Why are capacitors needed and where are they used? Why are capacitors needed? Capacitor connection

Constant voltage and set the voltage on his crocodiles to 12 Volts. We also take a 12 Volt light bulb. Now we insert a capacitor between one probe of the power supply and the light bulb:

Nope, it doesn't burn.

But if you do it directly, it lights up:

This begs the conclusion: DC current does not flow through the capacitor!

To be honest, at the very initial moment of applying voltage, the current still flows for a split second. It all depends on the capacitance of the capacitor.

Capacitor in AC circuit

So, to find out whether AC current is flowing through the capacitor, we need an alternator. I think this frequency generator will do just fine:

Since my Chinese generator is very weak, instead of a light bulb load we will use a simple 100 Ohm one. Let’s also take a capacitor with a capacity of 1 microfarad:

We solder something like this and send a signal from the frequency generator:

Then he gets down to business. What is an oscilloscope and what it is used for, read here. We will use two channels at once. Two signals will be displayed on one screen at once. Here on the screen you can already see interference from the 220 Volt network. Do not pay attention.

We will apply alternating voltage and watch the signals, as professional electronics engineers say, at the input and output. Simultaneously.

It will all look something like this:

So, if our frequency is zero, then this means constant current. As we have already seen, the capacitor does not allow direct current to pass through. This seems to have been sorted out. But what happens if you apply a sinusoid with a frequency of 100 Hertz?

On the oscilloscope display I displayed parameters such as signal frequency and amplitude: F is the frequency Ma – amplitude (these parameters are marked with a white arrow). The first channel is marked in red, and the second channel in yellow, for ease of perception.

The red sine wave shows the signal that the Chinese frequency generator gives us. The yellow sine wave is what we already get at the load. In our case, the load is a resistor. Well, that's all.

As you can see in the oscillogram above, I supply a sinusoidal signal from the generator with a frequency of 100 Hertz and an amplitude of 2 Volts. On the resistor we already see a signal with the same frequency (yellow signal), but its amplitude is some 136 millivolts. Moreover, the signal turned out to be somewhat “shaggy”. This is due to the so-called ““. Noise is a signal with small amplitude and random voltage changes. It can be caused by the radio elements themselves, or it can also be interference that is caught from the surrounding space. For example, a resistor “makes noise” very well. This means that the “shaggyness” of the signal is the sum of a sinusoid and noise.

The amplitude of the yellow signal has become smaller, and even the graph of the yellow signal shifts to the left, that is, it is ahead of the red signal, or in scientific language, it appears phase shift. It is the phase that is ahead, not the signal itself. If the signal itself was ahead, then we would have the signal on the resistor appear in time earlier than the signal applied to it through the capacitor. The result would be some kind of time travel :-), which, of course, is impossible.

Phase shift- This difference between the initial phases of two measured quantities. In this case, tension. In order to measure the phase shift, there must be a condition that these signals same frequency. The amplitude can be any. The figure below shows this very phase shift or, as it is also called, phase difference:

Let's increase the frequency on the generator to 500 Hertz

The resistor has already received 560 millivolts. The phase shift decreases.

We increase the frequency to 1 KiloHertz

At the output we already have 1 Volt.

Set the frequency to 5 Kilohertz

The amplitude is 1.84 Volts and the phase shift is clearly smaller

Increase to 10 Kilohertz

The amplitude is almost the same as at the input. The phase shift is less noticeable.

We set 100 Kilohertz:

There is almost no phase shift. The amplitude is almost the same as at the input, that is, 2 Volts.

From here we draw profound conclusions:

The higher the frequency, the less resistance the capacitor has to alternating current. The phase shift decreases with increasing frequency to almost zero. At infinitely low frequencies its magnitude is 90 degrees orπ/2 .

If you plot a slice of the graph, you will get something like this:

I plotted voltage vertically and frequency horizontally.

So, we have learned that the resistance of a capacitor depends on frequency. But does it only depend on frequency? Let's take a capacitor with a capacity of 0.1 microfarad, that is, a nominal value 10 times less than the previous one, and run it again at the same frequencies.

Let's look and analyze the values:

Carefully compare the amplitude values ​​of the yellow signal at the same frequency, but with different capacitor values. For example, at a frequency of 100 Hertz and a capacitor value of 1 μF, the amplitude of the yellow signal was 136 millivolts, and at the same frequency, the amplitude of the yellow signal, but with a capacitor of 0.1 μF, was already 101 millivolts (in reality, even less due to interference ). At a frequency of 500 Hertz - 560 millivolts and 106 millivolts, respectively, at a frequency of 1 Kilohertz - 1 Volt and 136 millivolts, and so on.

