage polarity.

Solid Capacitors: instead of a traditional electrolyte, they use an organic polymer that conducts current, or a semiconductor, as a lining.

Variable Capacitors: capacitance can be changed mechanically, by electrical voltage or by temperature.

Film Capacitors: The capacitance range of this type of capacitor is approximately 5pF to 100uF.

There are other types of capacitors.

Ionistors

These days, ionistors are gaining popularity. An ionistor (supercapacitor) is a hybrid of a capacitor and a chemical current source, the charge of which accumulates at the interface between two media - an electrode and an electrolyte. The creation of ionistors began in 1957, when a capacitor with a double electric layer on porous carbon electrodes was patented. The double layer as well as the porous material helped increase the capacitance of such a capacitor by increasing the surface area. In the future, this technology was supplemented and improved. Ionistors entered the market in the early eighties of the last century.

With the advent of ionistors, it became possible to use them in electrical circuits as voltage sources. These supercapacitors have long term service life, light weight, high charge-discharge rates. In the future, this type of capacitor can replace conventional batteries. The main disadvantages of supercapacitors are lower specific energy (energy per unit weight) than electrochemical batteries, low operating voltage and significant self-discharge.

Ionistors are used in Formula 1 cars. In energy recovery systems, during braking, electricity is generated, which is stored in the flywheel, batteries or ionistors for further use.

In consumer electronics, ionistors are used to stabilize the main power supply and as a backup power source for devices such as players, flashlights, automatic utility meters and other battery-powered devices with varying loads, providing power at increased load.

In public transport, the use of ionistors is especially promising for trolleybuses, since it becomes possible to implement autonomous running and increase maneuverability; ionistors are also used in some buses and electric vehicles.

Electric cars are currently produced by many companies, for example: General Motors, Nissan, Tesla Motors, Toronto Electric. The University of Toronto has partnered with Toronto Electric to develop an all-Canadian A2B electric vehicle. It uses ionistors along with chemical power sources, the so-called hybrid electrical energy storage. The engines of this car are powered by batteries weighing 380 kilograms. Also for recharging are used solar panels installed on the roof of the electric vehicle.

Capacitive touch screens

Modern devices are increasingly using touch screens, which allow you to control devices by touching indicator panels or screens. Touch screens are different types: resistive, capacitive and others. They can respond to one or more simultaneous touches. The principle of operation of capacitive screens is based on the fact that a large-capacity object conducts alternating current. In this case, this object is the human body.

Surface capacitive screens

Thus, a surface capacitive touch screen is a glass panel coated with a transparent resistive material. As a resistive material, an alloy of indium oxide and tin oxide, which has high transparency and low surface resistance, is usually used. Electrodes supplying the conductive layer with a small AC voltage, located at the corners of the screen. When touching such a screen with a finger, a current leakage appears, which is registered in four corners by sensors and transmitted to the controller, which determines the coordinates of the touch point.

The advantage of such screens is durability (about 6.5 years of clicks with an interval of one second, or about 200 million clicks). They have high transparency (about 90%). Thanks to these advantages, capacitive screens have been actively replacing resistive screens since 2009.

The disadvantage of capacitive screens is that they do not perform well when negative temperatures, there are difficulties with using such screens with gloves. If the conductive coating is located on the outer surface, then the screen is quite vulnerable, so capacitive screens are used only in those devices that are protected from the weather.

Projected capacitive screens

In addition to surface capacitive screens, there are projected capacitive screens. Their difference lies in the fact that inside the screen is covered with a grid of electrodes. The electrode that is touched together with the human body forms a capacitor. Thanks to the grid, you can get the exact coordinates of the touch. Projection-capacitive screen responds to touch in thin gloves.

Projected capacitive screens also have high transparency (about 90%). They are durable and strong enough, so they are widely used not only in personal electronics, but also in vending machines, including those installed on the street.

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