From here the conclusion suggests itself: As the value of a capacitor decreases, its resistance increases.

Using physical and mathematical transformations, physicists and mathematicians have derived a formula for calculating the resistance of a capacitor. Please love and respect:

Where, X C is the resistance of the capacitor, Ohm

P - constant and equals approximately 3.14

F– frequency, measured in Hertz

WITH– capacitance, measured in Farads

So, put the frequency in this formula at zero Hertz. A frequency of zero Hertz is direct current. What will happen? 1/0=infinity or very high resistance. In short, a broken circuit.

Conclusion

Looking ahead, I can say that in this experiment we obtained (high-pass filter). Using a simple capacitor and resistor, and applying such a filter to the speaker somewhere in the audio equipment, we will only hear squeaky high tones in the speaker. But the bass frequency will be dampened by such a filter. The dependence of capacitor resistance on frequency is very widely used in radio electronics, especially in various filters where it is necessary to suppress one frequency and pass another.

In electrical stores, capacitors can most often be seen in the form of a cylinder, inside of which there are many strips of plates and dielectrics.

Capacitor - what is it?

A capacitor is part of an electrical circuit consisting of 2 electrodes that are capable of accumulating, focusing or transmitting current to other devices. Structurally, the electrodes are capacitor plates with opposite charges. In order for the device to work, a dielectric is placed between the plates - an element that prevents the two plates from touching each other.

The definition of condenser comes from the Latin word “condenso”, which means compaction, concentration.

Elements for soldering containers are used to transport, measure, redirect and transmit electricity and signals.

Where are capacitors used?

Every novice radio amateur often asks the question: what is a capacitor for? Beginners do not understand why it is needed and mistakenly believe that it can fully replace a battery or power supply.

All radio devices include capacitors, transistors and resistors. These elements make up a board or an entire module in circuits with static values, which makes it the basis for any electrical appliance, from a small iron to industrial devices.

The most common uses of capacitors are:

1. Filter element for HF and LF interference;
2. Levels sudden surges in alternating current, as well as for static and voltage on the capacitor;
3. Voltage ripple equalizer.

The purpose of the capacitor and its functions are determined by the purposes of use:

1. General purpose. This is a capacitor, the design of which contains only low-voltage elements located on small circuit boards, for example, devices such as a television remote control, radio, kettle, etc.;
2. High voltage. The capacitor in the DC circuit supports high-voltage industrial and technical systems;
3. Pulse. Capacitive generates a sharp voltage surge and supplies it to the receiving panel of the device;
4. Launchers. Used for soldering in those devices that are designed to start, turn on/off devices, for example, a remote control or control unit;
5. Noise suppressing. The capacitor in the AC circuit is used in satellite, television and military equipment.

Types of capacitors

The design of the capacitor is determined by the type of dielectric. It comes in the following types:

1. Liquid. Dielectric in liquid form is rare; this type is mainly used in industry or for radio devices;
2. Vacuum. There is no dielectric in the capacitor, but instead there are plates in a sealed housing;
3. Gaseous. Based on the interaction of chemical reactions and used for the production of refrigeration equipment, production lines and installations;
4. Electrolytic capacitor. The principle is based on the interaction of a metal anode and an electrode (cathode). The oxide layer of the anode is the semiconductor part, as a result of which this type of circuit element is considered the most productive;
5. Organic. The dielectric can be paper, film, etc. It is not able to accumulate, but only slightly level out voltage surges;
6. Combined. This includes metal-paper, paper-film, etc. The efficiency increases if the dielectric contains a metal component;
7. Inorganic. The most common ones are glass and ceramic. Their use is determined by durability and strength;
8. Combined inorganic. Glass-film, as well as glass-enamel, which have excellent leveling properties.

Types of capacitors

The elements of the radio board differ in the type of capacitance change:

1. Permanent. The cells maintain a constant voltage capacity until the end of their shelf life. This type is the most common and universal, as it is suitable for making any type of device;
2. Variables. They have the ability to change the volume of the container when using a rheostat, varicap or when the temperature changes. The mechanical method using a rheostat involves soldering an additional element onto the board, while when using a variconde, only the amount of incoming voltage changes;
3. Trimmers. They are the most flexible type of capacitor, with which you can quickly and efficiently increase the throughput of the system with minimal reconstruction.

Operating principle of a capacitor

Let's look at how a capacitor works when connected to a power source:

1. Charge accumulation. When connected to the network, the current is directed to the electrolytes;
2. Charged particles are distributed on the plate according to their charge: negative ones - into electrons, and positive ones - into ions;
3. The dielectric serves as a barrier between the two plates and prevents particles from mixing.

The capacitance of a capacitor is determined by calculating the ratio of the charge of one conductor to its potential power.

Important! The dielectric is also capable of removing the resulting voltage on the capacitor during operation of the device.

Capacitor Characteristics

The characteristics are conventionally divided into points:

1. The amount of deviation. Before entering the store, each capacitor must undergo a series of tests on the production line. After testing each model, the manufacturer indicates the range of permissible deviations from the original value;
2. Voltage value. Mostly elements with a voltage of 12 or 220 Volts are used, but there are also 5, 50, 110, 380, 660, 1000 and more Volts. In order to avoid capacitor burnout and dielectric breakdown, it is best to purchase an element with a voltage reserve;
3. Permissible temperature. This parameter is very important for small devices operating on a 220 Volt network. As a rule, the higher the voltage, the higher the permissible temperature level for operation. Temperature parameters are measured using an electronic thermometer;
4. Availability of direct or alternating current. Perhaps one of the most important parameters, since the performance of the designed equipment completely depends on it;
5. Number of phases. Depending on the complexity of the device, single-phase or three-phase capacitors can be used. To connect an element directly, a single-phase one is sufficient, but if the board is a “city”, then it is recommended to use a three-phase one, as it distributes the load more smoothly.

What does capacity depend on?

The capacitance of the capacitor depends on the type of dielectric and is indicated on the case, measured in uF or uF. It ranges from 0 to 9,999 pF in picofarads, while in microfarads it ranges from 10,000 pF to 9,999 µF. These characteristics are specified in the state standard GOST 2.702.

Note! The larger the electrolyte capacity, the longer the charging time, and the more charge the device can transfer.

The greater the load or power of the device, the shorter the discharge time. In this case, resistance plays an important role, since the amount of outgoing electrical flow depends on it.

The main part of the capacitor is the dielectric. It has the following number of characteristics that affect the power of the equipment:

1. Insulation resistance. This includes both internal and external insulation made from polymers;
2. Maximum voltage. The dielectric determines how much voltage the capacitor is capable of storing or transmitting;
3. The amount of energy loss. Depends on the configuration of the dielectric and its characteristics. Typically, energy dissipates gradually or in sharp bursts;
4. Capacity level. In order for a capacitor to store a small amount of energy for a short period of time, it needs to maintain a constant volume of capacitance. Most often, it fails precisely because of the inability to pass a given amount of voltage;

Good to know! The abbreviation “AC” located on the element body denotes alternating voltage. The accumulated voltage on the capacitor cannot be used or transmitted - it must be extinguished.

Capacitor Properties

The capacitor acts as:

1. Inductive coil. Let's take the example of a regular light bulb: it will light up only if you connect it directly to an AC source. This leads to the rule that the larger the capacity, the more powerful the luminous flux of the light bulb;
2. Charge storage. Properties allow it to quickly charge and discharge, thereby creating a powerful impulse with low resistance. Used for the production of various types of accelerators, laser systems, electric flashes, etc.;
3. The battery received charge. A powerful element is capable of maintaining the received portion of current for a long time, while it can serve as an adapter for other devices. Compared to a rechargeable battery, a capacitor loses some of its charge over time, and is also not able to accommodate a large amount of electricity, for example, for industrial scale;
4. Charging the electric motor. The connection is made through the third terminal (operating voltage of the capacitor is 380 or 220 Volts). Thanks to the new technology, it has become possible to use a three-phase motor (with a phase rotation of 90 degrees), using a standard network;
5. Compensator devices. It is used in industry to stabilize reactive energy: part of the incoming power is dissolved and adjusted at the output of the capacitor to a certain volume.

Video

In Fig. Figure 4.11 shows an electric generator circuit containing a capacitor. Once the circuit is turned on, a voltmeter connected to the circuit will show the full voltage of the generator. The ammeter needle will be set to zero - no current can flow through the capacitor insulation.

But let's carefully follow the ammeter needle when turning on an uncharged capacitor. If the ammeter is sensitive enough and the capacitance of the capacitor is large, then it is not difficult to detect the oscillation of the needle: immediately after switching on, the needle will go from zero, and then quickly return to its original position.

Rice. 4.11. Electric generator circuit containing a capacitor

This experience shows that when the capacitor was turned on (while charging), a current flowed in the circuit - charges moved in it: electrons from the plate connected to the positive pole of the source moved to the plate connected to the negative pole.

As soon as the capacitor is charged, the movement of charges stops.

By turning off the generator and reconnecting it to the capacitor, we will no longer detect the movement of the needle: the capacitor remains charged, and when turned on again, there is no movement of charges in the circuit.

In order to observe the needle deflection again, you need to short-circuit the generator to the discharged capacitor. To this end, having previously turned off the generator, we close the capacitor plates with a wire, and a spark will jump between the terminals of the capacitor and the wire brought to them, thereby making it easy to verify that when the capacitor is discharged, current flows again in its circuit.

If the circuit is made with a wire so that the path of the charges passes through the ammeter, then it is easy to see that its needle will briefly deviate. The deflection of the arrow should now occur, of course, in the other direction.

After discharging the capacitor, you can repeat the first experiment - the ammeter needle will again show that electric charges are moving in the capacitor circuit (current is passing).

Let's try to calculate the current flowing in the wires connected to the capacitor.

If over a period of time the voltage of the capacitor increases by , then, during the same time, its charge will increase by

i.e., the charge of the capacitor increases by the product of the capacitance and the voltage increment.

Suppose that the voltage across a capacitor with a capacitance increases by 50 V in a time of one tenth of a second. In this case, during the same time, the charge on the positive plate of the capacitor increased by

But in order for such a charge to pass through the wires in time c, an average current must flow through them

Charging a capacitor through a resistor. Let's imagine that a generator with a constant voltage is connected through a resistor with resistance to an uncharged capacitor with a capacitance (Fig. 4.12, a).

At the initial moment, while the capacitor is not yet charged, its voltage is zero.

This means that all the source voltage falls on the resistance R. This means that, according to Ohm’s law, current will flow in the circuit

Over time, on the contrary, the capacitor will charge, its voltage will be equal to the voltage of the generator, there will be no current in the circuit, and there will be no voltage across the resistor.

Rice. 4.12. a - charge of capacitor C through a resistor with a resistance. On the left is an electrical circuit in which the generally accepted image of a capacitor is used; on the right is shown how the voltage on capacitor c increases over time and how the current g gradually decreases. These graphs are plotted on the assumption that the capacitor with a capacitance 100 µF is charged from a 100 V DC source through a 10,000 ohm resistance. In this case, charging occurs very slowly. If the capacitance were only 1 µF and the resistance 1 ohm, everything would happen a million times faster. In order for the given graphs to be suitable for the second case, it must be assumed that time is expressed not in seconds, but in millionths of a second (in the general case, for any R and C, the time values ​​​​indicated on the graph should be multiplied by the product of C and R). If the source voltage remains 100 V, then the current values ​​must be increased by 10,000 times. For example, at the initial moment a current will flow not 10 mA, but 100 A. The duration and nature of the process do not depend on the source voltage; b - discharge of capacitor C through a resistor with resistance R. An electrical circuit is shown on the left. After charging, the capacitor turns off. On the right is how the capacitor current and voltage change over time. The graphs are plotted for the case. Reducing the capacitance and resistance to 1 ohm would increase the discharge rate a million times. Initial; the current value (with the initial voltage unchanged) would increase 10,000 times and would be 100 A instead of 10 mA. For other values ​​of R and C, the time shown on the graph must be multiplied by the product

In this case, the charge of the capacitor must be equal to

Let us pose the following question: how quickly can a charge of one hundredth of a coulomb be imparted to a capacitor?

If the current in the circuit did not decrease, but remained equal, i.e., 10 mA, then this would require a time equal to only 1 s:

But let us consider whether such a current can flow for a long time. If such a current flowed for a quarter of a second, it would already impart a quarter of the full charge to the capacitor, and therefore would raise its voltage to a quarter of the full 100 V.

But when the capacitor voltage increases to 25 V, the current should decrease to 7.5 mA. In fact, if the generator voltage is 100 V and the voltage across the capacitor is 25 V, then the difference between them is accounted for by the resistor.

Again according to Ohm's law

But such a current will charge the capacitor more slowly than a current of 10 mA would charge it.

From the above discussion it is clear that:

the voltage across the capacitor will increase, gradually slowing down;

the current, having reached its highest value at the initial moment, then gradually decreases;

The greater the capacitance (the greater the charge) and the greater the resistance of the circuit, the slower the capacitor charges.

Discharging a capacitor to a resistor. If you turn off the generator and close the capacitor plates through a resistor with resistance R, the process of discharging it will begin. In Fig. Figure 4.12, b shows the current and voltage curves of the capacitor during its discharge.

Electric field energy in a capacitor. A charged capacitor has a certain amount of energy contained in its electric field.

This can be judged by the fact that a charged capacitor, disconnected from the network, is capable of maintaining an electric current for some time - this can also be judged by the spark observed when the capacitors are discharged.

The energy contained in the capacitor is supplied to it while it is being charged by the generator. In fact, during its charging, current flows in the circuit and voltage is applied to its terminals, which means that energy is imparted to it. The total amount of energy stored by the capacitor can be expressed by the formula

Energy is equal to half the square of the voltage times the capacitance.

If the voltage is expressed in volts and the capacitance is in farads, then the energy will be expressed in joules.

Thus, the energy stored in a capacitor with a capacity of 100 μF at a voltage of 1000 V,

This, of course, is not very much energy (this energy is absorbed by a 50 W light bulb every second). But if the capacitor discharges quickly (say, in one thousandth of a second), then the power of the resulting energy discharge is, of course, very large:

Therefore, it is clear that when a large capacitor is discharged, the sound is similar to a gunshot.

A rapid discharge of energy stored in a capacitor is sometimes used to weld small metal products.

When a capacitor is discharged onto a resistor, the energy contained in the electrical capacitor is converted into heat from the heated resistor.

Application of capacitors. The applications of capacitors in electrical engineering are very diverse.

Let's look at some of them here.

1. Capacitors are widely used for the purpose of isolating two circuits at direct voltage while maintaining the connection between them at alternating current. Capacitors isolate DC voltage without allowing DC current to pass through. At the same time, the slightest change in voltage changes their charge and, therefore, passes a corresponding alternating current through them (Fig. 4.13).

Rice. 4.13. At the input of the circuit between points a and b, a constant voltage and a small, time-varying voltage are applied - its shape corresponds to the transmitted signal. The capacitor does not pass direct current (corresponding to ). A small changing voltage A changes the charge of the capacitor. The flowing charging current creates a voltage drop across a high resistance circuit. This voltage drop is very close to the value of the AC voltage. Thus, the voltage at the output of the circuit between points c and d is approximately equal to

2. Smoothing devices (filters that do not pass alternating voltage) are based on the properties of a capacitor to pass current under the influence of a changing voltage and not allow current to pass under the influence of a constant voltage. In Fig. Figure 4.14 shows such a device - alternating current passes through the first resistor and capacitor, but due to the large capacitance of the capacitor, the voltage fluctuation across it is very small. At the output of the circuit, the voltage is smoothed - it is close to constant.

Even stronger smoothing can be achieved by including inductive coils L instead of resistors.

Rice. 4.14. A smoothing device containing R and C. Voltage fluctuations at the input of the circuit are not transmitted to the output. The output voltage is close to constant

As was shown in Chap. 2, when a changing current flows, an emf is induced in them, preventing current fluctuations. Such a smoothing device is shown in Fig. 4.15.

3. In Fig. Figure 4.16 schematically shows a device for igniting a combustible mixture in the cylinders of a car engine.

Rice. 4.15. A smoothing device containing L and C. A voltage is applied to the input, which fluctuates noticeably over time. Load voltage is almost constant

Current from the battery passes through the primary winding of the coil. At the right moment it is interrupted by special moving contacts. A rapid change in current induces an emf of mutual induction in the secondary winding of the coil. The number of turns of the secondary winding is very large, and the current is broken quickly. Therefore, the EMF induced in the secondary winding can reach 10-12 thousand V. At this voltage, a spark discharge occurs between the electrodes of the “candle”, igniting the working mixture in the cylinder. Contact interruption occurs very often: for example, in a four-cylinder engine, one contact break occurs for every engine revolution.

In the diagram in Fig. Figure 4.16 shows a capacitor connected to the breaker terminals.

Let's explain its purpose.

In the absence of a capacitor, a circuit break would be accompanied by the formation of a spark between the contacts of the breaker.

Rice. 4.16. Diagram of a circuit used to electrically ignite a combustible mixture in the cylinders of a car engine: - breaker. Below is a cross-section of a cylinder with a piston, above which a mixture of air and gasoline is ignited by an electric spark jumping between the electrodes of the spark plug

Not to mention the fact that a frequently appearing spark would quickly lead to wear of the contacts, the presence of a spark prevents a sharp break in the current: the current, after the contacts separate, still remains closed through the spark and only gradually drops to zero.

If a capacitor is connected between the breaker contacts (as shown in Fig. 4.16), the picture will be different. When the contacts begin to diverge, the current circuit does not break - the current closes through the not yet charged capacitor. But the capacitor quickly charges, and further current flow is impossible.

The voltage on a charged capacitor can greatly exceed 12 V, since a decrease in the current in the primary winding of the coil induces a large self-induction emf in it.

Despite this, a spark no longer occurs between the contacts of the breaker, since by this moment the contacts of the breaker have time to move far enough away from each other.

When the breaker contacts close again, the capacitor will quickly discharge and will be ready for use when the contacts open again.

Thus, the capacitor protects the contacts from burning and improves the operation of the ignition system.

In the diagram in Fig. 4.16, an additional resistance can be connected next to the capacitor. Its purpose will become clear after we consider electrical oscillations in the inductance-capacitor system.

Rice. 4.17. Discharge of a capacitor into inductance. Electrical oscillations occur in such a circuit (see Fig. 4.18)

4. One of the very important applications of capacitors is in alternating current circuits (improving the “cosine phi”). It is discussed in Chap. 6.

The use of capacitors in the oscillatory circuits of generators is described in Chapter. 8.

These applications of capacitors are based on electrical fluctuations in the LC (inductance and capacitance) system.

Discharge of a capacitor into inductance. Electrical vibrations. Let's consider what happens if a charged capacitor is connected to a coil that has inductance and very low resistance (Fig. 4.17).

Let's take a capacitor C, charged to a voltage in its electric field, while energy is stored

We connect the capacitor to the inductive coil. Obviously, the capacitor will begin to discharge. However, due to the emerging EMF of self-induction, the current in the coil increases gradually (§ 2.16 and 2.18). The current was initially zero, but gradually it increases. As current flows, the capacitor discharges; its tension decreases.

But we know that the rate of rise of current - or in general the rate of change of current - in inductance is proportional to the voltage applied to it (carefully consider, if necessary, § 2.16).

As the voltage across the capacitor decreases, the rate of current rise decreases.

We said that the rate of rise of the current decreases, but this does not mean at all that the current itself decreases.

Rice. 4.18. Changes in the voltage across the capacitor and the discharge current in the circuit shown in Fig. 4.17. The current and voltage values ​​​​given here correspond to the discharge of a capacitor with a capacity of C = 4 μF, pre-charged to voltage . Coil inductance L = 1.6 mH. These data correspond to the period

Indeed, consider the graphs of capacitor voltage and current presented in Fig. 4.18.

At first, the current was zero, but it increased very quickly (this can be seen from the steepness of the rise of the curve line depicting the dependence of the current on time). At the end of the discharge of the capacitor, when its voltage became zero, the current stopped increasing - it reached its maximum value and no longer increases.

We can express all this with the following equation:

The voltage across the capacitor is always equal to the voltage across the inductance, equal to the rate of current rise multiplied by the inductance L.

The capacitor has discharged.

The energy contained in the capacitor's electric field has left the capacitor. But where did she go?

In the event of a capacitor discharge into a resistance, the energy is converted into heat from the heated resistance. But in the example we are considering now, the circuit resistance is negligible (we neglected it completely). Where is the energy contained in the capacitor now?

The energy transferred from the electric field of the capacitor to the magnetic field of the inductance.

In fact, at the beginning of the process there was no current in the inductance; when the current in the inductance reached a value, energy appeared in its magnetic field

Based on the law of conservation of energy, it is not difficult to find the highest value that is achieved by the current at the moment the voltage on the capacitor becomes zero.

At this moment, there is no energy in the capacitor, which means that all the energy initially stored in it has turned into the energy of the magnetic field. Equating their expressions, we find

Obviously, at any point in time, when the voltage across the capacitor is less than and the current is less than, the total energy is equal to the sum of the energies of the electric and magnetic fields:

This total energy is equal to the initial energy reserve. Let's check what has been said on those numerical values ​​that are easy to find from the graph shown in Fig. 4.18.

Each division along the time axis corresponds to 50 μs (microseconds). Let us find from the graph the values ​​of current and voltage at a time of 50 μs. They are approximately equal

This means that the energy of the electric field at this moment is

The energy of the magnetic field at the same moment is equal to

The total energy at this moment in time (as at any other) is equal to the energy originally contained in the capacitor:

So, we have explained what happens during the period of time it takes for the capacitor to completely discharge.

In Fig. 4.18 this corresponds to the current and voltage curves relating to the interval indicated by the number I (time from 0 to 125 μs).

But the matter does not end there. Although the capacitor is completely discharged, a large current flows in the circuit. This current cannot immediately disappear, since its existence is associated with the energy of the magnetic field.

This current continues to flow in the circuit and recharges the capacitor: it continues to carry electrons away from the negative plates and transfer them to the positive plates, or rather, transfer them from the plates that were negative to the plates that were positive. The sign of charge on the plates now changes.

A voltage appears on the capacitor, preventing further current flow, and the current gradually begins to decrease.

By the end of the time period indicated by number II (at the time of 250 μs), the current drops to zero. But by this moment the capacitor will again be fully charged; all the energy that went into the magnetic field has now turned back into the energy of the electric field.

The current is zero. The capacitor has the same voltage as at the beginning (only of a different sign). Everything starts again, as described: the capacitor begins to discharge, the current begins to increase, etc.

The only difference is in the sign of the voltage on the capacitor and, accordingly, in the direction of the current: the current remains negative for periods of time indicated by numbers III and IV.

At the end of interval IV (i.e. after 500 μs has passed), everything will return to its original state - the capacitor is positively charged and there is no current.

From this moment on, everything repeats all over again.

The considered picture represents electrical oscillations in the LC circuit.

The time required for everything to return to its original state after the start of the discharge is called period (T).

At the values ​​of capacitance and inductance for which the graphs in Fig. 4.18, one period is 500 μs. The greater the inductance and capacitance, the longer the oscillation period.

The relationship between these three quantities is expressed by the equality

The considered oscillations are called free (as opposed to forced), since they occur in the absence of an extraneous source of energy that could cause the voltage to change according to some other law.

Such fluctuations will be discussed below, in Chapter. 5 and 6. The following will be shown there: one source (generator) produces a voltage that varies according to a law similar to that shown in Fig. 4.18, and if an inductor is connected to the source, then current will flow in it

here are the highest values ​​of fluctuating voltage and current; - a value equal to the number divided by the oscillation period:

We examined the oscillations that occur when a capacitor is discharged, neglecting the circuit resistance. In fact, in any oscillatory circuit the resistance cannot be considered zero.

The presence of a small resistance in the circuit leads to a gradual attenuation of the oscillations, since the energy of the electromagnetic field is dissipated in the resistance - it turns into heat in accordance with the Joule-Lenz law.

Rice. 4.19. Damped oscillatory discharge. The given graph of the voltage on the capacitor corresponds to the data: , initial voltage on the capacitor.

Therefore, each time all the energy is again concentrated in the electric field of the capacitor, the voltage across the capacitor turns out to be less:

In Fig. Figure 4.19 shows the voltage curve across a capacitor in an RLC circuit (that is, in a circuit containing, in addition to inductance and capacitance, also resistance).

If the resistance in the circuit is sufficiently large, oscillations do not occur at all. The capacitor discharge occurs, as they say, aperiodically. Such a discharge is shown in Fig. 4.20. The discharge can be made aperiodic and by connecting a resistor in parallel with the capacitor.

The concept of various applications of an oscillatory system (oscillatory circuit) will be given in Chapter. 6 and 8.

Rice. 4.20. Aperiodic capacitor discharge. The graph shows the voltage and current in the capacitor circuit with the same inductance and capacitance (L = 1.6 MH, C = 4 μF) and with a circuit resistance of 64 Ohms

For now we will limit ourselves to pointing out that the presence of a capacitor between the contacts of the breaker in a car (Fig. 4.16) can serve as a source of oscillations that interfere with radio reception. These oscillations can be “damped” if an additional resistor is introduced (in accordance with the diagram in Fig. 4.20).

A capacitor, conder, air conditioner - this is what experienced specialists call it - one of the most common elements used in various electrical circuits. A capacitor is capable of storing an electric current charge and transferring it to other elements in an electrical circuit.
The simplest capacitor consists of two plate electrodes separated by a dielectric; an electric charge of different polarity accumulates on these electrodes; one plate will have a positive charge and the other will have a negative charge.

The principle of operation of a capacitor and its purpose- I will try to answer these questions briefly and very clearly. In electrical circuits, these devices can be used for various purposes, but their main function is to store electrical charge, that is, a capacitor receives electric current, stores it and subsequently transfers it to the circuit.

When a capacitor is connected to an electrical network, an electrical charge begins to accumulate on the electrodes of the capacitor. At the beginning of charging, the capacitor consumes the greatest amount of electric current; as the capacitor is charged, the electric current decreases and when the capacitor’s capacity is filled, the current will disappear completely.

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, itself, as it were, becoming a power source.

The main technical characteristic of a capacitor is its capacity. Capacitance is the ability of a capacitor to accumulate electrical charge. The larger the capacitance of the capacitor, the more charge it can accumulate and, accordingly, release back into the electrical circuit. The capacitance of a capacitor is measured in Farads. Capacitors vary in design, materials from which they are made and areas of application. The most common capacitor is - constant capacitor, it is designated as follows:

Constant-capacity capacitors are made from a wide variety of materials and can be metal-paper, mica, or ceramic. Such capacitors as an electrical component are used in all electronic devices.

Electrolytic capacitor

The next common type of capacitors is polar electrolytic capacitors, its image on the electrical diagram looks like this -

An electrolytic capacitor can also be called a permanent capacitor because its capacitance does not change.

But eh electrolytic capacitors have a very important difference, the (+) sign near one of the electrodes of the capacitor indicates that this is a polar capacitor and when connecting it to the circuit, polarity must be observed. The positive electrode must be connected to the plus of the power source, and the negative (which does not have a plus sign) correspondingly to the negative - (on the body of modern capacitors the designation of the negative electrode is applied, but the positive electrode is not designated in any way).

Failure to follow this rule can lead to capacitor failure and even an explosion, accompanied by scattering of foil paper and a bad smell (from the capacitor, of course...). Electrolytic capacitors can have a very large capacity and, accordingly, accumulate quite a large potential. Therefore, electrolytic capacitors are dangerous even after the power is turned off, and if handled carelessly, you can receive a strong electric shock. Therefore, after removing the voltage, for safe work with an electrical device (electronics repair, setup, etc.), the electrolytic capacitor must be discharged by short-circuiting its electrodes (this must be done with a special discharger), especially for large capacitors that are installed on power supplies where there is high voltage.

Variable capacitors.

As you understand from the name, variable capacitors can change their capacitance - for example, when tuning radio receivers. More recently, only variable capacitors were used to tune radio receivers to the desired station; rotating the receiver tuning knob thereby changed the capacitance of the capacitor. Variable capacitors are still used today in simple, inexpensive receivers and transmitters. The design of a variable capacitor is very simple. Structurally, it consists of stator and rotor plates, the rotor plates are movable and enter the stator plates without touching the latter. The dielectric in such a capacitor is air. When the stator plates enter the rotor plates, the capacitance of the capacitor increases, and when the rotor plates exit, the capacitance decreases. The designation of a variable capacitor looks like this -

APPLICATION OF CAPACITORS

Capacitors are widely used in all areas of electrical engineering; they are used in various electrical circuits.
In an alternating current circuit they can serve as capacitance. Let's take this example: when a capacitor and a light bulb are connected in series to a battery (direct current), the light bulb will not light up.

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 qualities, capacitors are used as filters in circuits that suppress high-frequency and low-frequency interference.

Capacitors are also used in various pulse circuits where the rapid accumulation and release of a large electrical charge is required, in accelerators, photo flashes, pulsed lasers, due to the ability to accumulate a large electrical charge and quickly transfer it to other elements of the network with low resistance, creating a powerful pulse.Capacitors are used to smooth out ripples during voltage rectification. The ability of a capacitor to retain a charge for a long time makes it possible to use them for storing information. And this is only a very short list of everything where a capacitor can be used.

As you continue your studies in electrical engineering, you will discover many more interesting things, including the work and use of capacitors. But this information will be enough for you to understand and move forward.

How to check a capacitor

To check capacitors you need a device, tester or otherwise multimeter. There are special devices that measure capacitance (C), but these devices cost money, and there is often no point in purchasing them for a home workshop, especially since there are inexpensive Chinese multimeters on the market with a capacitance measurement function. If your tester does not have such a function, you can use the usual dialing function - to how to ring with a multimeter, as when checking resistors - what is a resistor. The capacitor can be checked for “breakdown”; in this case, the resistance of the capacitor is very large, almost infinite (depending on the material from which the capacitor is made). Electrolytic capacitors are checked as follows - It is necessary to turn on the tester in the continuity mode, connect the probes of the device to the electrodes (legs) of the capacitor and monitor the reading on the multimeter indicator, the multimeter reading will change downward until it stops completely. After which you need to swap the probes, the readings will begin to decrease almost to zero. If everything happened as I described, the Conder is working. If there is no change in the readings or the readings immediately become large or the device shows zero, the capacitor is faulty. Personally, I prefer to check the “air conditioners” with a dial gauge; the smooth movement of the needle is easier to track than the flashing of numbers in the indicator window.

Capacitor capacity measured in Farads, 1 farad is a huge value. Such a capacity will have a metal ball whose dimensions will exceed the size of our sun by 13 times. A sphere the size of planet Earth would have a capacity of only 710 microfarads. Typically, the capacitance of capacitors that we use in electrical devices is indicated in microfarads (mF), picofarads (nF), nanofarads (nF). You should know that 1 microfarad is equal to 1000 nanofarads. Accordingly, 0.1 uF is equal to 100 nF. In addition to the main parameter, the permissible deviation of the actual capacity from the specified one and the voltage for which the device is designed are indicated on the body of the elements. If it is exceeded, the device may fail.

This knowledge will be enough for you to start and to independently continue studying capacitors and their physical properties in special technical literature. I wish you success and perseverance!